Open and breathable apparatus. Formation of a flammable environment inside process equipment during normal operation Valves made of solid crushed materials
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In production conditions, various flammable liquids are obtained, processed or participate in the technological process as auxiliary materials in a cold and heated state, at different pressures and in devices of different design. In modern production facilities, technological processes are sealed, i.e. substances are enclosed in apparatus or pipelines, the interior of which can serve as a place for a fire.
Apparatus with a fixed liquid level.
Inside a closed apparatus with a stationary liquid level, a combustible medium can form only if there is a volume free of liquid (gas space) in the apparatus, which communicates with the atmosphere and is saturated to one degree or another with liquid vapor.
The following technical solutions contribute to the prevention of the formation of a combustible medium in closed apparatus with a fixed liquid level:
1. Elimination of the gas space is achieved:
· The maximum filling of the apparatus or container with liquid. In this case, there may be the following emergency situations:
Overflow;
· Destruction of the apparatus;
· Overflow when the temperature rises.
· Storage of liquid under a protective layer of water; (eg H 42 0S);
· The use of tanks with a floating roof; the use of tanks with a fixed roof and a floating pontoon;
· The use of containers with flexible inner shells.
2. Maintaining a safe temperature regime. This is achieved through control and regulation systems. The operating temperature is maintained below the lower or above the upper temperature limit for the spread of the liquid flame.
3.Reduction of the concentration of vapors of a combustible liquid at a given temperature below the lower concentration limit of flame propagation. This is achieved:
* the use of highly resistant foams, emulsions and hollow micro-balls, floating on the surface of the liquid and preventing its evaporation;
Apparatus and pipelines with fire and explosion hazardous substances used in various technologies, under certain conditions, can become a place of fire or explosion. To identify the possibility of combustion inside the technological equipment, it is necessary, first of all, to assess the possibility of the formation of a combustible medium in it. A combustible medium is understood as a mixture of a combustible substance with an oxidizer in such ratios that the occurrence and further development of combustion is possible.
To assess the possibility of the formation of a combustible medium inside the process equipment, it is necessary to know the main operating parameters (operating temperature, pressure, concentration), and for devices with liquids, it is also necessary to have information about the presence of free volume. This information is contained in the technological documentation.
The conditions for the formation of a combustible medium in devices with flammable gases, liquids, solids and dusts are somewhat different.
Apparatus with gases most often they are filled with clean combustible gases without oxidizing agents. Such devices are always under excess pressure, therefore, the flow of air into them is impossible, and therefore, the formation of a combustible medium is also impossible.
In rare cases, according to the conditions of technology, it is necessary to supply a mixture of combustible gas with air or oxygen to the apparatus (for example, when hydrogen is produced by conversion of methane or when acetylene is obtained by
Table 2.2 - Fire hazard analysis of devices
thermal oxidative pyrolysis of natural gas). In such situations, the possibility of the formation of a combustible medium is assessed by comparing the working concentration j p with the lower and upper concentration limits of flame propagation. A combustible environment will take place if the condition is met:
In closed devices with liquids a combustible medium can form only when there is a free volume above the surface (mirror) of the liquid. In this case, any liquid in the apparatus will evaporate, and its vapors will gradually be distributed in the free space. If there is air or any other oxidizing agent in the free space of the apparatus, then liquid vapors, mixing with it, can form a flammable medium.
The presence of a free space above the liquid mirror is a necessary, but not sufficient condition for the formation of a combustible medium. In order to find out the presence of a combustible vapor-air mixture in the apparatus, it is necessary, as in the case with gases, to check condition (2.3).
However, it should be borne in mind that the concentration of vapors over the height of the free space is distributed unevenly. Above the surface of the liquid, it is close to the saturation concentration, and at the roof of the apparatus, its values \u200b\u200bare minimal. Even at the same height, the concentration will differ at different times from the beginning of evaporation. This is primarily due to the peculiarities of the process of vapor diffusion into the free space of the apparatus. That is, for technological equipment with flammable liquids, it is characteristic that only a certain concentration region can be present in the free space, which is between the lower and upper concentration limits of ignition. The height of the location of the zone of hazardous concentrations changes over time. Methods for calculating the concentration of vapors in the free space of devices with liquids can be found in special literature.
For devices with a stationary liquid level (for example, for continuous operation), the assessment of the possibility of the formation of a combustible medium can be facilitated. The operation of such devices is characterized by constant values \u200b\u200bof the working concentration at a constant temperature and pressure in the device. Taking this into account, an assessment of the possibility of the formation of a combustible medium can be carried out by comparing the working temperature of the liquid t p with the values \u200b\u200bof the temperature limits of flame propagation. A combustible medium in devices with a stationary liquid level will be formed if the condition is met:
(2.4)
Condition (2.4) can also be used for devices with a moving liquid level during the period of their filling after downtime. This is due to the fact that when the liquid level in the apparatus rises, the saturated concentration of the vapor-air mixture above the liquid mirror does not change. In the case of emptying such devices, the state of saturation of the free space with liquid vapors is disturbed due to the flow of additional air through the breathing armature. In this case, the concentration of vapors above the liquid mirror decreases and can become dangerous. Therefore, the assessment of the possibility of the formation of a combustible medium during the period of emptying the apparatus is made only by condition (2.3).
So, in the general case, the possibility of the formation of a flammable environment in closed apparatus with flammable and flammable liquids can be estimated by:
1) checking the presence of a free vapor-air volume above the liquid mirror;
2) comparison of the working concentration of liquid vapors with the concentration limits of ignition;
3) comparison of the working temperature of the liquid in the apparatus with the values \u200b\u200bof the temperature limits of ignition.
In technological equipment with solid combustible substances and materialsa combustible medium can form when exposed to heat on the latter or as a result of their self-heating. As you know, solid combustible substances and materials themselves are not capable of forming a combustible medium in a mixture with air. However, in the process of heating them to certain temperatures, the decomposition process may begin with the release of volatiles. So, in the process of pyrolysis of wood at temperatures of 150 - 275 ° C, decomposition of its less heat-resistant components occurs with the release of carbon monoxide, acetic acid, methane, hydrogen and other substances. The released decomposition products in a mixture with an oxidizing agent can form a combustible mixture under certain conditions. In such cases, the assessment of the possibility of the formation of a combustible medium in technological equipment is carried out, as in the case of gases, according to condition (2.3).
Technological apparatus with combustible dust are characterized by a significant fire hazard. When operating mills, crushers, cotton baking powder, centrifugal classifiers, pneumatic conveying systems, a very large amount of dust is generated. Dust in such devices can be suspended in air (aerosol) and in a settled state (airgel). In the first case, the fire hazard of dust is considered both for gases and vapors, in the second case, as for solids and materials.
Dust suspended in the air can form explosive concentrations. To assess the possibility of the formation of a combustible medium inside technological equipment with dusty materials, in practice, the value of the lower concentration limit of flame propagation j n is used. The upper concentration limits for dust are so high that they have no practical value for assessing fire hazard. In addition, dust-air mixtures are more prone to stratification than steam and gas-air mixtures. Therefore, in the equipment, even at very high concentrations, local zones with a concentration below the VCPR can always form.
When determining the working (actual) concentration of dust inside the process equipment, it is necessary to take into account the mass of suspended and settled dust. A combustible medium in devices with dust will form if the following condition is met:
Explosions and fires inside process equipment often occur in transient periods... Such periods include start-up of devices into operation and their stop for routine inspection or repair. During these periods, the danger of the formation of a flammable environment inside the process equipment is very high. So, the period of starting up the equipment is characterized by the flow of combustible components into the volume of the apparatus filled with air, and the exit of the apparatus to the specified operating mode. In this case, the concentration of flammable substances in the apparatus increases and can become flammable if it exceeds the LEL value.
The reasons for the formation of a flammable environment when stopping technological equipment are:
· Lowering the temperature regime in devices with a working temperature of the liquid exceeding the value of VTPR. In this case, the temperature, decreasing, will enter the temperature range of ignition;
· Intake of outside air through the breathing fittings when the apparatus is emptied or through open hatches when they are depressurized;
· Incomplete removal of combustible substances from the apparatus;
· Leaky disconnection of devices from pipelines with flammable substances. In this case, combustible substances through leaks will enter the apparatus and form a combustible mixture mixed with air.
All these features must be taken into account when assessing the possibility of the formation of a combustible environment inside technological equipment and the development of fire prevention measures.
After the analysis of the possibility of the formation of a combustible medium inside each technological apparatus, it is necessary to give an appropriate conclusion and make an entry in column 6 of Table 2.2.
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Fire safety of the object - The state of the object, in which, with a regulated probability, the possibility of the occurrence and development of a fire and exposure of people to hazardous fire factors is excluded, and the protection of material values \u200b\u200bis provided.
Fire safety of industrial enterprises in accordance with GOST 12.1.004 - 91 “Fire safety. General requirements "is provided by fire prevention and fire protection systems, as well as organizational and technical measures. The development of such systems is carried out based on the analysis of fire hazard and protection of technological processes. The method of analysis of fire hazard and protection of technological processes of production is based on identifying the causes of the occurrence of a combustible environment, sources of ignition and ways of fire propagation in production conditions, without knowing which it is impossible to conduct a fire-technical examination of design materials.
Analysis of fire hazard and protection of production processes is carried out in stages. It includes the study of production technology; assessment of the fire hazardous properties of substances circulating in technological processes; identification of possible reasons for the formation of a combustible environment in industrial conditions, ignition sources and ways of fire propagation; development of fire prevention and fire protection systems, as well as organizational and technical measures to ensure fire safety.
Ensuring fire safety of high-risk technological processes
The feasibility study of construction and projects of high-risk technological processes are subject to a state fire safety examination carried out by the bodies of the state fire service in accordance with their competence.
The state examination is carried out in order to establish the compliance of design materials with the requirements of the legislation, norms and rules of fire safety and to assess the completeness, validity and sufficiency of the envisaged measures to ensure fire safety.
Based on the results of the examination, an expert opinion is drawn up, containing an assessment of the admissibility and the possibility of making a decision on the implementation of the examination object.
The implementation of the technological process (including construction and construction) should be carried out according to projects that have a positive conclusion of the state examination.
Fire safety requirements for the design, manufacture and operation of equipment for technological processes of increased fire hazard are established by the rules and regulations of fire safety.
The manufacturer of the equipment establishes in the technical documentation the conditions and restrictions on the use of equipment, requirements for its maintenance, repair, disposal and other measures to ensure fire-safe operation of the manufactured equipment.
Categories of buildings and premises for explosion and fire hazard
Article 27 Determination of the category of buildings, structures, structures and premises for fire and explosion hazard.
Categories of buildings, structures and structures for fire and explosion hazard are determined based on the share and total area of \u200b\u200bpremises of a particular hazard category in this building, structure, structure.
A building does not belong to category A if the total area of \u200b\u200bthe premises of category A in the building does not exceed 25 percent of the total area of \u200b\u200ball premises located in it (but not more than 1000 square meters) and these premises are equipped with automatic fire extinguishing installations.
A building belongs to category B if the following conditions are simultaneously met: the building does not belong to category A and the total area of \u200b\u200bpremises of categories A and B exceeds 5 percent of the total area of \u200b\u200ball premises or 200 square meters.
A building does not belong to category B if the total area of \u200b\u200brooms of categories A and B in the building does not exceed 25 percent of the total area of \u200b\u200ball rooms located in it (but not more than 1000 square meters) and these rooms are equipped with automatic fire extinguishing installations.
A building belongs to category C if the following conditions are simultaneously met: the building does not belong to category A or B and the total area of \u200b\u200bpremises of categories A, B, B1, B2 and C3 exceeds 5 percent (10 percent if there are no premises of categories A and B in the building ) of the summed area of \u200b\u200ball rooms.
A building does not belong to category C if the total area of \u200b\u200bpremises of categories A, B, B1, B2 and B3 in the building does not exceed 25 percent of the total area of \u200b\u200ball premises located in it (but not more than 3500 square meters) and these premises are equipped with automatic fire extinguishing installations.
A building belongs to category G, if at the same time the highest warehouse designation for fire and explosion hazard is indicated in the design documentation for capital construction and reconstruction facilities.
According to fire and explosion hazard, industrial and warehouse premises, regardless of their functional purpose, are divided into the following categories:
1) increased fire and explosion hazard (A);
2) explosion and fire hazard (B);
3) fire hazard (B1 - B4);
4) moderate fire hazard (D);
5) reduced fire hazard (D).
Buildings, structures, structures and premises for other purposes are not subject to division into categories.
Categories of premises for fire and explosion hazard are determined based on the type of combustible substances and materials in the premises, their quantity and fire hazard properties, as well as on the basis of the space-planning solutions of the premises and the characteristics of the technological processes carried out in them.
| Category premises |
Characterization of substances and materials, located (circulating) in the room |
| AND
increased explosion and fire hazard |
Combustible gases, flammable liquids with a flash point of not more than 28 ° C in such an amount that they can form explosive vapor-gas-air mixtures, when ignited, the calculated excess explosion pressure in the room develops in excess of 5 kPa, and (or) substances and materials that can explode and burn when interacting with water, atmospheric oxygen or with each other, in such an amount that the calculated excess pressure of the explosion in the room exceeds 5 kPa |
| B
explosion and fire hazard |
Flammable dusts or fibers, flammable liquids with a flash point of more than 28 ° C, flammable liquids in such an amount that they can form explosive dust-air or vapor-air mixtures, when ignited, the design overpressure of the explosion in the room exceeding 5 kPa |
| B1-B4
fire hazard |
Flammable and hardly combustible liquids, solid combustible and hardly combustible substances and materials (including dust and fibers), substances and materials that can only burn when interacting with water, air oxygen or with each other, provided that the premises in which they are located (apply), do not belong to category A or B |
| D
moderate fire hazard |
Non-flammable substances and materials in a hot, incandescent or molten state, the processing of which is accompanied by the release of radiant heat, sparks and flames, and (or) flammable gases, liquids and solids that are burned or disposed of as fuel |
| D
reduced fire hazard |
Non-combustible substances and materials in a cold state |
The determination of the categories of premises should be carried out by sequential verification of the belonging of the premises to the categories shown in Table 1, from the most dangerous (A) to the least dangerous (D).
Fire hazard analysis of technological processes
The assessment of fire safety of technological processes of increased fire hazard is carried out using the criteria:
- individual risk;
- social risk;
- regulated parameters of fire hazard of technological processes.
Fire safety of technological processes is considered to be unconditionally fulfilled if:
- the individual risk is less than 10 -8;
– social risk is less than 10 -7.
Operation of technological processes is unacceptable if the individual risk is more than 10 -6 or the social risk is more than 10 -5.
Operation of technological processes at intermediate risk values \u200b\u200bcan be allowed after additional justification has been carried out, which will show that all possible and sufficient measures have been taken to reduce the fire hazard.
The assessment of the fire hazard of technological processes should be carried out on the basis of their risk assessment.
If it is impossible to carry out such an assessment (for example, due to the lack of necessary data), it is allowed to use other criteria for fire safety of technological processes (permissible values \u200b\u200bof the parameters of these processes).
When assessing the fire hazard of a technological process, it is necessary to evaluate by calculation or experiment:
- excess pressure developed during combustion of gas-vapor-air mixtures in the room;
- the size of the zones limited by the lower concentration limit of flame propagation (LEL) of gases and vapors;
- the intensity of thermal radiation in case of fires of spills of flammable and combustible liquids for comparison with the critical (maximum permissible) values \u200b\u200bof the intensity of the heat flux for humans and structural materials;
- the size of the zone of propagation of a cloud of combustible gases and vapors during an accident to determine the optimal placement of people and equipment when extinguishing a fire and to calculate the time the cloud reaches their locations;
- the possibility of occurrence and damaging effect of a "fireball" in an accident for calculating the radii of the affected areas of people from thermal effects, depending on the type and mass of fuel;
- wave parameters, pressure during combustion of gas-vapor-air mixtures in open space;
- damaging factors in case of rupture of technological equipment due to the impact on it of a fire source;
- the intensity of evaporation of flammable liquids and liquefied gases in open space and indoors;
- temperature regime of fire to determine the required limit of fire resistance of building structures;
- the required limit of fire resistance of building structures, ensuring the integrity of the enclosing and supporting structures of the fire compartment with the technological process with the free development of a real fire;
- the size of the drain holes for flammable liquids in pallets, compartments and sections of production areas. In this case, the area of \u200b\u200bthe drain hole should be such as to exclude overflow of liquid through the side of the limiting device and spreading of liquid outside of it;
- parameters of steam curtains to prevent contact of steam-gas mixtures with ignition sources. In this case, the curtain must exclude the slip of the combustible mixture into the protected area of \u200b\u200bthe object;
- the concentration of phlegmatizers for combustible mixtures in technological devices and equipment;
- other indicators of the fire and explosion hazard of the technological process, necessary for analyzing their hazard and calculated according to the methods developed in specialized organizations.
The choice of the necessary parameters of fire hazard for a given technological process is determined based on the considered variants of accidents (including major, design and maximum) and the properties of hazardous substances.
The values \u200b\u200bof the admissible parameters of the fire hazard should be such as to exclude the death of people and limit the spread of the accident beyond the considered technological process to other facilities, including hazardous production facilities.
Measures to reduce the consequences of a fire should include:
- limiting the spreading of flammable liquids in the shop or production site;
- decrease in the intensity of evaporation of flammable liquids;
- emergency discharge of flammable liquids into emergency containers;
- installation of flame arresters;
- limiting the mass of hazardous substances during storage and in technological devices;
- water irrigation of technological devices;
- phlegmatization of combustible mixtures in apparatuses and technological equipment;
- removal of fire hazardous equipment to isolated rooms;
- the use of devices that reduce the pressure in the apparatus to a safe value during the combustion of gas and vapor-air mixtures;
- installation of high-speed disconnecting devices in the technological equipment;
- limiting the spread of fire with the help of fire breaks and barriers;
- the use of fire retardant paints and coatings;
- protection of technological processes with fire extinguishing installations;
- application of fire alarm;
- training of the personnel of enterprises in the ways of liquidation of accidents;
- creation of conditions for the earliest possible commissioning of fire protection units by arranging access roads, fire reservoirs and an external fire-fighting water supply system.
The results of the analysis of the parameters of fire and explosion safety and measures to reduce the consequences of a fire should be taken into account when developing plans for the localization and elimination of fire hazardous situations and accidents.
The assessment of social and individual risk in an accident is carried out on the basis of calculating the damaging factors of a fire and the measures taken to reduce their probability and consequences.
The calculation of the individual and social risk must be carried out for the possible loss of life both at the enterprise and outside it. At the same time, it is necessary to consider all possible ways to reduce it and justify the accepted minimum risk.
The procedure for ensuring fire safety of technological processes other than processes of increased fire hazard
The design of a technological process should be preceded by an analysis of its fire hazard.
The commissioning of an industrial facility is allowed provided that the fire safety requirements provided for by the project are met and that meet the current fire safety standards and rules.
Fire hazard analysis of technological processes should include:
- determination of the fire hazard of substances and materials used in the technological process (according to the reference data of the federal data bank on the fire and explosion hazard of substances and materials or experimentally in accordance with the requirements of GOST 12.1.044 on metrologically certified equipment);
- study of the technological process in order to identify equipment, areas or places where combustible materials are concentrated or the formation of dust and steam-gas-air combustible mixtures is possible;
- determination of the possibility of the formation of a combustible environment inside premises, apparatus and pipelines;
- determination of the possibility of the formation of ignition sources in a combustible medium;
- investigation of various variants of accidents, paths of fire propagation and selection of design basis accidents;
- determination of the composition of fire prevention systems and fire protection of technological processes;
- development of measures to improve the fire safety of technological processes and its individual sections.
The fire hazard of technological processes is determined on the basis of studying:
- technological regulations;
- technological scheme of production;
- indicators of fire and explosion hazard of substances and materials used in the technological process;
- design features of devices, machines and aggregates;
- layouts of hazardous equipment in a workshop, on a site or in an open area.
Technological regulations should determine:
- recipe and main characteristics of products, raw materials, materials and intermediates (composition, physical and chemical properties, indicators of fire and explosion hazard, toxicity, etc.);
- production wastes and air emissions;
- parameters of the technological regime (pressure, temperature, composition of the oxidizing medium, etc.);
- the procedure for carrying out technological operations;
- means of control over the technological process;
- basic rules for the safe conduct of the technological process, excluding the possibility of fires.
When studying the technological regulations, all stages of the technological process should be considered, from the preparation of raw materials to the production of products.
The principal technological scheme for the production of products should determine the sequence of technological operations for converting raw materials into finished products, the parameters of the technological regime, the places where raw materials and auxiliary substances are introduced into the process, the places where semi-finished products and finished products are obtained.
Data on fire hazard properties are presented for all hazardous substances, materials, mixtures, intermediate products and finished products available in production, taking into account the characteristics and parameters of the technological process (pressure, temperature, composition of the oxidizing medium, etc.).
If the necessary data on the fire hazard properties are absent, then they should be determined empirically on installations that have passed certification for the right to obtain experimental data in the prescribed manner, or using standardized calculation methods.
The design of technological devices, machines and aggregates must provide for sufficient fire protection measures to ensure the safety of their work.
Assessment of the danger of a fire and its propagation paths is carried out using the layouts of hazardous equipment built on the basis of plans for industrial buildings, installations, shelves and rooms.
The diagrams and maps indicate:
- places of possible formation of a fire and explosive combustible environment;
- areas of possible accidents and their causes;
- probable sources of ignition;
- paths of fire propagation in case of fire;
- measures provided for by the project to protect areas, units and apparatus from fire.
Based on the analysis of the documentation, a system of measures is developed to prevent fire and fire protection of technological processes in accordance with the requirements of the current regulatory documents.
In this case, it is necessary to additionally consider:
- the possibility of the formation of local concentrations of combustible mixtures at the places where vapors and gases exit into the room at devices that are constantly or temporarily connected with the external environment through open hatches, breathing lines, safety valves or having open evaporation surfaces;
- the presence and efficiency of the suction system, purging with inert gas and blocking in batch-type devices, the loading and unloading of which is accompanied by the opening of hatches and covers;
- the efficiency of the branch lines at the apparatus and containers equipped with breathing devices, safety valves, manual release devices;
- operability and efficiency of systems for capturing gases and vapors, devices against overfilling and spreading of liquids, devices for monitoring and regulating temperature when operating open containers filled with flammable liquids;
- reliability of the adopted methods of sealing glands, the need to use local suction and blocking of exhaust ventilation during the operation of pumps for pumping flammable liquids and liquefied gases and compressors.
In the presence of devices and equipment operating under vacuum or in which, according to the conditions of the technological process, there are mixtures of combustible substances with an oxidizer, it is necessary to determine:
- the possibility and conditions for the formation of combustible mixtures in the apparatus;
- actual concentrations of combustible gases in mixtures;
- the need to control the composition of the medium in the apparatus;
- the need for automatic means of warning about the formation of mixtures;
- the possibility of localizing combustible mixtures;
- the reliability and effectiveness of the available means of protection.
To develop measures to ensure fire safety of technological processes, it is advisable to consider all types of ignition sources that may occur in the production process.
In this case, it is necessary:
- to establish what technical solutions are provided so that this apparatus or device itself is not the cause of a fire, to assess their efficiency and reliability;
- in the presence of devices and gas pipelines with a high temperature of the outer surface of the walls, to determine the possibility of ignition of combustible mixtures in areas that do not have thermal insulation;
- to establish a list of substances and materials that, according to the conditions of the technological process, heat up above the autoignition temperature and, in the event of emergency emissions from the apparatus, are capable of igniting upon contact with the ambient air;
- determine if the process uses substances that can ignite on contact with water or other substances;
- to analyze the possibility of formation and accumulation of pyrophoric deposits;
- to identify the presence in the technological process of substances that decompose with ignition during heating, impact, friction or spontaneously igniting in air under normal conditions;
- to prevent the ingress of metal and stones into machines and apparatus with rotating mechanisms (mixers, mills, crushers, augers, etc.), and in the presence of a combustible medium in them, evaluate the effectiveness and reliability of the applied protection;
- provide, where necessary, the use of intrinsically safe and explosion-proof electrical equipment;
- provide means of control and protection against overheating of moving parts of machines and devices (bearings, shafts, etc.);
- to evaluate the possibility of ignition of combustible mixtures from the thermal manifestation of electrical energy (sparks and arcs of opening, short circuits, overload currents, overheating of electrical contacts, heating of equipment elements by induction currents and high frequency currents, lightning strikes and discharges of static electricity);
- to determine the compliance of power, lighting and other equipment with the nature of the impact on it of the environment and the class of explosive and fire hazardous zones of the premises under consideration in accordance with the PUE;
- to exclude the possibility of penetration of gases and vapors from explosive areas into rooms with a normal environment, in which electrical equipment is used in an open design, and provide appropriate protection measures;
- to develop technical solutions to prevent the formation of flammable media and ignition sources to protect technological processes from fires.
If the fire prevention system used in the technological process cannot exclude its occurrence and spread to neighboring areas and equipment, then it is necessary to develop measures for its fire protection.
Fire protection of technological processes should be provided:
- the use of fire extinguishing means and appropriate types of fire fighting equipment;
- using automatic fire alarm and fire extinguishing systems;
- devices limiting the spread of fire beyond specified limits;
- the use of building structures with regulated fire resistance and fire propagation limits;
- organization of timely evacuation of people and provision of service personnel with means of collective and individual protection from dangerous factors of fire;
- the use of building and technological structures with regulated fire resistance and fire propagation limits.
Restriction of the spread of fire beyond the combustion source should be ensured:
- the device of fire barriers;
- establishing the maximum permissible areas of fire compartments and sections;
- device for emergency shutdown and switching of installations and communications;
- the use of means to prevent or restrict the spill and spread of liquids during a fire;
- the use of flame retardants in equipment.
The choice of fire extinguishing agents, compositions and automatic fire alarm systems, the number, speed and performance of fire extinguishing installations should be carried out at the design stage of technological processes, depending on the physical and chemical properties of the processed substances and extinguishing agents.
At the same time, the types of fire fighting equipment used must ensure effective fire extinguishing and be safe for people.
If, according to the conditions of the technological process in an accident, a one-time fire of several different combustible substances and materials that differ from each other in fire hazardous properties and extinguishing characteristics is possible, then the calculation and design of fire extinguishing installations should be made for the most unfavorable substance or product for extinguishing a fire.
If, according to the conditions of compatibility of fire extinguishing substances with combustible materials, the appointment of a common fire extinguishing agent for all is impractical, then the use of several fire extinguishing substances is permissible. In this case, groups of combustible substances compatible with one of the fire extinguishing compositions must be spatially separated or taken out into separate rooms.
Factors characterizing the explosion and fire hazard of technological processes
Combustible environment
Under production conditions, various flammable and combustible liquids in a cold and heated state, at different pressures and in devices of different design are obtained, are processed or participate in the technological process as auxiliary solid combustible materials.
Apparatus, tanks and containers with flammable liquids are usually not filled to the limit, i.e., they have a certain free volume. Since liquids have the property of evaporating at any temperature, the free space of closed apparatus is gradually saturated with vapors. If there is air (or other oxidizer) in this space, liquid vapors, mixing with it, can form combustible mixtures
In the vapor-air volume of closed apparatus, a combustible vapor mixture is formed only in certain temperature intervals of heating the liquid, which are called temperature limits of flame propagation. It follows from this that the prerequisites for the formation of explosive (flammable) vapor concentrations in closed apparatus and containers with liquids are:
a) the presence of a vapor-air space in the apparatus;
b) the presence of a combustible liquid in the apparatus, the operating temperature of which is in the interval between the lower and upper temperature limits of flame propagation.
The following technical solutions allow ensuring the operation of devices and containers without the formation of explosive vapor concentrations in them.
- Liquidation of the vapor-air volume. If there is no vapor-air volume in tanks and reservoirs, even with a changing level of liquids, then there will be no conditions for the formation of flammable concentrations.
the device of storage facilities in which flammable liquids are located under a protective layer of water or above a layer of water (naturally, in this way it is possible to store flammable liquids that are practically immiscible with water, for example, carbon disulfide, oil products);
using tanks with floating roof and floating pontoons. The roof floating on a liquid is a hollow disc made of steel sheets 2-5 mm thick. To make the roof flood-proof, it is divided by partitions into a series of compartments. The floating roof diameter is smaller than the inner diameter of the tank. The existing gap between the roof and the walls of the tank is sealed with special closures that ensure adequate tightness when the roof moves up and down.
- The use of substances and materials that can, without collapsing, float on the surface of the flammable liquid of the tank, creating the required layer thickness and impenetrable sealing with the body.
- Creation of temperature conditions, excluding the formation of explosive concentrations. In this case, constant operating conditions of the apparatus with a temperature regime below the lower or above the upper temperature limits of flame propagation must be ensured.
- Introduction of non-combustible gases into the vapor-air volume of apparatus or containers. If the device has conditions for the formation of an explosive concentration of vapors and it is impossible to change the temperature mode of operation, then the safety of operation of the device can be ensured by supplying it with any non-flammable gas or water vapor (if the operating temperature of the device is above 100 ° C)
In production conditions, a variety of combustible gases are obtained or used in the technological process at different temperatures and pressures.
The following gases are widely used as chemical raw materials or fuel: natural, petroleum, coke oven, ethylene, acetylene, butylene, off-gases, ammonia, hydrogen, etc.
Incorrect operation of devices with flammable gases can cause fires and explosions. Superheated vapors of liquids also have the same properties as gases; the conditions for the formation of flammable concentrations of gases inside the apparatus apply to superheated vapors.
The actual gas concentration is determined by analysis or established according to the data of the technological regulations.
It is possible to ensure the operation of devices with flammable gases without the formation of explosive concentrations in them using the following technical solutions:
a) in the presence of a mixture of combustible gas with an oxidizer, the working concentration in the apparatus must be set above the upper or below the lower limits of flame propagation, taking into account the safety margin;
b) the accepted safe ratio of the fuel-oxidizer mixture must not be violated, for which automatic ratio regulators and automatic gas pressure regulators are installed on the lines feeding the apparatus;
c) violation of automatic regulation of the ratio of components or cessation of the supply of one of them should be accompanied by automatic shutdown of the lines feeding the apparatus with the simultaneous launch of non-combustible gas into the system;
d) in the presence of a mixture of a combustible gas with an oxidizing agent, which is within the range of ignition or close to them, phlegmatizing additives should be used.
e) for continuous monitoring of the working concentration of a mixture of gases with an oxidizer, the devices are equipped with stationary gas analyzers that automatically signal a deviation from the norm.
Under production conditions, finely ground solid combustible substances can be the final product (pulverized fuel, wood flour, powdered sugar, etc.) or waste and by-products of production (flour, tobacco, wood dust, etc.). The sizes of dust particles vary widely. Dust can be suspended or settled depending on particle size and air velocity. With an increase in the speed of movement of air flows, settled dust (airgel) easily passes into a suspended state (aerosol) and vice versa.
Many dusts in suspension are capable of creating explosive concentrations with air.
A large amount of suspended dust is formed during the operation of machines and units with impact mechanisms (crushers, mills, openers, shells, centrifugal classifiers, etc.), as well as machines and installations, the action of which is associated with the use of air flows (pneumatic conveying systems, air classifiers, separators, etc.) or with the fall of crushed products from a height (gravity pipes, places of pouring from one conveyor to another, nodes for loading and unloading crushed products, etc.).
Accumulation of settled dust is a significant hazard to the apparatus. Settled dust can create explosive mixtures when swirling; self-igniting dust - cause spontaneous combustion centers. Sparks from impacts from metal particles caught in the machine can cause smoldering spots that ignite the airborne dust. A local flash can cause a large amount of dust to swirl and cause a second explosion of great destructive power.
Settled dust in machines and devices accumulates in stagnant areas, dead ends, surface defects, in places of sharp changes in diameters and sharp junctions. The accumulation of settled dust is facilitated by the increased air humidity and moisture condensation on the walls of the apparatus and pipelines.
It is possible to reduce the fire hazard of devices and pipelines with the presence of dust in the following ways:
a) the use of less “dusty” grinding processes (for example, vibration grinding, wet grinding, wet processing of solid and fibrous substances);
b) introducing non-combustible gases into the apparatus during the entire period of operation or only at the most dangerous moments (for example, during the periods of starting and stopping mills and similar machines) or adding mineral substances to the flammable dust;
c) installation of systems for suction of dust from cars;
d) using non-combustible gases for pneumatic transportation of the most dangerous dusts, when drying powder materials by spraying and in a suspended layer;
e) establishing the optimal speed / air or non-combustible gas and systematic monitoring of its value during pneumatic transportation of crushed materials (to avoid dust settling);
f) constructive solutions of devices and pipelines, ensuring the minimum accumulation of settled dust;
g) using vibrators to prevent dust plugging in bins and pipelines;
h) protection of the walls of apparatus and pipelines from moisture.
Explosions and fires often occur during periods of shutdown of technological devices for preventive inspection and repair, when they are put into operation.
The formation of fire hazardous concentrations during the shutdown of devices or pipelines occurs as a result of incomplete removal of combustible substances, and during start-up as a result of insufficient air removal.
To avoid the formation of explosive concentrations inside the devices and pipelines, when they stop, flammable liquids are completely drained and flammable gases are released, pipelines with flammable substances are reliably disconnected and the internal volume of the devices is blown so that no vapors of liquids and gases remain in them.
Ignition sources
Ignition sources found in production conditions are very diverse in their reasons for their appearance, in nature, as well as in their parameters.
Most of the ignition sources are formed as a result of a violation of the fire regime by the maintenance personnel, as well as by the repair and installation teams, due to the violation of the established parameters of the technological regulations, in case of malfunctions and accidents of production devices.
To facilitate the process of identifying and studying all the variety of ignition sources, based on the nature of their appearance (formation), can be divided into the following groups: open fire, incandescent combustion products and surfaces heated by them; thermal manifestation of mechanical energy; thermal manifestation of electrical energy; thermal manifestation of chemical reactions (open fire and combustion products are separated from this group into an independent one).
Fires caused by open fire are very common. This is due not only to the fact that open fire is widely used for industrial purposes, during emergency and repair work, and therefore conditions are often created for accidental contact of the flame with a combustible medium, but also by the fact that the temperature of the flame, as well as the amount of heat released during this process, are sufficient for ignition of almost all combustible substances. In production conditions, there may be permanently or periodically operating fire furnaces, reactors, torches for burning vapors and gases, during repair work, they often use the flame of burners and blowtorches, use torches to warm up frozen pipes, fires to warm up the soil or incinerate waste, there are cases smoking in places where it is not allowed, etc.
The combustion temperature of substances is very high.
So, when burning gaseous substances, the actual combustion temperature ranges from 1200-1400 ° C, liquids - 1100-1300 ° C, dust and other solids 1000-1200 ° C. At such a temperature of the apparatus of fire action, any damage and accidents of adjacent apparatus, accompanied by the release of flammable liquids, vapors or gases and their spread towards the furnaces, will inevitably lead to an outbreak and fire.
In the oil refining, petrochemical, and chemical industries, flare installations are still used to burn gas emissions in the form of by-products, the use of which is impractical, as well as gases obtained during the period of setting up production, during emergency shutdowns of devices. Flare installations can be of continuous or periodic action. Incorrect flare installation can lead to the thermal effect of the flame torch on nearby buildings, structures and devices with flammable gases and liquids, to the danger of igniting local accumulations of vapors and gases in the air, to the possibility of sparking, as well as to gas-out of the territory in case of sudden attenuation torch.
The design of the flare burner must ensure the continuous combustion of the supplied gas by means of a permanently burning burner.
A significant fire hazard is posed by hot repair and installation work. Hot work includes electric and gas welding, cutting, soldering, repair and installation work associated with heating parts, equipment, structures and communications with an open fire; fire spraying on the surface of various materials, etc. The fire hazard of hot work is caused not only by an open flame, but also by the presence of hot and molten metal, sparks in the form of small burning drops of metal scattering in all directions, incandescent stumps of electrodes and heated sections of the apparatus, pipeline or other structural elements processed by the flame. When gas welding and cutting metals and gasoline cutting works, they strive to obtain a flame with the highest possible temperature, for this purpose the fuel is burned in pure oxygen. The flame temperature in this case reaches 2000-3000 ° C. The arc flame temperature when using carbon electrodes is 3200-3900 ° C, and when using steel electrodes 2400-2600 ° C.
The greatest amount of splashes and sparks is generated during gas or air-arc cutting of metals. In this case, a significant part of the molten mass of metal is blown out of the cut groove by an air jet at a distance of 10 or more around the place of work. When welding metals, sparks and spatter are emitted less, but in this case, about 10% of the metal of the electrodes and some part of the base metal is spent on the formation of sparks and spatter. Drops and sparks in the form of partially molten metal have a temperature of 1700 ° C or more. Naturally, getting on combustible materials, they ignite them.
The fire hazard from sparks and incandescent residues (cinders) of electrodes arises most often during hot work at height. In this case, sparks and stubs, falling on the floor and scaffolding below the welding site, can ignite waste, combustible materials and structures. Fireplaces are often discovered several hours after the end of hot work.
Often, sparks through unprotected openings and openings enter the underlying or adjacent rooms, causing fires in them.
With an increase in the height of the welding place above the level of the floor or platform, the area of \u200b\u200bspread of sparks increases
Often, fires occur when elementary requirements are violated, i.e. when using torches to heat a frozen product in pipes, lighting when inspecting devices, containers, when measuring the level of liquids, smoking and using matches in unauthorized places, making fires on the territory of an object, burning combustibles deposits in devices, pipelines, etc. Gross violations of the established fire safety rules are still observed, despite extensive explanatory work and administrative measures taken.
Ignition of many substances is also possible from such "low-calorie" sources as a smoldering cigarette or cigarette butt. Facts and studies have shown that a smoldering cigarette and a cigarette have a temperature of 350-400 ° C, and the duration of smoldering reaches 12 minutes or more. Contact of a burning cigarette butt with a solid and fibrous substance or dust causes the appearance of a smoldering center, which, with sufficient air access and under conditions conducive to the accumulation of the released heat, causes a fiery combustion of the substance. So, a smoldering cigarette or cigarette, in the presence of optimal conditions, causes ignition:
- shavings and wood after 1-1.5 and 2-3 hours, respectively (the flame appears at a temperature of 450-500 ° C);
- waste paper, hay and straw in 0.25-1 hours (depending on their density);
- cotton fabrics in 0.5-1 hours (depending on the volumetric weight of the fabric).
When solid, liquid or gaseous substances are burned in furnaces and internal combustion engines, a large amount of gaseous combustion products with a high temperature are formed. The temperature of the flue and exhaust gases depends on many factors and reaches 800-1200 ° C and above. Even if we take into account the decrease in the temperature of gases as they move in pipes and channels (the temperature decrease is 2-6 ° C per 1 m of a brick channel and 15-45 ° C per 1 w of a metal pipe), then the temperature of the gases will also be quite high. At this temperature of the flue gases, the outer surface of the walls can be heated above the autoignition temperature of the substances in production. This is especially true for metal exhaust pipes. A significant fire hazard is posed by the release of hot gases through faults in the masonry of furnaces, smoke channels and in case of damage to the exhaust pipes of internal combustion engines.
Therefore, when operating furnaces and engines, they monitor the state of the masonry of the smoke channels and hogs, preventing leaks and burnout of exhaust pipes, as well as contamination of their surface with combustible dust or the presence of any combustible substances near heated surfaces. Highly heated surfaces of metal pipes are protected with thermal insulation made of non-combustible materials or blown-through casings. The maximum permissible surface temperature of uninsulated metal pipes should not exceed 80% of the autoignition temperature of substances in the room (except for those cases where combustible substances are capable of thermal spontaneous combustion).
Sparks also pose a fire hazard as an ignition source.
Sparks are solid smoldering particles in a gas stream, which are formed as a result of incomplete combustion or mechanical entrainment of combustible substances. The temperature of such a solid particle is quite high, usually higher than the autoignition temperature of almost all combustible substances, but the supply of thermal energy is small, since in the overwhelming majority of cases the mass of the spark is very small.
Consequently, a spark is capable of igniting only substances that are sufficiently prepared for combustion and have a short induction period. Such substances include gas and vapor-air mixtures, especially at concentrations close to stoichiometric, as well as settled dust and fibrous materials.
The main reasons for the formation of sparks during the operation of furnaces are:
- large mechanical carryover of fuel as a result of design flaws in the furnace, the use of the wrong type of fuel for which the furnace is designed, reinforced shurovka and blast;
- incomplete combustion of fuel due to insufficient air supply, excessive fuel supply, insufficient atomization of liquid fuel;
- violation of the terms of cleaning furnaces and chimneys from soot.
The main reasons for the formation of sparks and carbon deposits during the operation of diesel and carburetor engines:
- incorrect adjustment of the fuel supply system and electric ignition;
- fuel contamination with lubricating oil and mineral impurities;
- long-term operation with engine overload;
- violation of the terms of cleaning the exhaust system from carbon deposits.
To avoid the occurrence of fires from sparks during fuel combustion, it is necessary to eliminate the causes of sparking, as well as to catch and extinguish those sparks that are still formed and thrown out.
The elimination of the causes of sparking is ensured by maintaining the furnaces and engines in good technical condition, observing the established fuel combustion modes, using only the type of fuel for which the furnace is designed, by regularly cleaning the surface from soot and carbon deposits, arranging chimneys of such a height that sparks burn out and extinguished, not reaching buildings and other places with combustible substances.
Capturing and extinguishing sparks during the operation of furnaces and internal combustion engines is achieved by using spark arrestors and spark arresters.
With a sufficiently strong impact of some solids against each other, sparks are struck, which are particles of metal or stone heated to glow. The dimensions of the impact or friction sparks depend on the fragility of the material of the colliding bodies, the impact force and usually do not exceed 0.1-0.5 mm. No visible sparks are generated when metals are struck in an oxygen-free atmosphere. Consequently, the high temperature of friction sparks is determined not only by the quality of the metal and the force of the impact, but also by its oxidation with atmospheric oxygen. The spark temperature of unalloyed low-carbon steels is within the melting point of the metal, i.e. about 1550 ° C. The temperature increases slightly with an increase in the carbon content in steel and decreases significantly with an increase in alloying additions, especially tungsten.
The spark temperature rises linearly with increasing load, and the sparks generated when steel on corundum impacts have a higher temperature than steel on steel. Despite the high temperature, impact and friction sparks, while cooling, can give off a small amount of heat, since their mass is very small.
Making its flight, the spark constantly comes into contact with new and new elementary volumes of the combustible medium and gives them its heat. Thus, the contact of each elementary volume of a combustible medium with a red-hot spark is counted in hundredths, and maybe even thousandths of a second, while the spark temperature will change all the time. It so happens that the spark does not enter the combustible medium immediately after its formation, but only after it has flown a certain distance and during this time it has cooled down somewhat. Consequently, of practical interest is the change in the spark temperature during its flight.
Impact and friction sparks are capable of igniting only mixtures such as acetylene, ethylene, hydrogen, carbon monoxide, and carbon disulfide.
More dangerous are not flying, but stationary sparks, that is, those that, after cutting, hit some surface (obstacle). In this case, the spark cools more slowly and will give off its heat to the same volume of the surrounding combustible medium; thus, the conditions for ignition will be more favorable.
A flying spark does not ignite dust-air mixtures, but if it hits the settled dust or fibrous substances, it causes the appearance of foci of smoldering.
This, apparently, explains that the largest number of flashes and ignitions from mechanical sparks occurs in such machines where there are fibrous materials or deposits of fine combustible dust. So, in the grinding shops of mills and groats factories, in the sorting-loosening and carbon monoxide shops of textile factories, as well as in cotton ginning plants, more than 50% of all ignitions and fires arise from sparks carved by solid objects.
The igniting ability of impact and friction sparks drops sharply with a decrease in the oxygen content in the mixture and, conversely, increases as the air is enriched with oxygen.
Extremely dangerous sparks are generated when aluminum bodies strike an oxidized steel surface. In this case, a chemical interaction occurs between the heated aluminum particle and iron oxides, with the release of a significant amount of heat.
Dangerous manifestations of impact and friction sparks are observed when steel tools are used in explosive workshops, foreign metal bodies or stones get into machines with rotating mechanisms or impact mechanisms, impacts of rotating mechanisms on stationary parts of the machine, as well as during accidents associated with the breakdown of fast-moving mechanisms or rupture of the case of the apparatus.
Sparks from metal or stones hitting machines. If the machines have a steel body and rapidly rotating mechanisms in the form of drums, blades, beaters, knives, wheels, discs, etc., the ingress of solid foreign objects in them in the form of pieces of metal or stones can lead to sparks.
Such machines and devices that pose a fire hazard include:
- agitators for dissolving or chemically treating solids in flammable solvents;
- centrifugal impact machines for crushing, loosening and mixing of solid combustible materials;
- mixing apparatus for mixing and composing powder compositions;
- centrifugal apparatus for moving gases, vapors and comminuted solids (eg fans, gas blowers, centrifugal compressors).
Solid objects can enter these machines along with the processed products or be the result of machine malfunction and breakdown.
The formation of sparks during the operation of these machines and devices is prevented by cleaning substances from metal impurities and stones
Sparks generated when moving mechanisms hit stationary parts of machines. In practice, machines and machine tools are often used whose moving and rapidly rotating mechanisms are located very close to stationary parts. Thus, the rotor of centrifugal fans is almost in contact with the vertical walls of the casing and is less than 1/100 of the diameter away from the discharge pipe.
Naturally, in this case, conditions are created under which the moving parts will hit the stationary ones. This can happen when the clearances are incorrectly adjusted, when the shaft is deformed and vibrated, bearings are worn, misaligned, the cutting tool is not properly attached to the shaft, etc. Such cases lead to the possibility of sparks, but also to breakdowns of individual parts of the machines. Breakdown of a machine unit or metal chipping, in turn, can be accompanied by the formation of sparks and the ingress of metal particles into the processed product.
Any movement of bodies in contact with each other requires the expenditure of energy to overcome the work of friction forces. This energy turns into warmth. The greatest amount of heat is released during dry and semi-dry friction
The most dangerous, if possible, overheating is the plain bearings of heavily loaded and high-speed shafts.
An increase in frictional forces, and, consequently, in the amount of heat released, can lead to a violation of the lubrication quality of the working surfaces, contamination, distortions, overloading of the machine and excessive tightening of the bearing.
Insufficient lubrication of a bearing can be caused by irregularities, a small amount of lubricating oil, a clogged hole or channel for supplying oil to the bearing, or the use of an oil of the wrong type for which the bearing is designed.
Deterioration of the conditions for heat transfer from the surface (from the bearing to the environment, surface contamination with a layer of low-heat-conducting substances, a malfunction of the additional cooling system of the bearing, additional insulation of the bearing or the entire machine with non-ventilated casings, etc.) can contribute.
Quite often, the outer surface of the bearings is contaminated with deposits of combustible dust (wood, flour, cotton), which creates conditions for its overheating and, at the same time, being exposed to prolonged exposure to heat, itself begins to oxidize. Forced cooling of bearings is most often provided by circulating oil or cold water through the cooling jacket of the bearing. An insufficient amount of oil or water supplied to the cooling system, as well as a strong contamination of the heat exchange surface, lead to an increase in the bearing temperature.
Overheating of the conveyor belt is caused by prolonged slippage of the belt or belt relative to the pulley. Such slippage, called slipping, occurs as a result of a mismatch between the transmitted force and the tension of the branches of the belt, tape.
When slipping, all the energy is spent on friction between the belt and the pulley, resulting in a significant amount of heat. Slipping is often due to overloading or loose belt tension. In bucket elevators, the reason for slipping the belt is most often a condition when bucket elevators cannot pass through the thickness of the transported substance.
Fibrous materials and strawy products are often wound on shafts near bearings. Winding is accompanied by a gradual compaction of the mass, and then strong heating of it with friction against the walls of the machine, charring and, finally, ignition.
Fires from this kind of reasons often occur in flax factories, hemp-jute factories, spinning mills, fiber dryers, in combines when harvesting grain crops.
Sometimes, ignition occurs as a result of the winding of fibrous materials on the shafts of conveyors that move waste and finished products. In spinning mills, ignitions often occur as a result of a break in the cord or webbing, with the help of which the spindles of the spinning machines are driven into rotation, and then wound around the necks of rapidly rotating drive shafts.
The winding of fibrous materials on rotating machine shafts is facilitated by: the presence of an increased gap between the shaft and the bearing (getting into this gap, the fiber is wedged, pinched, the process of winding it on the shaft with an ever stronger compaction of layers begins), the presence of bare sections of the shaft with which the fibrous materials, passing wet and contaminated raw materials through machines.
Overheating during machining of solid combustible materials. Mechanical processing (cutting, planing, milling, grinding) of hard materials is associated with overcoming significant friction forces and, as a result, with heating the material, waste, and cutting tools. Under normal cutting conditions and correct sharpening of the cutting tool, the developing temperatures are not dangerous, however, a deviation from the norm can cause them to rise significantly. The main factors influencing the heating of the material during its machining are: cutting speed, tool feed (chip thickness), tool sharpening quality, mechanical and thermal technical qualities of the material. The faster the cutting speed, the thicker the chips and the duller the tool, the more heat will be generated.
If the machining regime is violated, plastics, rubber, magnesium alloys and other similar materials pose a fire hazard.
Heating gases by compressing them in compressors.
Changing the volume of gaseous bodies or the shape of plastic materials requires the expenditure of mechanical energy, while heat is released, which heats the substance, as well as the structural elements of compressors and presses. The processes of gas compression and pressing of plastics are widely used in the national economy. Compressors create pressures necessary for transporting gases through pipelines and for carrying out production processes. Many production operations can only take place at elevated and high gas pressures (hydrogenation of fats requires a hydrogen pressure of 4-5 atm, filling the cylinders with acetylene is carried out at a pressure of 25-30 atm, the production of ethyl alcohol from ethylene requires a pressure of 100 atm, synthesis of ammonia takes place at a pressure nitrogen-hydrogen mixture up to 300 atm, obtaining high pressure polyethylene - up to 1500 atm, etc.).
Almost all manufacturing plants have air compressors to produce compressed air (for squeezing, mixing, spraying or pneumatic transportation of substances, driving braking or conveying devices, etc.).
The practice of operating compressors has shown that in the event of malfunctions and disruption of normal operation, flashes, fires and explosions can occur, not only during compression of the gppuichu gya.chov. but also when compressing air.
Despite the fact that air compressors compress and supply air, rather than combustible gas, into the pipelines, in practice, they explode with subsequent fires. Explosions in air compressors occur as a result of the formation of explosive concentrations of vapors and decomposition products of oil with air, with the simultaneous presence of foci of spontaneous combustion of deposits on the pipe surface. The formation of vapors of oil and its decomposition products is caused by high temperatures, the cause of which is the adiabatic compression of air.
Under the influence of a relatively high temperature (150 ° C), part of the oil evaporates, decomposes and is oxidized by the oxygen of the compressed air. Evaporation and oxidation is also facilitated by the developed surface of the oil film and suspension. A further increase in the temperature in the compressor sharply increases the intensity of the oxidation process.
Studies have established that in the temperature limit of 150 ° C for every subsequent 10 ° C increase in temperature, the oxidation process is accelerated by 2-3 times. The heat generated during this process contributes to an even more intense evaporation, decomposition and oxidation of the oil. The products of decomposition, oxidation and evaporation of oil are carried away by the air from the compressor, and some of them are deposited on the pipe surface in the form of oil sludge. Thus, more favorable conditions for explosion are formed in the discharge air duct, since the process of oil oxidation continues in the layer deposited on the pipe walls. As a result of oxidation, the temperature of the deposits gradually increases. This leads not only to additional release of oil vapors and products of its oxidation into compressed air with the formation of explosive concentrations, but also to the formation of spontaneous combustion centers of deposits on pipes, i.e. to an explosion. The most dangerous are the pipeline sections from the compressor to the air collector and the air collector itself. Explosions most often occur when compressors are operating at elevated pressures.
Repeated cases of explosions were observed during the operation of oxygen compressors. In this case, the main cause of the explosion was a violation of the established lubrication system, i.e. when not distilled water was used, but oil or a soap emulsion with a significant content of fats in it Oxygen compressors should be lubricated only with distilled water with the addition of no more than 10% glycerin.
Chemical reactions taking place in air with the release of a significant amount of heat pose a potential danger of a fire or explosion, since this can lead to the heating of reacting, newly formed or nearby combustible substances to their autoignition temperature. In the conditions of production and storage of chemicals, a large number of such compounds are found, the contact of which with air or water, as well as the mutual contact of substances with each other, can cause a fire.
Often, according to the conditions of the technology, the substances in the apparatus are heated to a temperature exceeding the temperature of their self-ignition. So, for example, pyrolysis gas in the production of ethylene from petroleum products has a self-ignition temperature in the range of 530-550 ° C, and leaves the pyrolysis furnaces with a temperature of 850 ° C; fuel oil with a self-ignition temperature of 380-420 ° С is heated up to 500 ° С in thermal cracking units; butane and butylene, having a self-ignition temperature of 420-439 ° C, respectively, when receiving butadiene are heated up to 550-650 ° C, etc. Naturally, that the appearance of leaks in apparatus and pipelines and contact with the air of a product leaving the outside, heated above the auto-ignition temperature , accompanied by; his sunburn. In some cases, the substances used in the technology have a very low autoignition temperature, even below the ambient temperature.
The ignition of such substances can be avoided only by ensuring good tightness of the apparatus with the exclusion of the mutual contact of these substances with air.
A large group of substances, in contact with air, are capable of spontaneous combustion. Spontaneous combustion of such substances begins either at ambient temperature, or after some preliminary heating. Such substances include vegetable oils and animal fats, coal and charcoal, sulphurous iron compounds, some types of soot, powdery substances (aluminum, zinc, titanium, magnesium, peat, waste nitroglyphthalic varnishes, etc.), drying oil, turpentine, varnished cloth, oilcloth, granitol, hay, silage, etc. Fires and explosions from spontaneous combustion of substances during storage, drying, transportation, when stopping devices for cleaning and repairs occur relatively often.
The duration of the process of spontaneous combustion of substances can be calculated as several minutes or many hours, since the rate of oxidation of combustible substances depends on many factors and, all other things being equal, on the amount of material (mainly on the height of the heap or stack), the initial temperature of the process and the conditions of removal into the environment of heat released during oxidation.
Contact of spontaneously combustible chemicals with air usually occurs when containers are damaged, liquids are poured, substances are packed, during drying, open storage of crushed solid, as well as fibrous, sheet and roll materials, when opening devices for inspection and repair, when pumping liquids from tanks, when there are spontaneously igniting deposits inside them, etc.
A significant amount of chemical compounds interact with water or moisture in the air with a certain exothermic effect. The heat released in this case can cause the ignition of the combustible substances formed or adjacent to the reaction zone. Substances that ignite or cause combustion when in contact with water include sodium, potassium, calcium carbide, alkali metal carbides, quicklime, rubidium, silanes, calcium phosphorus, sodium phosphide, sodium sulfide, sodium hydrogen sulfite, etc.
Many of these substances (alkali metals, carbides, etc.), when interacting with water, form combustible gases that can be ignited by the heat of reaction.
When a small amount (3-5 g) of potassium and sodium interacts with water, a temperature develops above 600-650 ° C. If larger pieces interact, molten metal splash explosions often occur. In a finely divided state, alkali metals ignite in humid air. Excessive heating can occur when calcium carbide interacts with water.
To decompose 1 kg of chemically pure calcium carbide, 0.562 kg of water is needed. With this or less amount of water in the reaction zone, the temperature develops up to 800-1000 ° C. In this case, pieces of calcium carbide can be heated to glow. Naturally, under such conditions, the acetylene formed in air ignites, since its autoignition temperature is 335 ° C. With a large amount of water, acetylene does not ignite, because the heat of reaction is absorbed by the water. Alkali metal carbides (Na 2 C 2 and K 2 C 2) react explosively on contact with water.
Some substances from this group are themselves non-flammable (for example, quicklime), but the heat of their reaction with water can heat the adjoining combustible materials to the autoignition temperature. So, when a small amount of water comes into contact with quicklime, the temperature of the mixture can reach 600 ° C, and the wood or burlap will ignite.
During the production and storage of various chemicals, fires and explosions can occur as a result of their interaction with each other. Most often, such cases occur when organic substances are exposed to oxidants.
Chlorine, bromine, fluorine, nitrogen oxides, nitric acid, sodium, barium and hydrogen peroxides, chromic anhydride, lead dioxide, bleach, liquid oxygen, nitrate (nitric acid salts, for example, potassium nitrate, sodium nitrate, barium nitrate, calcium nitrate), chlorates (chloric acid salts, for example, berthollet's salt), perchlorates (perchloric acid salts, for example sodium perchloric acid), permanganates (manganic acid salts, for example potassium permanganate), chromic acid salts, etc.
The listed oxidants in most cases cause the ignition of organic substances when mixed or in contact with them. Many of them (saltpeter, chlorates, perchlorates, permanganates, chromic acid salts), in addition, are capable of forming mixtures with other organic substances, exploding from minor mechanical or thermal effects. Some mixtures of oxidants and combustible substances can ignite when exposed to sulfuric or nitric acid or a small amount of moisture.
The organoaluminum compounds, which have already been mentioned in the previous paragraphs, react explosively with acids, alcohols and alkalis. The impossibility of extinguishing organoaluminium compounds by conventional fire extinguishing means has led to the need to develop a special fire extinguishing powder composition.
Many initiators, catalysts and vaporizers from the commonly used synthetic resins, plastics, synthetic fibers, and rubber cause fires and explosions when interacting with other substances.
The reactions of interaction of oxidants with combustible substances are facilitated by the fineness of substances, the quality of mixing, an increased initial temperature, as well as the presence of initiators of the chemical process of interaction of substances. In some cases, the reactions are explosive.
Oxidizers of this kind cannot be stored together with other combustible substances, no contact between them should be allowed, unless this is due to the nature of the technological process.
Some chemicals are inherently unstable and can degrade over time or under the influence of temperature, friction, shock and other factors. This group of substances includes, as a rule, endothermic compounds, therefore the process of their decomposition is associated with the release of more or less heat. This group includes explosives, nitrates, peroxides, hydroperoxides, carbides of certain metals, acetylenides, acetylene, diacetylene, porophores and many other substances.
Violation of technological regulations during the production, use or storage of such substances, exposure to them near located heat sources (for example, heating devices, hot product pipelines), and especially the heat of a possible fire, can lead to explosive decomposition of compounds and initiate a new or support a developed fire.
Cases are known when a fire that has arisen led to the explosive decomposition of the product in the apparatus, causing powerful explosions of the equipment with complete destruction of the installation and damage to the apparatus of neighboring installations.
Ignition sources from the thermal manifestation of electrical energy can arise when electrical equipment (machines, motors, networks, converters, ballasts, electrical devices in technological devices, etc.) does not match the nature of the environment acting on it; in case of non-observance of the rules for the design and operation of electrical equipment; in case of malfunctions and damage caused by mechanical causes, as well as by the action of chemically active substances, moisture, etc. The thermal effect of an electric current can manifest itself in the form of electric sparks and arcs (in case of short circuits, breakdowns of the insulation layer, etc.), excessive overheating of motors, machines, contacts, individual sections of electrical networks and electrical equipment, as well as devices during overloads and large transition resistances, in the form of overheating as a result of the thermal manifestation of induction and self-induction currents, during spark discharges of static and atmospheric electricity, as a result of heating the substance and materials from dielectric energy losses.
The maximum temperature on the bulb of an incandescent light bulb depends on the power: 25 W - 100 ° C, 40 W - 150 ° C, 75 W - 250 ° C, 100 W - 300 ° C, 150 W - 340 ° C, 200 W - 360 ° С, 750 W - 370 ° С,
When electrical conductors are short-circuited or ground faults, arcs, sparks and large amounts of heat are generated. The scattering area of \u200b\u200bparticles during a short circuit at a wire height of 10 m ranges from 5 (probability of hitting 92%) to 9 (probability of hitting 6%) m; when the wire is located at a height of 3 m - from 4 (96%) to 8 1%); when located at a height of 1 m - from 3 (99%) to 6 m (6%).
A short circuit can cause ignition of the insulation, melting of conductors or parts of electrical machines and devices with splashing of molten metal particles.
Short circuits and spark breakdowns between capacitor plates, between the electrodes of devices and devices can lead to damage to sealed devices and ignition of flammable substances.
Overloading of electrical networks and machines is caused by an increase in the mechanical load on electric motors, as well as by connecting additional current collectors to electrical networks, for which the networks are not designed. An increase in current strength in networks and machines leads to the release of a large amount of heat, ignition of insulation. Dangerous consequences of overload are observed with incorrectly selected or faulty network protection by fuse-links or automatic devices.
Transient resistances arise most often in places where wires and cables are poorly connected to machines and devices or conductive conductors are connected to each other by cold (twisting), as well as in places of poor contact of metal objects through which leakage currents flow. significant amount of heat Heating of the places of transition resistances can ignite electrical insulation, as well as nearby combustible substances.
Discharges of static electricity can form when moving liquids, gases and dusts, during impacts, grinding, spraying and similar processes of mechanical action on materials and substances that are dielectrics. Electrostatic sparks can ignite steam, gas, and dust / air mixtures. The accumulation of high potentials of static electricity and the formation of spark discharges are facilitated by the absence or ineffectiveness of special measures of protection against static electricity, inefficiency or malfunction of grounding devices, the formation of an electrically insulating layer of deposits on grounded surfaces, disruption of the operation modes of devices (increase in the speed of movement of substances, falling of a jet from a height, contamination moving liquids or the presence on their surface of any floating bodies, etc.).
Sparks of static electricity generated when working with moving dielectric materials reach values \u200b\u200bfrom 2.5 to 7.5 mJ.
Lightning and spark discharges from atmospheric electricity. The absence, malfunction or improper operation of lightning protection systems for buildings, structures and outdoor installations in zones of active manifestation of thunderstorm activity can cause them to be struck by direct lightning strikes, especially in the presence of massive high metal structures or devices with bleed lines and air vents.
The temperature of a lightning discharge is 30,000 ° C at a current strength of 200,000 A and an action time of about 100 μs. The energy of the spark discharge of the secondary effect of lightning exceeds 250 mJ and is sufficient to ignite combustible materials with a minimum ignition energy of up to 0.25 J. The energy of spark discharges when a high potential is brought into a building through metal communications reaches - values \u200b\u200bof 100 J and more, which is sufficient to ignite all combustible materials.
The induction and electromagnetic effects of atmospheric electricity contribute to the appearance of significant electrical potentials on production equipment, pipelines, and building structures. The absence or malfunction of the grounding systems of devices and structures, the absence of jumpers between the pipelines can lead to the formation of dangerous spark discharges.
In some cases, the ignition of combustible substances occurs as a result of induction and dielectric heating. So, when exposed to alternating magnetic fields, metal particles are heated to a high temperature, which, for example, in wood when it is dried by high-frequency currents. In addition, there may be local overheating of dielectrics that have come under the influence of an alternating electric field (for example, the presence of highly knotty, resinous boards when drying wood with high frequency currents).
Conditions conducive to the spread of fire
The experience of operating production facilities where fire safety rules are observed shows that accidents do not lead to complex and protracted fires with grave consequences: human casualties and great material damage. At such enterprises, as a rule, there are no conditions for the development of a fire that has begun, i.e. rapid spread of fire through technological devices and communications, through combustible substances and materials, through production facilities.
The spread of a fire that has begun is facilitated by:
- accumulation of excessive amounts of combustible substances and materials in warehouses and production sites;
- delayed detection of a fire that has arisen and notification of the fire department, absence or malfunction of fire extinguishing equipment and systems, improper actions of people to extinguish the fire;
- the sudden appearance in the process of a fire of factors that accelerate its development;
- the presence of paths along which the spread of flame and incandescent combustion products to neighboring rooms and workshops, to adjacent installations and production sites is possible.
A great danger of the occurrence and development of a fire in an operating production is the accumulation of combustible substances and materials (raw materials, semi-finished products, finished products, waste). To ensure the uninterrupted and continuous operation of enterprises, the necessary stocks of raw materials and semi-finished products are created, the quantity of which and the location are determined, on the one hand, by production needs, and on the other, by fire safety. The creation of excess and unaccounted stocks due to poor organization of production, the fuzzy operation of ridiculous enterprises-suppliers of raw materials leads to overloading of workplaces with combustible substances, necessitates the creation of temporary warehouses that do not have the necessary fire protection.
Processing of plastics, wood, cotton, agricultural products, processing of solid combustible materials are accompanied by the formation of scraps, shavings, sawdust and other waste directly at the workplace. Sources of ignition also appear here: discharges of static electricity, frictional heat, friction sparks, the tendency of substances to thermal spontaneous combustion is manifested. All this taken together creates an increased fire hazard.
In many industries, all kinds of antifreezes, antifriction fluids and coolants, detergents, solvents and thinners prepared on the basis of flammable and combustible liquids are widely used in the processes of mechanical processing and processing of materials, degreasing products, preparing adhesive and rubber compositions, obtaining chemical fibers and other materials.
In the course of a fire, factors may suddenly appear that contribute to its development and make it difficult to extinguish. The entry of additional amounts of combustible substances and materials into the fire site occurs when the apparatus and pipelines are damaged and destroyed. The equipment is destroyed due to an excessive increase in the pressure of the environment in devices heated in a fire, explosion of combustible mixtures or thermal decomposition of products, the appearance of thermal stresses and deformations, deterioration of the mechanical properties of the equipment material and loss of stability of structures. It must be remembered that the use of compact jets of water in a fire can accelerate the destruction of devices when they fall on highly heated surfaces.
The typical paths of flame propagation include: the surface of spilling flammable liquids, deposits of combustible materials on the floor of the room, equipment surfaces, air ducts and other surfaces, deposits of varnishes, paints, polymeric materials, dust and other combustible substances and materials in the duct boxes; steam, gas or dust-air explosive mixtures in industrial premises or in open areas, freshly painted surfaces of products; solid combustible materials, semi-finished products and products (including combustible building structures); heat insulation of apparatus and pipelines impregnated with flammable liquids; pipelines, material pipelines, sewage systems and other transport communications in the presence of a combustible medium in them. Through open doorways and technological openings in walls and ceilings, flammable liquids, combustible mixtures, and incandescent combustion products can penetrate into adjacent rooms.
Reasons for the formation of a combustible environment inside technological devices and production facilities
Industrial enterprises store and process liquid, gaseous and solid substances of various chemical and fire hazard properties. Each of these groups of substances has its own characteristics. For example, liquids can be kept both in hermetically sealed and open containers, and gases, including liquefied ones, can only be in hermetically sealed apparatus. The vapor elasticity of the liquid above its mirror in the apparatus is close to or equal to the saturated vapor pressure at a given temperature, while the concentration of gases in the apparatus does not depend on the operating temperature.
Naturally, the most dangerous for production are cases of damage and accidents of devices. In the event of damage to devices and pipelines with gases, the latter, due to their high fluidity and diffusion capacity, go out, mix with air and can quickly form explosive concentrations in large volumes. If the devices and pipelines with liquids are damaged, the liquids spread and evaporate. In this case, explosive mixtures of liquid vapors with air can form only under certain temperature conditions. Leakage of liquids most often leads to fires and less often to explosions.
Most solids are handled openly, that is, without special shelters and insulation. In the case when the substances ignite spontaneously in air or the process of their processing is accompanied by the formation of dust and decomposition products, the processing of solid substances is also carried out in isolation from the air or in closed apparatus with local dust suction.
Consequently, the conditions for the formation of hazardous concentrations in devices with dusts are somewhat different from the conditions in devices with vapors and gases. At the same time, the causes of damage to devices and pipelines with flammable gases, liquids and dusts are largely similar and can be considered together.
After assessing the fire and explosion hazard of the environment inside the production apparatus, it is necessary to establish which of these apparatus can be sources of the release of flammable substances outside.
Combustible gases, vapors and liquids leave the apparatuses and pipelines into the production room or into an open area not only in case of damage and accidents, but also in the presence of serviceable apparatuses with an open surface of liquid evaporation or breathing devices, if periodical apparatuses are used, with stuffing box seals etc. Even from hermetically sealed devices operating under increased pressure, small leaks also occur due to the presence of leaks in the seams, flange connections and fittings.
During operation of these devices, flammable concentrations may form at the places where vapors and gases escape. The dimensions of the zones of flammable mixtures, i.e. The real danger of such devices is determined not only by the fire hazardous properties of the substances in them, but also, mainly, by their quantity, which can come out in a certain period of time.
The amount of substances released outside can be found empirically or determined by calculation.
Evaporation from an open surface occurs during storage of liquids in open tanks, in the presence of painting baths, impregnation of fabrics and paper with dissolved resins in baths, rinsing and drying of parts with solvents, etc. accidents of apparatus and pipelines.
An explosive concentration of a mixture of vapors with air above the surface of an open apparatus is formed if the temperature of the liquid t slave is higher than its flash point t flash. Taking into account the reliability factor, this is conventionally expressed by the ratio:
t slave ≥ t flash - 10 ° C.
The size of the resulting explosive vapor zone is determined by the evaporation conditions.
The amount of liquid evaporating from a free surface depends on its physical properties, temperature conditions of evaporation, area of \u200b\u200bthe evaporation mirror, time of evaporation and air mobility.
In technological processes of production, devices of periodic action are often used. Such devices include solvents of synthetic resins and other substances, xanthogenerators, glue mixers, mixers, extractors, filter presses, etc. All other things being equal, batch devices pose a greater fire hazard than continuous devices.
Batch devices are loaded with solid or liquid combustible substances before the start of the working cycle, during operation it becomes necessary to take samples of the processed substances for analysis, and at the end of the process the device must be unloaded and prepared for the next cycle of work. Thus, the operation of even hermetically sealed devices of periodic action is associated with the need to open hatches, covers, loading and unloading devices and the release of a certain amount of combustible substances.
Opening the mixer cover during unloading, loading the apparatus will lead to the release of flammable liquids vapors outside, the formation of local fire hazardous concentrations near the apparatus, as well as inside the apparatus when air enters it.
The process of loading flammable substances into the apparatus is accompanied by the displacement of a predetermined amount of the gas-air mixture outside.
In this case, if the apparatus is connected by a loading pipe to a bunker located in another room, flammable liquid vapors displaced during loading of such an apparatus can enter the bunker and from it into the room, creating a danger of explosion.
When operating closed apparatus and containers under pressure, even when they are in good condition, small leaks of flammable substances always occur. This is due to the fact that even with the most careful processing of adjacent surfaces, it is impossible to create absolute impermeability.
The amount of leakage will depend mainly on the mode of operation of the apparatus and the condition of the seals.
Leaks from normally sealed pressure vessels occur, although continuously, but usually do not cause a real fire hazard, since small jets of gas or steam escaping are most often dispersed over the surface of the device and, in the presence of air exchange, are immediately dispersed and removed from the place their allocation.
A significant number of pressure devices have moving mechanisms (stirrer blades, pump and compressor wheels, auger screws, etc.), the shafts or rods of which pass through the device body with appropriate stuffing box seals.
Seals for rotating shafts and reciprocating rods must be low friction, wear resistant, have the required tightness and be easy to replace.
It is very difficult to create the proper tightness of the stuffing boxes, therefore, when working with devices with the presence of stuffing box seals, there is always a leak of vapors, gases or liquids.
The greatest danger for production is represented by damage and accidents of technological equipment and pipelines, as a result of which a significant amount of combustible: substances go outside, causing a dangerous accumulation of vapors and gases in rooms, gas pollution of open areas, spillage of liquids over large areas.
The consequences of damage or an accident will depend on the size of the accident, as well as the fire hazardous properties of substances escaping outside, on their temperature and pressure.
During the operation of production devices, not only damage to gaskets, oil seals, seams, etc. is possible, but also damage to the body material and even complete destruction of the devices with the release of a large amount of liquids and gases.
If in damaged apparatus or pipelines combustible substances are heated above the autoignition temperature, then when they go outside and come into contact with air, they will ignite and a stable torch of a burning gaseous product or spilled liquid is formed. In this case, the ignition may be accompanied by a small pop. The same phenomenon will be observed when there are sources of open fire or devices with a surface temperature equal to or higher than the autoignition temperature of the product falling on them in the immediate vicinity of the damaged area.
If the flammable substance coming out of the damaged apparatus or pipelines is heated below the self-ignition temperature, but above the flash point (for liquids), then the formation of flammable concentrations of vapors or gases with air will inevitably occur. In this case, not only local, but also in the entire volume of the production facility or on the territory of open areas, explosive concentrations can form.
Damage to devices and pipelines can be of a local, i.e. local, character (formation of cracks, fistulas, through holes from corrosion, burnout of the heat exchange surface, squeezing of flange joint gaskets, etc.), but complete destruction of the device or pipeline can also occur ... In the first case, the product in the form of jets of steam, gas or liquid will go out through the formed hole under almost constant pressure. In the second case, the entire contents of the apparatus will immediately come out and, in addition, the outflow of gas or liquid from the pipelines connected to it will continue.
Reasons for the formation of a combustible environment inside industrial premises
The working conditions of technological devices are varied. The material of devices, pipelines, their fittings and gaskets are constantly exposed to the processed substances, often heated to high temperatures and under high pressure. The devices are also affected by the environment, which often, especially in chemical industries, has aggressive properties.
The material of the apparatus and the thickness of the walls during the design are selected so that they can withstand the effects of the processed substances, temperature, internal pressure and the external environment. Consequently, accidents and damage can arise from design flaws (unsuccessful selection of material, incorrect calculation) or operational nature (violation of the accepted operating modes of the apparatus) or due to the action of both causes simultaneously. It is not always easy to determine the true cause of damage, since a seemingly obvious cause of damage in reality may be the result of a number of other interrelated phenomena.
Naturally, preventive measures should be aimed at preventing the actual causes of damage and accidents.
To somewhat facilitate the process of investigating the causes of damage to devices, it is necessary to study the most characteristic of them. It is almost impossible to consider these reasons simultaneously and in interrelation, so we will consider them separately.
The causes of damage to production equipment can be divided into three groups:
- damage resulting from mechanical stress on the material of devices and pipelines;
- damage arising from temperature effects on the material of apparatus and pipelines;
- damage resulting from chemical wear of the material.
The mechanical strength of technological equipment is a prerequisite for ensuring its safe operation.
The strength of the technological equipment is ensured by the correct selection of material, taking into account the nature and magnitude of external loads acting on the apparatus. In this case, they always leave the most unfavorable operating conditions of the device.
When designing and manufacturing devices, all measures are taken to prevent the possibility of damage due to insufficient mechanical strength. At the same time, at industrial enterprises, damage and accidents of apparatus and pipelines are often observed.
This happens for many reasons, including as a result of the impact of loads not provided for by the calculation, the presence of hidden internal defects in the material, the absence or malfunction of effective means of protecting the apparatus from overloads, as well as poor-quality technical supervision of the equipment during its operation. As a result of the mechanical effect not provided for by the calculation, the material of the body of the apparatus or pipeline may experience excessively high internal stresses that can cause not only the formation of leaks in the seams and detachable joints, but also the complete destruction of the apparatus or pipeline at the weakest section.
The reasons for the appearance of high internal stresses can be internal pressures in the devices that are too high against the norm (from material balance disturbance, thermal expansion of substances, termination of vapor condensation, etc.) and dynamic loads, for which the device is not designed.
Danger of damage to intermittent capacitors can arise during the filling period. Absence. regulating devices, as well as measuring the level of liquids or gas pressure, insufficient control over the filling process can cause overfilling of devices, the formation of increased pressure in them and damage to the case.
More than once, cases of high pressure formation were observed when gas was supplied to the gas tanks, when the moving part of the gas tank was jammed due to skewing or icing.
The above reasons, in the absence of automatic systems for cutting off the gas supply to the gas holder, led to the release of gas outward through the hydraulic sealing valve.
In order to prevent overfilling of the apparatus with liquids or gases, the filling operations are monitored, and the apparatus and containers are equipped with appropriate control and protection devices.
Increased pressures are often formed with an increase in resistance in the lines behind pumps, compressors or apparatuses. This happens with incomplete opening of the valves, as well as with a decrease in the section of the lines as a result of deposits on the walls of salts, dirt, coke, polymers, crystalline hydrates, etc.
A significant number of accidents accompanied by explosions and fires occur when compressors are started up with closed valves on gas flow lines.
A decrease in the internal section of the pipeline can occur as a result of various kinds of deposits.
For example, at low operating temperatures or low ambient temperatures in gas and liquid lines with the presence of moist hydrocarbon products, ice and crystalline hydrate plugs may form.
Often there is an accumulation and freezing of water in the drainage lines, which leads to overfilling of devices, tanks and an increase in pressure in them.
Deposits in pipes can occur as a result of the formation of solid products of thermal decomposition of organic substances (coking of pipes) and the formation of polymer compounds (especially when the temperature regime is violated and the speed of the product is reduced). In pipelines there can also be deposits of mechanical impurities and salts contained in the transported product.
An increase in pressure in gas lines often occurs due to the ingress of liquid (gas distillate, water condensate) into them, which forms plugs in the elbows, bends and the lowest sections.
In measuring tanks, reservoirs and other capacitive devices, increased pressure can form due to the lack of conditions for the timely removal of the displaced air-vapor mixture when filling the devices with liquid.
A very dangerous case is the formation of increased pressures in devices and pipelines when the liquids and gases contained in them are heated above the established limit. This can happen in the absence or malfunction of instrumentation, oversight of the maintenance personnel, and in some cases from the radiant energy of neighboring devices and even from an increase in the ambient temperature. With an increase in the working temperature, the pressure in the apparatus increases due to the volumetric expansion of substances and an increase in the elasticity of vapors and gases. It is necessary to distinguish between two types of apparatuses in which the indicated phenomena will affect differently - these are apparatuses that are completely isolated, and apparatuses that are connected to others. Naturally, the first devices will be the most dangerous, since in them, with the same heating, the pressure rises much more intensively and to higher limits than in the second.
When filling devices, containers and cylinders with liquid, cases of overflow may occur. In this case, the liquid will occupy the entire volume of the apparatus and there will be no vapor space in it. In completely filled devices and containers, especially if they are hermetically disconnected from other devices and containers, liquids, when heated, tend to increase in volume, but the walls of the devices prevent this. Since liquids are practically not compressed, heating them even to low temperatures causes very high internal pressures, leading to damage and rupture of the walls. The same is obtained when heating containers and cylinders, completely filled with liquefied gases. In practice, there were many cases when improper filling of barrels and tanks with liquids, as well as tanks and cylinders with liquefied gases during subsequent heating ended in accidents.
Some widespread industries are based on the evaporation processes of flammable and flammable liquids. Such processes include: distillation and rectification of solutions in atmospheric and vacuum installations, concentration and evaporation of solutions, evaporation of solvents from solutions of solids, etc. In all these technological operations, evaporation of liquids is combined with the subsequent condensation of the resulting vapor. Violation of the normal vapor condensation process can lead to the formation of increased pressure in the system.
If steam condensation decreases or stops altogether, and the vaporization process continues, then the amount of steam in the column, condenser and condensate receiver will increase. In this case, the pressure in the apparatus will increase, in addition, a part of the non-condensed steam will go out through the open air on the condensate tank.
If, for any reason, a liquid enters an apparatus with a high operating temperature, the boiling point of which is significantly lower than the temperature in the apparatus, then intensive evaporation of the liquid and an increase in pressure will occur.
Water or other low-boiling liquid can enter high-temperature devices along with the incoming product, through leaks in the heat exchange surface of refrigerators located inside the devices, if the lines are incorrectly switched, and also in the form of condensate from steam process and purge lines. The latter takes place in the case when live steam is supplied to insufficiently heated apparatuses, if the steam is watered or the steam line is not freed from condensate before steam is put into the apparatus. There have been cases when a part of the water remained in the apparatus after washing or testing and a highly heated product was supplied without its preliminary evaporation.
Many chemical processes proceed with the release of significant amounts of heat, with the formation of by-products in the form of vapors and gases. Excess heat, as well as excess gaseous products, are promptly removed from the apparatus, due to which normal operating pressure is maintained in it.
During the operation of production equipment, leaks and damage may appear as a result of the formation of temperature stresses not provided for by the calculation in the material of the walls of apparatus and pipelines, as well as as a result of changes in the mechanical properties of metals under the influence of temperature.
Dangerous temperature stresses in the material arise when there are sharp changes in the operating temperature of the apparatus or the environment, under the influence of the uneven effect of temperature on the structural elements of the apparatus, as well as under the action of changing or unequal temperatures on rigidly fixed structures and units of the apparatus. The total internal stress that appears in the material from the action of the payload and from the temperature effects can exceed the yield and strength limits and cause the appearance of irreversible deformations, ruptures of the walls of the apparatus, the pipeline.
The mechanical properties of the metal can change for the worse when the apparatus is not provided for by the calculation of both high and low temperatures. However, even normal working loads can lead to irreversible deformations and damage to devices or pipelines.
Chemical wear is understood as a decrease in the thickness or strength of the walls of the apparatus as a result of the chemical interaction of the material with the processed substances or with the external environment.
Substances in devices and pipelines, chemical impurities in them, catalysts, initiators or inhibitors used, as well as the environment surrounding the devices, can chemically interact with the housing material, causing its destruction.
The destruction of a metal from the action of a medium in contact with it is called corrosion.
Over the past quarter of a century, the fight against corrosion has become especially important, since high temperatures and pressures, high speeds, aggressive media, etc. are being used more and more. Protection of production equipment from corrosion is a kind of engineering and technical measure that reduces fire danger of the process.
Measures aimed at limiting the spread of combustion in production facilities and their rapid elimination
Limiting the spread of fires in industrial buildings is achieved:
- reducing the amount of combustible substances and materials simultaneously circulating in the technological process;
- selection of the operating mode of the production process;
- a decrease in the amount of combustible production waste, their timely disposal;
- replacement of flammable substances circulating in production, non-flammable;
- emergency discharge of flammable liquids and technological devices and pipelines;
- pumping flammable substances from a hazardous area to a less hazardous area;
- the use of fire-retardant devices on industrial communications;
- protection of pipelines from combustible deposits.
Reducing the amount of combustible substances and materials simultaneously circulating in the technological process not only creates conditions for limiting the possibility of fire spread, but also reduces the likelihood of its occurrence.
The task of reducing the amount of combustible substances and materials circulating in production is solved at all stages of the design of an industrial facility and largely depends on the choice of the technological scheme of production.
Naturally, the technological scheme of production should not only pursue fire and explosion-proof goals, but also be economically profitable.
All other things being equal, such a technological scheme of production is chosen, in which less fire-and-explosive raw materials are used, a lower consumption of raw materials and other fire-and-explosive substances per unit of the product obtained is provided, and the technological process itself consists of a smaller number of production operations and at the same time the amount of generated by-product fuels is reduced products and waste. Evaluation of options for fire and explosion hazard of any technological process is done by comparing the amount of combustible substances per unit of manufactured products.
There are some general conditions that reduce the fire and explosion hazard of the production process flow chart. So, instead of periodically operating apparatuses and processes, it is advisable to use continuously operating apparatuses and processes, since at the same performance, continuously operating apparatuses contain a smaller amount of combustible substances and the apparatuses themselves occupy a smaller area.
Great opportunities in terms of increasing the fire safety of production (reducing the amount of combustible substances) have design and research organizations at the stage of developing a technological scheme. On the basis of technological calculations, the size and number of devices are determined so that there is no unreasonable increase in the amount of combustible substances in them.
The technological scheme, as a rule, should exclude pressure tanks, intermediate tanks, measuring tanks, reflux tanks and the like. Instead, use automatic pressure and flow regulators, continuous metering measuring tanks, automatic feeders, etc.
If technically feasible, flammable absorbers and solvents, catalysts and initiators, as well as heat carriers and coolants, should be replaced in the technological process with less flammable or non-flammable substances. For example, instead of propane, ammonia, isopentane and other flammable substances used to cool the apparatus, it is advisable to use non-combustible freons and brines.
When choosing a particular technological scheme of production, it should be taken into account that the placement of technological equipment in open areas and shelves in all cases when it is possible due to climatic conditions and operating conditions contributes to the reduction of fire and explosion hazard.
When placing technological devices both in buildings and in open areas, it should be borne in mind that production communications (connections between devices) should be as simple as possible, have a short length and a small number of counter flows. Rational placement of production apparatus and pipelines reduces the amount of combustible substances circulating in them.
One of the directions used to limit the extent of a possible fire is to limit the production areas of buildings and open installations. Thus, building codes and regulations establish the maximum permissible floor area between the fire walls of one-story and multi-story buildings, depending on the category of production (fire hazard), the number of floors and the fire resistance of the building. The area of \u200b\u200bfree-standing open installations is also limited depending on the maximum height of the equipment or stack and the type of product to be processed. Warehouses for storing combustible materials are divided by fire walls into compartments that allow in the event of a fire to extinguish it with minimal damage.
When storing various materials and products in one warehouse, division into compartments is carried out based on the signs of the uniformity of the fire extinguishing agents used and the admissibility of their joint storage.
It should be borne in mind that the maximum permissible areas of industrial buildings, warehouses and open installations are large and many valuable combustible substances and materials can be concentrated on them. Therefore, one must not lose sight of the need to isolate fire and explosion hazardous areas from less hazardous areas, even within the limits of production areas permissible. So, devices and equipment, during the operation of which a large amount of flammable gases, vapors or dust can be released, as well as reactors with especially hazardous substances or reactors operating under very high pressure, are usually placed in separate rooms. Areas of production related to fire hazard to different categories are isolated from each other.
Outdoor installations are recommended to be placed on the side of the blank wall of the workshop building or in the end part of it in order to impede the transition of fire in case of fire. Some apparatuses with liquefied flammable gases, flammable liquids, as well as individual apparatuses with flammable gases, taken out from the workshop premises, but connected with the workshop equipment, are placed at the ends or on the side of the blank wall of the building. Equipment with fire and explosion hazardous substances must not be located above and below auxiliary rooms.
Many manufacturing processes require small shop floor warehouses. In this case, based on the needs and requirements of fire safety, the maximum storage capacity is set and they are isolated from the technological process.
Operating mode of the technological process of production
The operating mode of the production process should be understood as the conditions under which this process is least of all fire and explosion hazardous. To comply with this condition, it is necessary that the devices, workplaces have a certain amount of raw materials and semi-finished products. Their accumulation is always fraught with fire and explosion hazard. To know how much raw materials and semi-finished products can accumulate at devices and workplaces, you need to be able to calculate the permissible rate of one-time loading of work premises.
There are a number of restrictions governing the operating mode of a particular technological scheme of production. So, there is a limit on the number of products simultaneously in production. This mainly applies to technological lines associated with the production of large-sized items - aircraft, cars, cars, etc.
The administration of the enterprise, together with representatives of the fire department, establishes how many, for example, aircraft, can be simultaneously in the workshop for their assembly.
There are restrictions on the amount of combustible materials (solid and liquid) based on the area they occupy. The corresponding production instructions establish: in a given workshop, the devices may have such and such a quantity of raw materials, semi-finished products and finished products. They should not obstruct passages and approaches to production equipment, fire-extinguishing equipment, and emergency exits. Taking into account these conditions, the places of possible placement of combustible substances (sites) are distinguished by drawing sharply noticeable lines on the floor. The width of the aisles and approaches, which must always be free, is taken on the basis of the relevant regulations.
There are restrictions on the amount of combustible materials (solid and liquid), depending on their capacity or their weight. Relevant regulatory enactments regulate what volume of certain combustible substances can be directly in the workshop or stored in a warehouse.
Based on the conditions of the technological process, in the corresponding instructions for a particular production, the norms for the consumption of a combustible substance per day, per shift and even half shift are indicated. Compliance with these standards is a sine qua non condition to help prevent fires and explosions in production.
Reducing the amount of combustible waste products, their disposal
The processes of processing wood, plastics, cotton, flax, the processes of grinding and grinding solids, cleaning cereals are accompanied by the formation of waste in the form of scraps, shavings, sawdust, chips, dust. A significant amount of combustible waste accumulates at workplaces, as well as on the territory of such facilities; production equipment, building structures are covered with a layer of combustible dust.
Reducing the amount of combustible production waste is a very important technological problem, which is solved in various ways. One of the simplest and most immutable ways is regular cleaning of workplaces, cleaning of the entire workshop (warehouse). Other ways - rational processing of solid combustible materials, catching combustible waste right at the point of their exit, replacing some technological processes with others: planing, cutting, chiselling, milling, grinding, etc. are replaced by pressing, casting, extrusion, bending, gluing, etc. . P.
But if it is impossible to avoid the formation of combustible waste in the technological process, they must be captured and removed in a timely manner. Waste collection can be periodic and continuous, manual and mechanized. The most effective is mechanized cleaning and, in particular, a centralized aspiration system. Local extractions of aspiration systems are located as close as possible to the waste generation sites. However, the aspiration system can itself be the source of the emergence and development of a fire. The accumulation of dust and crushed materials both inside the aspiration system and in the area of \u200b\u200bits laying is especially dangerous. Therefore, the aspiration pipelines must not be located in basements under industrial premises.
The collection of combustible waste is well established in the case when the enterprise is economically interested in this, that is, when the production waste is disposed of: used as fuel, chemical raw materials, for the manufacture of building materials, etc. For the convenience of transporting waste to other enterprises, their usually pressed.
Replacement of flammable substances circulating in production with non-flammable
The spread of fire can be limited by replacing the combustible substances in production with non-combustible ones. This work is going mainly in the following directions. During the design development of a particular production, the designers should include in the technological process substances less hazardous in terms of fire, and the most fire and explosion hazardous - celluloid, nitrocellulose, etc. materials - should not be used at all; replace flammable liquids with fire-safe detergents; use fireproof varnishes, paints, resins, fireproof plastics.
The industry has mastered the production of water-soluble varnishes, paints and impregnating compounds instead of similar materials based on volatile solvents (for example, water-soluble bakelite varnish, water-based impregnating oil-mica varnishes, etc.). Water-soluble resins are used in the manufacture of getinax, textolite, fiberglass, insulating paper and other materials. Enterprises and construction sites use water-based or water-soluble paints instead of varnish-based paints (for painting internal surfaces). For gluing fibrous materials, rubberizing and making artificial leather, instead of rubber glue (a solution of rubber in gasoline), latex is used, which is a non-flammable polydisperse suspension of rubber in water.
In industrial and agricultural enterprises, fire-safe technical detergents are used to clean and degrease machine parts, and in some enterprises, cleaning is performed using ultrasound.
Emergency discharge of liquids
One of the ways to prevent the development of a fire and turn it into a large or especially large fire is an emergency discharge of flammable liquids from technological devices and pipelines that are in a hazardous area. Emergency drainage can be carried out using special devices or using conventional technological communications and containers. The need for emergency drains is determined by the relevant regulations.
When justifying the device for emergency drains, it should be borne in mind that:
- capacitive devices, as a rule, have a large volume;
- the liquid contained in the apparatus is flammable (or toxic); its entry into the fire zone sharply complicates the situation;
- capacitive apparatus with a flammable liquid is usually located at a height.
When justifying emergency discharge, the design features of the apparatus itself and its supports, as well as its contents, should be taken into account; the possible consequences of emptying or keeping the apparatus full in a fire should also be considered.
It is very important in an emergency or emergency fire situation to quickly drain the liquid from the apparatus, overheating of which may result in spontaneous thermal decomposition of products and an explosion.
An emergency forced evacuation of the liquid is necessary from the coils of the reaction apparatus, heat exchangers and tubular heaters when the movement of the liquid stops, since overheating of the stagnant product leads to its thermal decomposition and coking of pipes.
It is known that the mass of liquid in the apparatus is capable of absorbing a significant amount of heat from a fire and thereby prevent overheating, deformation, and destruction of the apparatus. An example is a tank with an oil product, the degree of damage to which by fire from its own or surrounding fire is inversely proportional to the level of the liquid. For this reason, an emergency draining device is in some cases impractical.
Emergency discharge of liquids from the capacitive equipment located inside the production building should be made into special emergency or drainage tanks of an underground or semi-underground type located outside the building. The distance from industrial buildings to emergency or drainage tanks is taken the same as for technological equipment located outside the building. The distance from the equipment of outdoor installations (or technological stacks) to emergency or drainage tanks, as a rule, is not standardized, but they must be located outside the dimensions of the installation (or stack). Emergency or drainage tanks should not be placed between buildings and outdoor installations (stacks) associated with these buildings.
Emergency discharge can be carried out either by gravity or by squeezing out the liquid with an inert medium (nitrogen, water vapor or carbon dioxide). Extrusion is preferable when the permissible duration of emergency operation does not exceed 10 ... 15 minutes. One emergency or drainage tank can be connected to several vessels. In this case, the capacity of the tank must be at least the volume of the largest of the devices.
Emergency tanks are closed and equipped with breathing tubes, led to a safe place and protected by fire arresters. Since water condensate can accumulate inside the emergency (or drainage) tank during operation, the discharge of highly heated liquids can lead to its rapid evaporation, which, in turn, will cause a sharp increase in internal pressure. Therefore, the accumulated water must be systematically removed. The bottom of the tank is made with a slope to ensure the most complete removal of water.
Emergency draining of highly heated liquids must be preceded by purging with water vapor (or inert gas) of the internal volume of the emergency tank and the drain line. Purging is needed to prevent the possibility of an explosion of a combustible mixture formed upon contact with air of a highly heated product discharged into a closed emergency container.
The pipelines of emergency discharge systems are laid with a one-sided slope (towards the emergency or drainage tank) and, if possible, straight (with a minimum number of turns). Installation of valves along the entire length of the "emergency" pipeline is not allowed (except for valve devices). The emergency drain line is protected from flame propagation by hydraulic seals.
Emergency valves are usually located outside the building (or on the ground floor), near exits. In the presence of a remote drive, the emergency valve is installed near the device (or installation) to be emptied; starter button - near exits, outside the building. The most successful solution is such an emergency drain, in which the activation of emergency valves is automated and interlocked with devices for emergency stop of devices or installations. Sensors of automatic valve opening systems are installed in the area of \u200b\u200bpossible combustion.
Use of an inert medium to increase the drainage rate. The use of an inert environment makes it possible to simultaneously solve another fire safety problem - to eliminate the possibility of an explosion inside the apparatus.
In industrial premises, when the volume of capacitive equipment (measuring tanks, distribution vessels, pressure and fuel tanks, quenching baths, etc.) is not large, special emergency reservoirs are not installed, and industrial tanks located outside the building (or in neighboring rooms behind a blank wall). In this case, the liquids are drained only by gravity.
Shop records should always contain instructions for activating the emergency drain system.
Sometimes, emergency drains may be impractical. Then, it is necessary to provide for the possibility of pumping flammable liquids from capacitive equipment located in the hazardous area to other devices and containers located in a less hazardous area. In this case, the evacuation of flammable liquids does not require the creation of special installations, since in the event of an emergency situation, existing technological communications and pumps can be used. This method is widely practiced in industrial enterprises, as well as in warehouses of flammable liquids, and not only when an emergency is created, but also when the equipment is stopped for preventive inspection or repair. However, here too there are some significant disadvantages due to the impossibility of performing an emergency drain in the absence of free containers, as well as the low speed of the system drive.
Fire-retardant devices on industrial communications
A fire and an explosion spread along industrial communications in cases where a combustible medium has formed inside pipelines, air ducts, trenches, tunnels or trays, when pipelines with this combustible medium operate with an incomplete section, if there is a layer of combustible liquid on the water surface in the plant sewage system, when there are flammable deposits on the surface of pipes, channels and air ducts, if the (System contains gases, gas mixtures or liquids that can decompose with ignition under the influence of high temperature or pressure.
To prevent the spread of fire through production communications, dry fire arresters, fire arresters in the form of hydraulic locks, locks made of solid crushed materials, automatic valves and dampers, water curtains, bulkheads, backfills, etc. are used.
Dry flame arresters - protective devices on pipelines that freely pass the flow of gases through a solid fire protection nozzle, but retain (extinguish) the flame. Their protective effect is based on the phenomenon of extinguishing the flame in narrow channels.
Flame arresters can be in the form of nets or attachments (Fig. 1). Packings made of granular bodies (balls, rings, gravel, etc.) or fibers (glass wool, asbestos fibers, etc.) form curved channels. Nozzles in the form of corrugated foil plates, spirally rolled tapes, etc. form channels of triangular, rectangular or other cross-sectional shape. Nozzles in the form of plates made of cermet and metal fiber have capillary channels.
Figure: 1. Schemes of flame arresters: a - with horizontal grids;
b - with vertical grids; c - with a nozzle made of gravel, balls, rings;
d - with a tape cassette with straight corrugations; d - with tape cassette with inclined corrugations;
e - with a cermet nozzle; 1 - case; 2 - flame extinguishing element
Dry flame arresters most often protect gas and vapor-air lines, in which, according to the conditions of technology or in case of a violation of normal operation, flammable concentrations can form (breathing lines of tanks, measuring tanks, intermediate tanks, pressure tanks and similar devices with flammable liquids, as well as with flammable liquids, heated to a flash point and above; bleed lines and blow-off plugs on devices with gases and flammable liquids; steam-air lines of recuperative installations; lines coming from devices and tanks to the torch; lines of gas piping of tanks with flammable liquids, etc.). Dry flame arresters also protect lines containing substances that can decompose under the influence of pressure, temperature or other factors.
Liquid flame arrester (such as a hydraulic seal) is called such a protective device, in which the flame is extinguished in the process of bubbling a gaseous mixture through a layer of liquid.
A hydraulic seal acting as a flame arrester must reliably extinguish the flame and delay the propagation of the blast wave, ensuring minimal entrainment of liquid by passing streams of steam or gas; have a small hydraulic resistance.
The reliability of extinguishing the flame in the hydraulic seal is ensured by the presence of a certain height of the liquid layer through which the burning mixture passes, and by crushing the gas flow into small streams or individual bubbles. This creates conditions for intense liquid cooling of the reaction products, as a result of which combustion stops.
Rational forms of hydraulic valves and their main dimensions are determined empirically.
A schematic diagram of a hydraulic safety valve on a low pressure gas line is shown in fig. 2. Liquid (most often water) is poured into the valve body so that the end of one of the pipes is immersed in it. Gases or vapors moving through the pipe freely pass through the liquid layer without encountering great resistance. The burning mixture also passes through the liquid layer; at the same time, it is intensively cooled, the flame goes out.
Figure: 2. Scheme of the hydraulic seal on the gas line:
1 - case; 2 - water; 3 - water supply line; 4 - supply pipe;
5-branch pipe; 6 - line for removing excess water; 7 - disk; 8 - slots
The constant flow of gas through the hydraulic seal results in loss of liquid due to evaporation and mechanical entrainment of particles. In this regard, measures are taken to maintain a constant liquid level in the water seal.
Hydraulic valves protect liquid and gas lines, industrial sewerage lines, trays, drain lines on railway and automobile loading and unloading racks
Flammable and combustible liquids, pipelines for emergency discharge of liquids, overflow lines for measuring tanks and tanks, filling and discharge lines of tanks, gas, acetylene lines, etc.
When using hydraulic valves, it is necessary to ensure that they are mounted in a strictly vertical position. The level of the shut-off liquid in them must not be lower than the level of the control (test) tap. Pour liquid and check its level in the hydraulic seal with the lines disconnected. It is impossible to put plugs instead of membranes.
When working in winter, the hydraulic locks should be placed in heated rooms, and if this cannot be done, they should be used as a locking liquid, for example, a solution of ethylene glycol or glycerin in water.
Valves made of solid crushed materials
To prevent the spread of fire through pipelines through which solid crushed materials or combustible waste are transported, devices are mounted on them that create tight plugs from the transported material (dry gates). With the help of such dry closures, the possibility of airspace formation in the pipeline is excluded.
As a dry shutter, a screw shutter is often used, on the endless screw of which (in front of the outlet pipe) several turns are missing, and immediately behind the pipe the screw turns are reversed. This design ensures the formation of a plug in the auger housing even when the material supply is completely interrupted.
Augers are used to supply wood waste to boiler furnaces at woodworking factories, etc. Augers are also installed in horizontal sections of gravity systems for transporting flammable crushed materials (coal dust, flour, press powders, etc.).
A sector doser is used to feed wood waste to the boiler furnace. The device has dampers that ensure the formation of a plug from the transported waste in this place, which will not let the flame from the furnace into the system. If all waste from the pipeline is discharged into the furnace, the dampers are tightly pressed against the metering blades.
The role of a dry seal can be played by hoppers, which are installed between cyclones and furnaces. In this case, make sure that there is always a sufficient amount of solid material inside the hopper.
Automatic dampers and dampers
The protective devices considered above are characterized by the fact that the movement of the medium through the pipelines does not stop at the moment of extinguishing the flame. Along with such protective devices, various types of valves and gates are widely used, which at the right time block the cross section of the pipe, thereby stopping both the movement of the mixture and the spread of the flame. The effectiveness of the fire-retardant action of dampers and dampers depends on the timeliness of their operation, the density of the pipe cross-section overlap, and their fire resistance. In order for the valve to be triggered even before the flame front approaches it, it should be equipped with an automatic drive, which consists of a sensor and an actuator. Sensors react to temperature rise, emission of hot particles (sparks) and smoke.
Diagrams of the simplest automatic dampers are shown in Fig. 3. They have a rotating or falling gate. In dampers with a rotating gate, the tightness of closing is achieved by a weight attached to it, a spring or a counterweight attached to the outside on the gate axis. In falling gate valves, the seal is achieved by its own weight.
Figure: 3. Schemes of automatic dampers and dampers:
and- with a load on the rotary damper; b- with counterweight on the rotary damper;
in- with a falling gate; 1 - pipeline; 2
- damper (gate); 3
- damper axis; 4
- weights;
5
-counterweight; 6
- damper drive with low-melting lock
Timeliness of dampers and dampers actuation is estimated by the duration of their actuation. The damper or gate valve will have time to close the section of the pipeline if the duration of its operation is less than the duration of the flame movement to the location of the gate valve.
Protection of pipelines from combustible deposits
In some shops, the inner surface of pipelines during long-term operation is contaminated with solid or liquid combustible deposits: particles of settling paint (air ducts of painting chambers in paint shops); oil condensate (air ducts of thermal shops); condensate of solvents and plasticizers (production of plastics and products from them, production of artificial leather, etc.); crystalline resin deposits (air ducts of workshops for the production and processing of caprolactam, phthalic anhydride, etc.); fibrous particles (air ducts of spinning factories, recuperation lines of factories of industrial rubber goods, artificial leather, etc.).
In the event of a flash in a car or at a workplace, the flame enters the air duct and spreads through the combustible deposits in the direction of the air flow. In such cases, measures are taken to reduce the contamination of pipes with combustible deposits, various methods are used to capture solid and liquid particles entrained in the air, prevent the possibility of condensation and crystallization of transported vapors on the surface of the pipes, and clean the pipe surface from combustible deposits.
They clean the air from entrained combustible dust, fluff and other solid production wastes with inertial catchers, cyclones and filters. To improve the efficiency of capturing solid particles from the air, water is supplied to inertial traps and cyclones.
Filters are varied in design. Self-cleaning oil filters, bag suction filters and foam washers are widely used. In oil filters, dust is trapped when air passes through a moving metal mesh soaked in oil. The efficiency of oil filters is 70 ... 85%.
Bag filters are highly efficient. They capture up to 95 ... 99% of dust. The continuity of their action is ensured by shaking the sleeves while simultaneously blowing the fabric back with air. It is desirable to treat sleeve fabric with fire retardants. To dissipate static electricity charges, elastic metal threads are woven into the fabric, connected to a grounding device.
Based on local conditions, operation of air ducts, it becomes necessary to periodically clean them manually from deposits. In this case, for the convenience of cleaning, the air ducts are made easily disassembled using flange connections, with ordinary or hinged bolts, or hatches are arranged in them at a distance of 4 ... 5 m from each other.
Avoid cleaning air ducts by burning off deposits. If burning is the only possible way to clean pipes from deposits, precautions are taken: burning is performed when the shop is not working, combustible materials are removed from the air ducts, a fire post is set up for the duration of burning, fire extinguishing means are kept ready, etc.
Combustible gases are stored or processed in sealed apparatus, often operating under high pressure or under vacuum. Inside sealed devices with flammable gases (or superheated vapors), WOCs are formed if air enters them or, according to the conditions of the technological process, an oxidizer (oxygen, air, chlorine, nitrogen oxides, etc.) is supplied when relation (1.2) is satisfied.
The working concentration of combustible gas j p is determined by the readings of stationary gas analyzers, by analyzing a sample of the medium from the apparatus in the laboratory, or calculated by the formula using the data of the material balance of the apparatus:
, (1.3)
where V r and V ok - volumes, respectively, of combustible gas and oxidizer in the apparatus, m 3; G r and G ok - volumetric flow rates of components, m 3 / s.
The values \u200b\u200bof j n and j in individual gases in the air at atmospheric pressure and a temperature of 25 ° C are given in the reference book "Fire and Explosion Hazard of Substances and Materials and their Extinguishing Agents", and in other oxidants - in the special literature. In the absence of data, as well as for individual gases in devices under conditions other than standard ones, or for mixtures of combustible gases and vapors, the values \u200b\u200bof j n and j in can be determined by calculation using special methods or experimentally.
If the technological process uses only a combustible gas, a mixture of combustible gases or a mixture of combustible gases with non-combustible gases, then the WOC is not formed in the devices, since there is no oxidizer in them and the hazard condition (1.2) is not met.
Due to the fact that in real production conditions, not chemically pure individual gases are used, the physicochemical properties of which are given in reference books, but technical products with different contents of the main component and impurities (depending on the type of product), fluctuations in the consumption of components (and as a consequence of the composition of the mixture) within the limits allowed by the technological regulations, and instrumentation and gas analyzers have a measurement error, then to determine the safe concentration of combustible gas in a mixture with an oxidizer, the so-called safety factor, or safety factor.
Explosion-proof operating conditions for devices with flammable gases are determined from the expressions:
(1.4)
, (1.5)
where and are the explosion-proof working concentrations of combustible gas (or superheated steam) in the apparatus, vol. share.
The main methods of ensuring the explosion-proof operation of sealed devices with flammable
gases.
1. Creation and maintenance of explosion-proof concentration of combustible gas in the mixture, for which it is necessary:
Use automatic regulators of flow and pressure of combustible gas and oxidizer;
Carry out automatic control of the composition of the medium in the apparatus using stationary gas analyzers with alarm about deviations from the norm;
Apply an automatic interlock to turn off the supply of one of the components when the supply of another component is interrupted while simultaneously turning on the supply of an inert gas to the apparatus.
2. Creation and maintenance of a safe pressure in the apparatus below the maximum permissible value, at which the spread of flame through the mixture is excluded (the mixture becomes explosion-proof).
It is known that the concentration limits of flame propagation depend on the pressure of the mixture: with increasing pressure, the area of \u200b\u200bflame propagation expands, and when the pressure drops below atmospheric pressure, it narrows. At a certain pressure much lower than atmospheric, a state occurs when j n and j in become equal, which characterizes the absence of a flame propagation region. The value of the maximum permissible pressure is determined experimentally, since it depends on the physicochemical properties of the combustible gas (steam), oxidizer, and also on the temperature of the mixture.
The condition for explosion-proof operation of the apparatus when the pressure in it drops below the maximum permissible value has the form:
R etc / K b.p, (1.6)
where is the safe working pressure of the medium in the apparatus; r pr - maximum permissible residual pressure of the mixture; K b.r - safety factor (safety margin), usually taken in the range of 1.2–1.5.
3. Creation and maintenance of a safe concentration of phlegmatizer in the mixture.
In practice, nitrogen, carbon dioxide (carbon dioxide), flue gases and water vapor (when the operating temperature of the medium in the apparatus is above 80 ° C) is used to phlegmatize the medium in the apparatus. The essence of the process of phlegmatization of a combustible mixture with an inert gas was considered in the course "Physicochemical Foundations of Combustion and Fire Fighting".
The maximum permissible explosion-proof concentration of the phlegmatizer can be found by the formula
PDVK f \u003d K b.f j f, (1.7)
where K b.f - safety factor (safety margin), without taking into account the errors of gas analysis and uneven distribution of concentrations, taken as follows:
for j f\u003e 0.15 vol. shares K b.f \u003d 1.2;
at j f 0.15 vol. shares K b.f \u003d 1.6;
j f - the minimum phlegmatizing concentration of the phlegmatizer; for some individual substances the values \u200b\u200bof j f are given in the handbook; in the absence of data, and for mixtures of combustible gases or
vapors, the value of j f can be determined by calculation:
j f \u003d 1 - 4.774, (1.8)
where - the minimum oxygen content in the mixture (MVSK), vol. shares; magnitude can be found in the reference book, and in the absence of data, determined by the formula
where β is the stoichiometric coefficient for oxygen in the combustion equation for 1 mole of combustible gas.
The condition for the explosion-proof operation of the apparatus during phlegmatization of the combustible mixture in it is as follows:
PDVK f, (1.9)
where is the actual (working) concentration of the phlegmatizer.
Depending on the specifics of certain technological processes, their safety is ensured by the following technical solutions:
a) when carrying out technological processes under vacuum:
Create and maintain a safe residual pressure in the apparatus below the maximum permissible value for the flammability of the mixture;
Automatic control of the composition of the outgoing medium from the apparatus for oxygen and oxygen-containing compounds (CO and CO 2) is carried out using stationary gas analyzers with an alarm about exceeding the maximum permissible amount;
An automatic blocking of the inert gas supply is applied when the oxygen or oxygen-containing compounds content in the apparatus exceeds the maximum permissible amount;
b) when using a combustible mixture in the process, which, according to the conditions of technology, cannot be phlegmatized with an inert gas (for example, in the production of formalin by oxidation of methanol, nitric acid - by oxidation of ammonia and some other chemical products):
The process is organized in such a way that the combustible gas is introduced into the oxidizer (or the oxidizer is introduced into the combustible gas) directly in the reaction zone;
Prevents the appearance of an ignition source in the combustible mixture;
Provide the supply of the combustible mixture to the reaction zone at a rate exceeding the speed of flame propagation through the combustible mixture;
Protect industrial communications with fire-prevention devices;
The apparatus is protected by an automatic explosion suppression system in case of a chemical reaction out of control or by a system for relieving excess pressure of the medium from the apparatus in the event of an explosion of a combustible mixture.
Similar information.
The mass of combustible gas going outward with the complete destruction of the apparatus m p is determined by the floor formula
m p \u003d (V ap * P P / 10 5 * ε + Σqiti + ΣЈјpr * fјpr * P P / 10 5) p r,
where P P is the working pressure of the medium in the apparatus, Pa; qi- the capacity of the i-th compressor or the flow capacity of the i-th pipeline feeding the device, m 3 / s; p r is the density of the combustible gas at the operating temperature of the medium in the apparatus, kg / m 3.
In the event of a spill of flammable liquids or liquefied flammable gases (LHG) in a room or on the territory of an industrial site, they evaporate with the formation of WOC zones (for a flammable liquid, the condition t p ≥ t flash must be met).
An explosive mixture can occupy the entire volume of a room and go beyond it. An explosive cloud can drift in the wind for considerable distances until it diffuses into the environment or meets an ignition source on its way that ignited it. The ignition of the cloud leads to the appearance hazardous factors of an explosion (overpressure of an explosion and a pulse of a pressure wave), as well as a fire of a spilled liquid. The defining parameters of the WOC zone are the distances X LEL, Y LEL, Z LEL (length, width and height), which limit the area of \u200b\u200bconcentrations exceeding the LEL of the flame, which depend on the mass, physical and chemical properties of the spilled products, temperature and mobility of the environment.
Methods for ensuring fire safety. Fire and explosion safety of production facilities is largely achieved by preventing damage and destruction of technological equipment, which is ensured by one of the following methods or their combination:
1.compliance with the technological regulations of the production process and safety measures;
2. maximum mechanization and automation of technological processes;
3. Realization of control over the geometric characteristics of technological equipment;
4. carrying out planned repairs, flaw detection and fluoroscopy of the most critical technological devices;
5. observance of temperature and pressure conditions during the operation of technological equipment;
6. Equipping the apparatus with independent level meters and pressure gauges for monitoring pressure modes;
7. regulation of the rate of filling (emptying) of the capacitive apparatus with liquid, which must not exceed the total throughput of the breathing devices installed on it;
8. ensuring the possibility of pumping products from one apparatus to another in an emergency;
9. use for condensate discharge when heating elements are located inside the apparatus (for example, a steam coil, all joints of which must be welded);
10. using double-walled apparatus with filling the inter-walled space with inert gases or non-combustible liquids (nitrogen, argon, antifreeze).
11. the use of devices to protect production equipment with flammable substances from damage and accidents, the installation of shut-off, cut-off and other similar devices, safety valves and bursting discs;
12 filling the hydraulic safety valves with a hard-to-challenge, non-crystallizing, non-polymerizing non-freezing liquid;
13.the use of flame arresters in equipment (spark arresters, flame arresters)
14. application of corrosion protection of equipment;
15. fulfillment of the requirements of the current norms, rules and standards in the field of fire and industrial safety.
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