Examination: Theory of combustion and explosion. Methods for calculating the rate of rise in the pressure of an explosion of gas and vapor-air mixtures Method for experimentally determining the conditions for thermal spontaneous combustion of solids and materials
The movement of the flame through the gas mixture called flame propagation. Depending on the speed of flame propagation, combustion can be deflagration at a speed of several m/s, explosive - at a speed of the order of tens and hundreds of m/s, and detonation - at thousands of m/s.
For deflagration or normal flame propagation characteristic is the transfer of heat from layer to layer, and the flame that occurs in the mixture heated and diluted with active radicals and reaction products moves in the direction of the initial combustible mixture. This is explained by the fact that the flame, as it were, becomes a source that releases a continuous flow of heat and chemically active particles. As a result, the flame front moves towards the combustible mixture.
deflagration combustion subdivided into laminar and turbulent.
Laminar combustion is characterized by a normal flame propagation speed.
The normal flame propagation speed, according to GOST 12.1.044 SSBT, is called flame front speed relative to unburned gas, in a direction perpendicular to its surface.
The value of the normal speed of flame propagation, being one of the indicators of fire and explosion hazard of substances, characterizes the danger of industries associated with the use of liquids and gases, it is used in calculating the rate of increase in the explosive pressure of gas, vapor-air mixtures, critical (extinguishing) diameter and in the development of measures , providing fire and explosion safety of technological processes in accordance with the requirements of GOST 12.1.004 and GOST 12.1.010 SSBT.
The normal speed of flame propagation - the physicochemical constant of the mixture - depends on the composition of the mixture, pressure and temperature and is determined by the rate of the chemical reaction and molecular thermal conductivity.
Temperature increases the normal speed of flame propagation relatively little, inert impurities reduce it, and an increase in pressure leads either to an increase or decrease in speed.
In a laminar gas flow the gas velocities are low, and the combustible mixture is formed as a result of molecular diffusion. The burning rate in this case depends on the rate of formation of the combustible mixture. turbulent flame It is formed with an increase in the speed of flame propagation, when the laminarity of its movement is disturbed. In a turbulent flame, the swirling of gas jets improves the mixing of the reacting gases, since the surface through which molecular diffusion occurs increases.
As a result of the interaction of a combustible substance with an oxidizing agent, combustion products are formed, the composition of which depends on the initial compounds and the conditions of the combustion reaction.
With the complete combustion of organic compounds, CO 2, SO 2, H 2 O, N 2 are formed, and with the combustion of inorganic compounds, oxides are formed. Depending on the melting temperature, the reaction products can either be in the form of a melt (Al 2 O 3, TiO 2), or rise into the air in the form of smoke (P 2 O 5, Na 2 O, MgO). The molten solid particles create the luminosity of the flame. During the combustion of hydrocarbons, the strong luminosity of the flame is provided by the glow of carbon black particles, which are formed in large quantities. A decrease in the content of carbon black as a result of its oxidation reduces the luminosity of the flame, and a decrease in temperature makes it difficult to oxidize carbon black and leads to the formation of soot in the flame.
In order to interrupt the combustion reaction, it is necessary to violate the conditions for its occurrence and maintenance. Usually, for extinguishing, violation of two basic conditions of a steady state is used - a decrease in temperature and a mode of movement of gases.
Temperature drop can be achieved by introducing substances that absorb a lot of heat as a result of evaporation and dissociation (eg water, powders).
Gas movement mode can be changed by reducing and eliminating the supply of oxygen.
Explosion, according to GOST 12.1.010 " Explosion proof”, - a fast transformation of matter (explosive combustion), accompanied by the release of energy and the formation of compressed gases capable of doing work.
An explosion, as a rule, leads to an intense increase in pressure. A shock wave is formed and propagates in the environment.
shock wave has a destructive capacity if the excess pressure in it is higher than 15 kPa. It propagates in the gas ahead of the flame front at a sound speed of 330 m/s. During an explosion, the initial energy is converted into the energy of heated compressed gases, which is converted into the energy of movement, compression and heating of the medium. Various types of initial explosion energy are possible - electrical, thermal, elastic compression energy, atomic, chemical.
The main parameters characterizing the danger of an explosion in accordance with GOST 12.1.010 are the pressure at the shock wave front, the maximum explosion pressure, the average and maximum rate of pressure increase during an explosion, crushing or high-explosive properties of an explosive environment.
General explosion effect manifests itself in the destruction of equipment or premises caused by a shock wave, as well as in the release of harmful substances (explosion products or contained in equipment).
Max Burst Pressure(P max) - the highest pressure that occurs during a deflagration explosion of a gas, vapor or dust-air mixture in a closed vessel at an initial mixture pressure of 101.3 kPa.
Explosion pressure rise rate(dР/dt) is the derivative of the explosion pressure with respect to time in the ascending section of the dependence of the explosion pressure of a gas, steam, dust-air mixture in a closed vessel on time. In this case, the maximum and average rates of pressure increase during the explosion are distinguished. When establishing the maximum speed, the pressure increment is used in the straight-line section of the dependence of the explosion pressure on time, and when determining the average speed, the section between the maximum explosion pressure and the initial pressure in the vessel before the explosion is used.
Both of these characteristics are important factors for explosion protection. They are used in establishing the category of premises and buildings in terms of explosion and fire hazard, in the calculation of safety devices, in the development of measures for fire and explosion safety of technological processes.
Detonation there is a process of chemical transformation of the oxidizer-reductant system, which is a combination of a shock wave propagating at a constant speed and exceeding the speed of sound, and following the front of the zone of chemical transformations of the initial substances. chemical energy, released in the detonation wave, feeds the shock wave, preventing it from decaying. The speed of the detonation wave is a characteristic of each specific system.
Federal Agency for Education of the Russian Federation
State educational institution of higher professional education
"Ufa State Petroleum Technical University"
Department of "Industrial safety and labor protection"
Control work on the subject:
Theory of combustion and explosion
1. Theoretical questions on explosion
In technological processes associated with the extraction, transportation, processing, production, storage and use of combustible gases (GH) and flammable liquids (flammable liquids), there is always a danger of the formation of explosive gas and vapor mixtures.
An explosive environment can be formed by mixtures of substances (gases, vapors, dusts) with air and other oxidizing agents (oxygen, ozone, chlorine, nitrogen oxides, etc.) and substances prone to explosive transformation (acetylene, ozone, hydrazine, etc.).
The most common causes of explosions are violation of the rules for the safe operation of equipment, gas leakage through leaks in connections, overheating of apparatuses, excessive pressure increase, lack of proper control over the technological process, rupture or breakage of equipment parts, etc.
The source of initiation of the explosion are:
open flames, burning and red-hot bodies;
electrical discharges;
Thermal manifestations of chemical reactions and mechanical effects;
sparks from impact and friction:
shock waves;
Electromagnetic and other radiation.
According to PB 09-540-03 Explosion is:
I. The process of transient release of potential energy associated with a sudden change in the state of matter and accompanied by a pressure jump or shock wave.
2. Short-term release of internal energy, creating excess pressure
An explosion can occur with or without combustion (oxidation).
Parameters and properties characterizing the explosive environment:
Flash point;
Concentration and temperature limits of ignition;
Self-ignition temperature;
Normal flame propagation speed;
Minimum explosive content of oxygen (oxidant);
Minimum ignition energy;
Sensitivity to mechanical action (impact and friction). Dangerous and harmful factors affecting workers
from the explosion are:
A shock wave in the front of which the pressure exceeds the allowable value;
Collapsing structures, equipment, communications, buildings and structures and their flying parts;
Harmful substances formed during the explosion and (or) released from damaged equipment, the content of which in the air of the working area exceeds the maximum permissible concentrations.
The main factors characterizing the danger of an explosion:
Maximum pressure and explosion temperature;
The rate of pressure increase during the explosion;
Pressure in the front of the shock wave;
Crushing and high-explosive properties of an explosive environment.
During an explosion, the initial potential energy of a substance is converted, as a rule, into the energy of heated compressed gases, which, in turn, when they expand, is converted into the energy of movement, compression, and heating of the medium. Part of the energy remains in the form of internal (thermal) energy of the expanded gases.
The total amount of energy released during the explosion determines the general parameters (volume, area) of destruction. The energy concentration (energy per unit volume) determines the intensity of destruction in the explosion site. These characteristics, in turn, depend on the rate of energy release by the explosive system causing the blast wave.
The explosions most frequently encountered in investigative practice can be divided into two main groups: chemical and physical explosions.
Chemical explosions include the processes of chemical transformation of matter, manifested by combustion and characterized by the release of thermal energy in a short period of time and in such volume that pressure waves are formed that propagate from the source of the explosion.
Physical explosions include processes that lead to an explosion and are not associated with chemical transformations of matter.
The most common cause of accidental explosions are combustion processes. Explosions of this kind most often occur during the storage, transportation and manufacture of explosives. They take place:
When handling explosives and explosive substances of the chemical and petrochemical industries;
With natural gas leaks in residential buildings;
in the manufacture, transportation and storage of volatile or liquefied combustible substances;
when flushing storage tanks for liquid fuels;
in the manufacture, storage and use of combustible dust systems and some spontaneously combustible solid and liquid substances.
Features of a chemical explosion
There are two main types of explosions: an explosion of condensed explosives and a volumetric explosion (explosion of vapors of dust-gas mixtures). Explosions of condensed explosives are caused by all solid explosives and a relatively small number of liquid explosives, including nitroglycerin. Such explosives usually have a density of 1300-1800 kg/m3, however, primary explosives containing lead or mercury have much higher densities.
Decomposition reactions:
The simplest case of an explosion is the process of decomposition with the formation of gaseous products. For example, the decomposition of hydrogen peroxide with a large thermal effect and the formation of water vapor and oxygen:
2H2O2 → 2H2O2 + O2 + 106 kJ/mol
Hydrogen peroxide is dangerous starting at a concentration of 60%.
Decomposition by friction or impact of lead azide:
Pb (N3) 2 → Pb - 3N2 + 474 kJ / mol.
Trinitrotoluene (TNT) is an "oxygen deficient" substance and therefore one of its main breakdown products is carbon, which contributes to the formation of smoke during TNT explosions.
Substances prone to explosive decomposition almost always contain one or more characteristic chemical structures responsible for the sudden development of the process with the release of a large amount of energy. These structures include the following groups:
NO2 and NO3 - in organic and inorganic substances;
N=N-N - in organic and inorganic azides;
NX3, where X is a halogen,
N=C in fulminates.
Based on the laws of thermochemistry, it seems possible to identify compounds whose decomposition process can be explosive. One of the decisive factors determining the potential danger of a system is the prevalence of its internal energy in the initial state compared to the final state. This condition is satisfied when heat is absorbed (endothermic reaction) in the process of formation of a substance. An example of a relevant process is the formation of acetylene from the elements:
2C + H2 → CH=CH - 242 kJ/mol.
Non-explosive substances that lose heat during formation (exothermic reaction) include, for example, carbon dioxide
C + O2 → CO2 + 394 kJ/mol.
It should be taken into account that the application of the laws of thermochemistry only makes it possible to reveal the possibility of an explosive process. Its implementation depends on the rate of reaction and the formation of volatile products. So, for example, the reaction of candle paraffin with oxygen, despite the high exothermicity, does not lead to an explosion due to its low speed.
The reaction 2Al+ 4AC2O2 → Al2O3 + 2Fe by itself, despite the high exothermicity, also does not lead to an explosion, since gaseous products are not formed.
Redox reactions, which form the basis of combustion reactions, for this reason, can lead to an explosion only under conditions conducive to achieving high reaction rates and pressure growth. The combustion of highly dispersed solids and liquids can lead to an overpressure of up to 8 bar under closed volume conditions. Relatively rare, for example in liquid air systems, where the aerosol is a mist of oil droplets.
In polymerization reactions accompanied by an exothermic effect and the presence of a volatile monomer, a stage is often reached at which a dangerous increase in pressure can occur, for some substances such as ethylene oxide, polymerization can begin at room temperature, especially when the starting compounds are contaminated with polymerization accelerating substances. Ethylene oxide can also isomerize to acetaldehyde by an exothermic route:
CH2CH2O - CH3HC \u003d O + 113.46 kJ / mol
Condensation reactions are widely used in the production of paints, varnishes and resins and, due to the exothermicity of the process and the presence of volatile components, sometimes lead to explosions.
To find out the general conditions that favor the onset of combustion and its transition to an explosion, consider the graph (Figure 1) of the dependence of the temperature developed in a combustible system on time in the presence of volumetric heat release due to a chemical reaction and heat loss.
If we represent the temperature T1 on the graph as a critical point at which combustion occurs in the system, it becomes obvious that in conditions where there is an excess of heat loss over heat gain, such combustion cannot occur. This process begins only when equality is reached between the rates of heat release and heat loss (at the point of contact of the corresponding curves) and can further accelerate with increasing temperature u. thus, the pressure before the explosion.
Thus, in the presence of conditions favorable for thermal insulation, the occurrence of an exothermic reaction in a combustible system can lead not only to combustion, but also to an explosion.
The resulting uncontrolled reactions that favor the explosion are due to the fact that the rate of heat transfer, for example, in vessels is a linear function of the temperature difference between the reaction mass and the coolant, while the rate of the exothermic reaction and, therefore, the influx of heat from it grows according to a power law with an increase in the initial concentrations of reagents and rapidly increases with increasing temperature as a result of the exponential dependence of the rate of a chemical reaction on temperature (Arrhenius' law). These regularities determine the lowest burning rates of the mixture and the temperature at the lower concentration ignition limit. As the concentration of the fuel and oxidizer approaches stoichiometric, the burning rate and temperature increase to their maximum values.
The concentration of gas of stoichiometric composition is the concentration of combustible gas in a mixture with an oxidizing medium, at which complete chemical interaction of the fuel and oxidizer of the mixture is ensured without residue.
3. Features of a physical explosion
Physical explosions, as a rule, are associated with explosions of vessels from vapor pressure and grooves. Moreover, the main reason for their formation is not a chemical reaction, but a physical process due to the release of the internal energy of a compressed or liquefied gas. The strength of such explosions depends on internal pressure, and destruction is caused by a shock wave from an expanding gas or fragments of a ruptured vessel. A physical explosion can occur if, for example, a portable pressurized gas cylinder falls and a pressure-reducing valve is blown off. The pressure of LPG rarely exceeds 40 bar (the critical pressure of most conventional LPG).
Physical explosions also include the phenomenon of so-called physical detonation. This phenomenon occurs when hot and cold liquids are mixed, when the temperature of one of them significantly exceeds the boiling point of the other (for example, pouring molten metal into water). In the resulting vapor-liquid mixture, evaporation can proceed explosively due to the developing processes of fine phlegmatization of melt droplets, rapid heat removal from them, and overheating of the cold liquid with its strong vaporization.
Physical detonation is accompanied by the appearance of a shock wave with excess pressure in the liquid phase, reaching in some cases more than a thousand atmospheres. Many liquids are stored or used under conditions where their vapor pressure is much higher than atmospheric pressure. These liquids include: liquefied combustible gases (eg propane, butane) liquefied refrigerants ammonia or freon stored at room temperature methane which must be stored at low temperature superheated water in steam boilers. If the container with the superheated liquid is damaged, then there is an outflow of steam into the surrounding space and rapid partial evaporation of the liquid. With a sufficiently rapid outflow and expansion of steam in the environment, explosive waves are generated. The causes of explosions of vessels with gases and vapors under pressure are:
Violations of the integrity of the body due to the breakdown of any node, damage or corrosion due to improper operation;
Overheating of the vessel due to violations in the electrical heating or the mode of operation of the combustion device (in this case, the pressure inside the vessel increases, and the strength of the body decreases to a state in which it is damaged);
The explosion of the vessel when the permissible pressure is exceeded.
Explosions of gas containers with subsequent combustion in the atmosphere basically contain the same causes that are described above and are characteristic of physical explosions. The main difference lies in the formation in this case of a fireball, the size of which depends on the amount of gaseous fuel released into the atmosphere. This amount depends, in turn, on the physical state in which the gas is in the container. When the fuel content is in the gaseous state, its amount will be much less than if it is stored in the same container in liquid form. The parameters of the explosion, which determine its consequences, are mainly determined by the nature of the distribution of energy in the explosion area and its distribution as the blast wave propagates from the source of the explosion.
4. Energy potential
The explosion has great destructive power. The most important characteristic of an explosion is the total energy of matter. This indicator is called the energy potential of explosiveness, it is included in all parameters characterizing the scale and consequences of an explosion.
In case of emergency depressurization of the apparatus, its full disclosure (destruction) occurs;
The area of the liquid spill is determined based on the design solutions of buildings or outdoor installation sites;
Evaporation time is taken no more than 1 hour:
E \u003d EII1 + EII2 + EII1 + EII2 + EII3 + EII4,
explosion firefighter room danger
where EI1 is the sum of the energies of adiabatic expansion and combustion of the vapor-gas phase (PGPC directly located in the block, kJ;
ЕI2 is the combustion energy of the HPF supplied to the depressurized section from adjacent objects (blocks), kJ;
EII1 - the energy of combustion of GTHF, formed due to the energy of the superheated liquid phase of the block under consideration and received from adjacent objects kJ;
EII2 is the energy of combustion of PHF formed from the liquid phase (LP) due to the heat of exothermic reactions that do not stop during depressurization, kJ;
EII3 is the combustion energy of PHF. formed from LF due to heat inflow from external heat carriers, kJ;
EII4 is the energy of combustion of PHF, which is formed from the LF spilled on a solid surface (floor, pallet, soil, etc.) due to heat transfer from the environment (from the solid surface and air to the liquid over its surface), kJ.
The values of the total energy potentials of explosiveness and are used to determine the values of the reduced mass and the relative energy potential that characterize the explosiveness of technological blocks.
The reduced mass is the total mass of combustible vapors (gases) of an explosive vapor-gas cloud, reduced to a single specific combustion energy equal to 46,000 kJ / kg:
Relative energy potential of explosion Qv of the technological block, which characterizes the total energy of combustion and can be calculated by the formula:
where E is the total energy potential of the explosion hazard of the technological unit.
According to the values of the relative energy potentials Rv to the reduced mass of the vapor-gas medium m, the categorization of technological blocks is carried out. The indicators of the explosion hazard category of technological blocks are given in Table 1.
Table No.
| Explosion category | Ov | m |
| I | >37 | >5000 |
| II | 27 − 37 | 2000−5000 |
| III | <27 | <2000 |
5. TNT equivalent. Excess pressure in the front of the shock wave
To assess the level of exposure to accidental and deliberate breakdowns, the method of assessment through the TNT equivalent is widely used. According to this method, the degree of destruction is characterized by the TNT equivalent, where the mass of TNT is determined, which is required to cause this level of destruction. chemically unstable compounds, is calculated by the formulas:
1 For steam-gas environments
q/ - specific calorific value of the vapor-gas medium, kJ kg,
qT is the specific explosion energy of TNT kJ/kg.
2 For solid and liquid chemically unstable compounds
where Wk is the mass of solid and liquid chemically unstable compounds; qk is the specific explosion energy of solid and liquid chemically unstable compounds. In production, an explosion of a gas-air, vapor-air mixture or dust produces a shock wave. The degree of resolution of building structures, equipment, machines and communications, as well as the damage to people, depends on the excess pressure in the shock wave front ΔРФ (the difference between the maximum pressure in the shock wave front and the normal atmospheric pressure in front of this front).
Calculations for assessing the action of combustible chemical gases and liquids are reduced to determining the excess pressure in the shock wave front (ΔРФ) during the explosion of a gas-air mixture at a certain distance from a container in which a certain amount of an explosive mixture is stored.
6. Calculation to determine the excess pressure of the explosion
The calculation of the overpressure of an explosion for flammable gases, vapors of flammable and combustible liquids is carried out according to the methodology set forth in NPB 105-03 "Determination of the categories of premises, buildings and outdoor installations in terms of explosion and fire hazard."
Task: to determine the excess pressure of the explosion of hydrogen sulfide in the room.
Initial conditions
Hydrogen is constantly in the apparatus with a volume of 20 m3. The device is located on the floor. The total length of pipelines with a diameter of 50 mm, limited by gate valves (manual) installed on the inlet and outlet sections of the pipelines, is 15 m. The flow rate of hydrogen sulfide in pipelines is 4·10-3 m3/s. The dimensions of the room are 10x10x4 m.
The room has emergency ventilation with an air exchange rate of 8 h-1. Emergency ventilation is provided by backup fans, automatic start-up when the maximum permissible explosive concentration is exceeded, and power supply according to the first category of reliability (PUE). Devices for removing air from the room are located in close proximity to the place of a possible accident.
The main building structures of the building are reinforced concrete.
Justification of the design option
According to NPB 105-03, the most unfavorable accident scenario, in which the largest number of substances that are most dangerous in relation to the consequences of an explosion, is involved, should be taken as the design version of an accident.
And as a design option, the option of depressurization of the tank with hydrogen sulfide and the exit from it and the inlet and outlet pipelines of hydrogen sulfide into the volume of the room was adopted.
1) Excess explosion pressure for individual combustible substances, consisting of atoms C, H, O, N, Cl, Br, I, F, is determined by the formula
(1)
where is the maximum explosion pressure of a stoichiometric gas-air or vapor-air mixture in a closed volume, determined experimentally or from reference data in accordance with the requirements of clause 3 of NPB -105-03. In the absence of data, it is allowed to take equal to 900 kPa;
Initial pressure, kPa (allowed to be taken equal to 101 kPa);
The mass of combustible gas (GG) or vapors of flammable (FL) and combustible liquids (GL) released into the room as a result of an accident, kg;
The coefficient of participation of fuel in the explosion, which can be calculated on the basis of the nature of the distribution of gases and vapors in the volume of the room according to the application. It is allowed to take the value according to the table. 2 NPB 105-03. I accept equal to 0.5;
Free volume of the room, ;
The maximum absolute air temperature for the city of Ufa equal to 39°C is taken as the design temperature (according to SNiP 23-01-99 "Construction climatology").
Below is a calculation of the quantities necessary to determine the overpressure of an explosion of hydrogen sulfide in a room.
Density of hydrogen sulfide at design temperature:
where M is the molar mass of hydrogen sulfide, 34.08 kg/kmol;
v0 is the molar volume equal to 22.413 m3/kmol;
0.00367 − coefficient of thermal expansion, deg -1;
tp is the design temperature, 390C (absolute maximum air temperature for Ufa).
The stoichiometric concentration of hydrogen sulfide is calculated by the formula:
;
where β is the stoichiometric coefficient of oxygen in the combustion reaction;
nc, nn, n0, nx, is the number of C, H, O atoms and halides in the fuel molecule;
For hydrogen sulfide (Н2S) nc= 1, nн = 4, n0 = 0, nх = 0, therefore,
We substitute the found value of β, we get the value of the stoichiometric concentration of hydrogen sulfide:
The volume of hydrogen sulfide that entered the room during a design accident consists of the volume of gas released from the apparatus and the volume of gas released from the pipeline before closing the valves and after closing the valves:
where Va is the volume of gas released from the apparatus, m3;
V1T - volume of gas released from the pipeline before its shutdown, m3;
V2T is the volume of gas released from the pipeline after its shutdown, m3;
where q is the flow rate of the liquid, determined in accordance with the technological regulations, m3/s;
T is the duration of gas inflow into the volume of the room, determined according to clause 38 of NPB 105-03 s;
where d is the internal diameter of pipelines, m;
Ln is the length of pipelines from the emergency apparatus to the gate valves, m;
Thus, the volume of hydrogen sulfide that entered the room during the considered variant of the accident:
Mass of hydrogen sulfide in the room:
If flammable gases, flammable or combustible gases, flammable or combustible liquids are used in the room, when determining the mass value, it is allowed to take into account the operation of emergency ventilation, if it is provided with backup fans, automatic start when the maximum permissible explosion-proof concentration is exceeded and power supply according to the first category of reliability (PUE ), provided that devices for removing air from the room are located in the immediate vicinity of the place of a possible accident.
In this case, the mass of combustible gases or vapors of flammable or combustible liquids heated to a flash point and above, which entered the volume of the room, should be divided by the coefficient determined by the formula
where - the multiplicity of air exchange created by emergency ventilation, 1 / s. This room has ventilation with an air exchange rate of 8 (0.0022s);
The duration of the entry of flammable gases and vapors of flammable and combustible liquids into the volume of the room, s, is assumed to be 300 s. (clause 7 of NPB 105-03)
The mass of hydrogen sulfide in the room during the considered variant of the accident:
Explosion calculation results
| option number | combustible gas |
Value, kPa | ||
| hydrogen sulfide | 5 | Medium building damage | ||
Table. Maximum permissible excess pressure during the combustion of gas, steam or dust-air mixtures in rooms or in open space
The initial and calculated data are summarized in Table 2.
Table 2 - Initial and calculated data
| No. p / p | Name | Designation | Value |
| 1 | Substance, its name and formula | hydrogen sulfide | H2S |
| 2 | Molecular weight, kg kmol-1 | M | 34,08 |
| 3 | Liquid density, kg/m3 | ρzh | - |
| 4 | Gas density at design temperature, kg/m3 | ρg | 1,33 |
| 5 | Temperatures of the environment (air before the explosion), 0С | T0 | 39 |
| 6 | Saturated vapor pressure, kPa | pH | 28,9 |
| 7 | Stoichiometric concentration, % vol. | Cst | 29,24 |
| 8 | Room dimensions − length, m − width, m − height, m |
||
| 9 | Pipeline dimensions: − diameter, m −length, m |
||
| 10 | Heptane consumption in the pipeline, m3/s | q | 4 10-3 |
| 11 | Valve closing time, s | t | 300 |
| 12 | Emergency ventilation rate, 1/hour | A | 8 |
| 13 | Maximum explosion pressure, kPa | Pmax | 900 |
| 14 | Initial pressure, kPa | P0 | 101 |
| 15 | Leakage and non-adiabatic coefficient | Kn | 3 |
| 16 | The coefficient of participation of fuel in the explosion | Z | 0,5 |
According to NPB 105-2003, the categories of premises for explosion and fire hazard are accepted in accordance with Table 4.
| Room category | Characteristics of substances and materials located (circulating) in the room |
And the explosive |
Combustible gases, flammable liquids with a flash point not exceeding 28 ° C in such an amount that they can form explosive vapor-gas-air mixtures, upon ignition of which an estimated overpressure of an explosion in the room develops, exceeding 5 kPa. Substances and materials capable of exploding and burning when interacting with water, atmospheric oxygen or with each other in such an amount that the calculated overpressure of the explosion in the room exceeds 5 kPa. |
|
explosive and fire hazardous |
Combustible dusts or fibres, flammable liquids with a flashpoint of more than 28 ° C, flammable liquids in such an amount that they can form explosive dust-air or vapor-air mixtures, upon ignition of which a calculated excess explosion pressure in the room develops in excess of 5 kPa. |
| B1-B4 fire hazardous | Combustible and slow-burning liquids, solid combustible and slow-burning substances and materials (including dust and fibers), substances and materials that can only burn when interacting with water, atmospheric oxygen or with each other, provided that the rooms in which they are in stock or in circulation, are not in category A or B. |
| G | Non-combustible 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; combustible gases, liquids and solids that are burned or disposed of as fuel. |
| D | Non-combustible substances and materials in a cold state, |
Conclusion: The room belongs to category A, since it is possible to release combustible gas (hydrogen sulfide) in such an amount that it can form explosive vapor-gas-air mixtures, upon ignition of which an estimated overpressure of the explosion in the room develops, exceeding 5 kPa.
8. Determination of the values of the energy indicators of the explosion hazard of the technological unit during an explosion
The explosive energy potential E (kJ) of a block is determined by the total energy of combustion of the gas-vapor phase located in the block, taking into account the value of the work of its adiabatic expansion, as well as the value of the energy of complete combustion of the evaporated liquid from the maximum possible area of its strait, while it is considered:
1) in case of emergency depressurization of the apparatus, its full disclosure (destruction) occurs;
2) the area of the spillage of liquid is determined based on the design solutions of buildings or outdoor installation site;
3) the evaporation time is assumed to be no more than 1 hour:
The sum of the energies of adiabatic expansion A (kJ) and the combustion of PHF located in the block, kJ:
q" = 23380 kJ/kg - specific heat of combustion of PHF (hydrogen sulfide);
26.9 - mass of combustible gas
.
For a practical determination of the energy of the adiabatic expansion of PGF, one can use the formula
where b1 - can be taken from Table. 5. With adiabatic index k=1.2 and a pressure of 0.1 MPa, it is equal to 1.40.
Table 5. The value of the coefficient b1 depending on the adiabatic index of the medium and the pressure in the process unit
| Indicator | System pressure, MPa | |||||||||
| adiabats | 0,07-0,5 | 0,5-1,0 | 1,0-5,0 | 5,0-10,0 | 10,0-20,0 | 20,0-30,0 | 30,0-40,0 | 40,0-50,0 | 50,0-75,0 | 75,0-100,0 |
| k = 1.1 | 1,60 | 1,95 | 2,95 | 3,38 | 3,08 | 4,02 | 4,16 | 4,28 | 4,46 | 4,63 |
| k = 1.2 | 1,40 | 1,53 | 2,13 | 2,68 | 2,94 | 3,07 | 3,16 | 3,23 | 3,36 | 3,42 |
| k = 1.3 | 1,21 | 1,42 | 1,97 | 2,18 | 2,36 | 2,44 | 2,50 | 2,54 | 2,62 | 2,65 |
| k = 1.4 | 1,08 | 1,24 | 1,68 | 1,83 | 1,95 | 2,00 | 2,05 | 2,08 | 2,12 | 2,15 |
0 kJ is the combustion energy of the PHF, which arrived at the depressurized section from adjacent objects (blocks), kJ. There are no adjacent blocks, so this component is zero.
0 kJ is the energy of combustion of the PHF, which is formed due to the energy of the superheated LF of the block under consideration and received from adjacent objects during the time ti.
0 kJ is the energy of combustion of PHF, which is formed from LF due to the heat of exothermic reactions that do not stop during depressurization.
0 kJ is the combustion energy of PHF, which is formed from the liquid phase due to the heat inflow from external heat carriers.
0 kJ is the combustion energy of PHF, which is formed from a liquid spilled on a solid surface (floor, pallet, soil, etc.) due to heat transfer from the environment (from a solid surface and air to a liquid over its surface.
The energy potential of the explosion hazard of the block is:
E=628923.51 kJ.
The values of the total energy potentials of explosiveness E are used to determine the values of the reduced mass and the relative energy potential that characterize the explosiveness of technological blocks.
The total mass of combustible vapors (gases) of an explosive vapor-gas cloud m, reduced to a single specific combustion energy equal to 46,000 kJ / kg:
Relative energy potential of explosiveness Qv of the technological unit is calculated by the formula
According to the values of the relative energy potentials Qb and the reduced mass of the vapor-gas medium m, the categorization of technological blocks is carried out. The indicators of the categories are given in Table. 5.
Table 4. Indicators of explosion hazard categories of technological blocks
| Explosion category | Qv | m, kg |
| I | > 37 | > 5000 |
| II | 27 - 37 | 2000 - 5000 |
| III | < 27 | < 2000 |
Conclusion: The room belongs to the III category of explosion hazard, since the total mass of the explosive vapor-gas cloud of hydrogen sulfide a reduced to a single specific combustion energy is 16.67 kg, the relative energy potential of the explosion is 5.18.
9. Calculation of the explosive concentration of the gas-air mixture in the room. Determination of the class of the premises for explosion and fire hazard according to PUE
Let us determine the volume of explosive concentration of hydrogen sulfide in the room:
where m is the mass of the vapor-air mixture in the room, kg,
NKPV - lower concentration limit of ignition, g/m3.
The concentration of the vapor-air mixture in the room will be:
where VCM is the volume of explosive concentration of hydrogen sulfide in the room, m3, VC6 is the free volume of the room, m3.
The calculation results are presented in Table 6.
Table 6. Results of calculating the concentration of the gas-air mixture
According to the PUE, the room in question belongs to class B-Ia - zones located in rooms in which, during normal operation, explosive mixtures of combustible gases (regardless of the lower ignition limit) or flammable liquid vapors with air are not formed, but are possible only as a result of accidents and malfunctions.
10. Determination of destruction zones during explosion. Classification of destruction zones
The radii of the destruction zones during the explosion of the gas-air mixture were determined according to the method described in Appendix 2 PB 09-540-03.
The mass of gas-vapor substances (kg) involved in the explosion is determined by the product
where z is the proportion of the reduced mass of hydrogen sulfide involved in the explosion (for GG it is 0.5),
t is the mass of hydrogen sulfide in the room, kg.
TNT equivalent can be used to assess the level of explosion exposure. The TNT equivalent of an explosion of a vapor-gas medium WT (kg) is determined according to the conditions for the adequacy of the nature and degree of destruction during explosions of vapor-gas clouds, as well as solid and liquid chemically unstable compounds.
For gas-vapor environments, the TNT equivalent of an explosion is calculated:
where 0.4 is the fraction of the explosion energy of the gas-vapor medium spent directly on the formation of a shock wave;
0.9 is the fraction of the explosion energy of trinitrotoluene (TNT) spent directly on the formation of a shock wave;
q" - specific calorific value of the vapor-gas medium, kJ/kg;
qT - specific explosion energy of TNT, kJ/kg.
The zone of destruction is the area with boundaries determined by the radii R, the center of which is the considered technological block or the most probable place of depressurization of the technological system. The boundaries of each zone are characterized by the values of excess pressures along the front of the shock wave AR and, accordingly, the dimensionless coefficient K. The classification of fracture zones is given in Table 6.
Table 7. The level of possible destruction during the explosive transformation of clouds of air-fuel mixtures
| Damage zone class | ΔР, kPa | To | Destruction zone | Characteristics of the affected area |
| 1 | ≥100 | 3,8 | full | Destruction and collapse of all elements of buildings and structures, including basements, the percentage of people's survival; For administrative - amenity buildings and control buildings of ordinary performance - 30%; For industrial buildings and structures of conventional design - 0%. |
| 2 | 70 | 5,6 | strong | The destruction of part of the walls and ceilings of the upper floors, the formation of cracks in the walls, the deformation of the ceilings of the lower floors. Possible Limited use of the remaining cellars after clearing the entrances. Human Survival Percentage: For administrative and amenity buildings and control buildings of conventional designs - 85%: For industrial buildings and structures of conventional design - 2% |
| 3 | 28 | 9,6 | medium | Destruction of mainly secondary elements (roofs, partitions and door fillings). Overlappings, as a rule, do not collapse. Part of the premises is suitable for use after clearing the debris and making repairs. Percentage of survival of people: - for administrative buildings and management buildings of ordinary performance - 94%. |
| 4 | 14 | 28 | weak | Destruction of window and door fillings and partitions. Basements and lower floors are fully preserved and suitable for temporary use after debris removal and sealing of openings. Percentage of survival of people: - for administrative buildings and control buildings of ordinary performance - 98%; industrial buildings and structures of conventional design - 90% |
| 5 | ≤2 | 56 | glazing | Destruction of glass fillings. Percentage of survivors - 100% |
The radius of the destruction zone (m) in general terms is determined by the expression:
where K is a dimensionless coefficient characterizing the impact of an explosion on an object.
The results of calculating the radii of the destruction zones during the explosion of the fuel-air mixture in the room are presented in Table 7.
Table 7 - Results of calculating the radii of the destruction zones
List of sources used
1. Beschastnov M.V. industrial explosions. Evaluation and warning. - M. Chemistry, 1991.
2. Life safety, Safety of technological processes and production (Labor protection): Textbook, Manual for universities / P.P. Kukin, V.L. Lapin, N, L. Ponomarev and others, - M.,: Higher. school 2001,
3. PB 09-540-03 "General explosion safety rules for fire and explosion hazardous chemical, petrochemical and oil refining industries".
4. GOST 12.1,010-76* Explosion safety
5. NPB 105-03 "Definition of categories of premises and buildings, outdoor installations in terms of explosion and fire hazard".
6. SNiP 23-01-99 Building climatology.
7. Fire and explosion hazard of substances and materials and means of extinguishing them. Ed. A. N. Baratova and A. Ya. Korolchenko. M., Chemistry, 1990. 8. Rules for the installation of electrical installations. Ed. 7th.
1 The method consists in determining the upper limits for the maximum and average rate of increase in the pressure of the explosion of gas and vapor-air mixtures in a spherical reaction vessel of constant volume.
The upper limit for the maximum rate of pressure rise in kPa s -1 is calculated by the formula
where p i- initial pressure, kPa;
S and. i- normal speed of flame propagation at initial pressure and temperature, m·s -1 ;
a- radius of the spherical reaction vessel, m;
Dimensionless maximum explosion pressure;
R - maximum absolute explosion pressure, kPa;
and- adiabatic index for the mixture under study;
is a thermokinetic exponent as a function of normal flame propagation velocity as a function of pressure and temperature. If the value unknown, it is taken equal to 0.4.
The upper limit for the average rate of pressure rise in kPa s -1 is calculated by the formula
,
(98)
where is a function of the parameters e , and , , the values of which are found using the nomograms shown in Fig. 26 and 27.
Values e and and are found by thermodynamic calculation or, in case of impossibility of calculation, are taken equal to 9.0 and 1.4, respectively.
The relative root-mean-square error of calculation by formulas (97) and (98) does not exceed 20%.
2. The maximum rate of increase in the explosion pressure of gas and vapor-air mixtures for substances consisting of atoms C, H, O, N, S, F, Cl is calculated by the formula
,
(99)
where V- volume of the reaction vessel, m 3 .
The relative root-mean-square error of calculation by formula (99) does not exceed 30%.
Method for experimental determination of the conditions of thermal spontaneous combustion of solid substances and materials
1. Hardware.
The equipment for determining the conditions of thermal spontaneous combustion includes the following elements.
1.1. Thermostat with a capacity of the working chamber of at least 40 dm 3 with a thermostat that allows you to maintain a constant temperature from 60 to 250 ° C with an error of not more than 3 ° C.
1.2. Baskets made of corrosion-resistant metal of cubic or cylindrical shape 35, 50, 70, 100, 140 and 200 mm high (10 pieces of each size) with lids. The diameter of the cylindrical basket should be equal to its height. The wall thickness of the basket is (1.0 ± 0.1) mm.
1.3. Thermoelectric transducers (not less than 3) with a maximum working junction diameter of not more than 0.8 mm.
2. Preparation for the test.
2.1. Carry out a calibration test to determine the correction ( t T) to the readings of thermoelectric converters 2 and 3 . To do this, a basket with a non-combustible substance (for example, calcined sand) is placed in a thermostat heated to a given temperature. Thermoelectric converters (Fig. 2) are installed in such a way that the working junction of one thermoelectric converter is in contact with the sample and is located in its center, the second one is in contact with the outer side of the basket, the third one is at a distance of (30 ± 1) mm from the basket wall. The working junctions of all three thermoelectric converters must be located at the same horizontal level, corresponding to the middle line of the thermostat.
1 , 2 , 3 - working junctions of thermoelectric converters.
A basket with a non-combustible substance is kept in a thermostat until a stationary regime is established, in which the readings of all thermoelectric
transducers for 10 minutes remain unchanged or fluctuate with a constant amplitude around average temperatures t 1 , t 2 , t 3 . Amendment t T is calculated by the formula
,
(100)
2.2. Samples for testing should characterize the average properties of the test substance (material). When testing sheet material, it is collected in a pile corresponding to the internal dimensions of the basket. In samples of monolithic materials, a hole with a diameter of (7.0 ± 0.5) mm is pre-drilled to the center for a thermoelectric converter.
The study of the combustion processes of combustible mixtures by Russian and foreign scientists made it possible to theoretically substantiate many of the phenomena that accompany the combustion process, including the speed of flame propagation. The study of the speed of flame propagation in gas mixtures makes it possible to determine the safe speeds of gas-air flows in pipelines of ventilation, recuperation, aspiration and in pipelines of other installations through which gas and dust-air mixtures are transported.
In 1889, the Russian scientist V.A. Michelson considered two limiting cases of flame propagation during normal or slow combustion and during detonation.
The theory of normal flame propagation and detonation was further developed in the works of N.N. Semenova, K.I. Shchelkina, D.A. Frank-Kamenetsky, L.N. Khitrina, A.S. Sokolika, V.I. Skobelkin and other scientists, as well as foreign scientists B. Lewis, G. Elbe and others. As a result, a theory of ignition of explosive mixtures was created. However, attempts to interpret the phenomena of flame propagation as diffusion of active centers or to explain the limits of flame propagation by chain termination conditions are not convincing enough.
In 1942, the Soviet scientist Ya.B. Zel'dovich formulated the provisions of the theory of combustion and detonation of gases. The theory of combustion provides an answer to the main questions: will a mixture of a given composition be combustible, what will be the burning rate of an explosive mixture, what features and forms of flame should be expected. The theory states that the explosion of a gas or vapor-air mixture is not an instantaneous phenomenon. When the ignition source is introduced into the combustible mixture, the oxidation reaction of the fuel with the oxidizer begins in the area of the ignition source. The rate of the oxidation reaction in some elementary volume of this zone reaches a maximum - combustion occurs. Combustion at the boundary of an elementary volume with a medium is called a flame front. The flame front looks like a sphere. The thickness of the flame front, according to Ya.B. Zel'dovich, is equal to 1 - 100 microns. Although the thickness of the combustion zone is small, it is sufficient for the combustion reaction to proceed. The temperature of the flame front due to the heat of the combustion reaction is 1000 - 3000 0 C and depends on the composition of the combustible mixture. Near the flame front, the temperature of the mixture also increases, which is due to heat transfer by thermal conduction, diffusion of heated molecules, and radiation. On the outer surface of the flame front, this temperature is equal to the self-ignition temperature of the combustible mixture. The change in the temperature of the mixture along the axis of the pipe at points in time is graphically shown in fig. 4.1. Gas layer QC 1, in which the temperature of the mixture rises, is the flame front. As the temperature rises, the flame front expands (up to QC 2) to the sides of the end walls of the pipe BUT and M, displacing the unburned mixture at a certain speed towards the wall M, and the burnt gas towards the wall BUT. After ignition of the combustible mixture, the spherical shape of the flame is very quickly distorted and more and more drawn towards the still unignited mixture. The extension of the flame front and the rapid increase in its surface is accompanied by an increase in the speed of movement
the center of the flame. This acceleration lasts until the flame touches the walls of the pipes or, in any case, does not come close to the wall of the pipe. At this moment, the size of the flame decreases sharply, and only a small part of it remains from the flame, covering the entire section of the pipe. The extension of the flame front and its intensive acceleration immediately after ignition by a spark, when the flame has not yet reached the walls of the pipe, are caused by an increase in the volume of combustion products. Thus, at the initial stage of the formation of the flame front, regardless of the degree of combustibility of the gas mixture, acceleration and subsequent deceleration of the flame occurs, and this deceleration will be the greater, the greater the flame speed.
Rice. 4.1. Temperature change in front of and behind the flame front: 1 - zone
combustion products; 2 - flame front; 3 - self-ignition zone;
4 - preheating zone; 5 - initial mixture
The process of development of the subsequent stages of combustion is influenced by the length of the pipe. Elongation of the pipe leads to the appearance of vibrations and the formation of a cellular structure of the flame, shock and detonation waves.
Consider the width of the heating zone in front of the flame front. In this zone, no chemical reaction takes place and no heat is released. Heating zone width l(in cm) can be determined from the dependence:
where a is the thermal diffusivity; v is the speed of flame propagation.
For a methane-air mixture, the width of the heating zone is 0.0006 m, for a hydrogen-air mixture it is much smaller (3 μm). Subsequent combustion occurs in a mixture whose state has already changed as a result of thermal conductivity and diffusion of components from neighboring layers. The admixture of reaction products does not have any specific catalytic effect on the speed of flame movement.
Let us now consider the velocity of the flame front in the gas mixture. Linear travel speed v(in m/s) can be determined by the formula
where is the mass burning rate, g / (cm × m 2), p is the density of the initial combustible mixture, kg / m 3.
The linear speed of the flame front is not constant, it varies depending on the composition of the mixture and the admixture of inert (non-combustible) gases, the temperature of the mixture, the diameter of the pipes, etc. The maximum speed of flame propagation is observed not at a stoichiometric mixture concentration, but in a mixture with an excess of fuel. When inert gases are introduced into the combustible mixture, the flame propagation speed decreases. This is explained by a decrease in the combustion temperature of the mixture, since part of the heat is spent on heating the inert impurities that do not participate in the reaction. The heat capacity of the inert gas affects the rate of flame propagation. The greater the heat capacity of an inert gas, the more it reduces the combustion temperature and the more it reduces the speed of flame propagation. Thus, in a mixture of methane and air diluted with carbon dioxide, the flame propagation velocity turns out to be approximately three times less than in a mixture diluted with argon.
When the mixture is preheated, the flame propagation speed increases. It has been established that the flame propagation velocity is proportional to the square of the initial temperature of the mixture.
With an increase in the diameter of the pipes, the flame propagation speed increases unevenly.
With an increase in the diameter of the pipes to 0.10 - 0.15 m, the speed increases quite quickly; with a further increase in the diameter of the pipes, it continues to increase, but to a lesser extent. The increase in temperature occurs until the diameter reaches a certain limiting diameter, above which the increase in speed does not occur. With a decrease in the diameter of the pipe, the flame propagation speed decreases, and at a certain small diameter, the flame does not propagate in the pipe. This phenomenon can be explained by an increase in heat losses through the pipe walls.
Therefore, in order to stop the spread of flame in a combustible mixture, it is necessary in one way or another to lower the temperature of the mixture by cooling the vessel (in our example, a pipe) from the outside or by diluting the mixture with cold inert gas.
The normal speed of flame propagation is relatively small (no more than tens of meters per second), but under certain conditions, the flame in pipes propagates at a tremendous speed (from 2 to 5 km / s), exceeding the speed of sound in a given environment. This phenomenon is called detonation. Distinctive features of detonation are as follows:
1) constant burning rate regardless of the pipe diameter;
2) high flame pressure caused by the detonation wave, which can exceed 50 MPa, depending on the chemical nature of the combustible mixture and the initial pressure; moreover, due to the high burning rate, the developed pressure does not depend on the shape, capacity and tightness of the vessel (or pipe).
Let us consider the transition from fast combustion to detonation in a long tube of constant cross section when the mixture is ignited at the closed end. Under the pressure of the flame front, compression waves arise in the combustible mixture - shock waves. In the shock wave, the gas temperature rises up to values at which the mixture spontaneously ignites far ahead of the flame front. This mode of combustion is called detonation. As the flame front moves, the movement of the layers adjacent to the wall is retarded and, accordingly, the movement of the mixture in the center of the pipe is accelerated; speed distribution
cross-sectional growth becomes uneven. Jets of gas mixtures appear, the speed of which is less than the average velocity of the gas mixture during normal combustion, and jets moving faster. Under these conditions, the speed of flame movement relative to the mixture increases, the amount of gas burning per unit time increases, and the movement of the flame front is determined by the maximum speed of the gas jet.
As the flame accelerates, the amplitude of the shock wave also increases, and the compression temperature reaches the self-ignition temperature of the mixture.
The increase in the total amount of gas burning per unit time is explained by the fact that in a jet with a velocity variable over the cross section, the flame front is bent; as a result of this, its surface increases and the amount of the burning substance increases proportionally.
One of the ways to reduce the burning rate of combustible mixtures is the action of inert gases on the flame, but due to their low efficiency, chemical combustion inhibition is currently used by adding halogenated hydrocarbons to the mixture.
Combustible gas mixtures have two theoretical combustion temperatures - at constant volume and at constant pressure, the first being always higher than the second.
The method for calculating the calorimetric combustion temperature at constant pressure is considered in Section 1. Let us consider the method for calculating the theoretical combustion temperature of gas mixtures at a constant volume, which corresponds to an explosion in a closed vessel. The calculation of the theoretical combustion temperature at a constant volume is based on the same conditions that are indicated in Sec. 1.7.
When gas mixtures are burned in a closed volume, the products of combustion do not do work; the energy of the explosion is spent only on heating the products of the explosion. In this case, the total energy is defined as the sum of the internal energy of the explosive mixture Q vn.en.cm and the heat of combustion of the given substance. The value of Q ext.cm is equal to the sum of the products of the heat capacities of the components of the explosive mixture at a constant volume and the initial temperature of the mixture
Q vn.en.cm \u003d s 1 T + s 2 T + ... + s n T,
where c 1 , c 2 , c n are the specific heat capacities of the components that make up the explosive mixture, kJ/(kg × K); T is the initial temperature of the mixture, K.
The value of Q int.en.cm can be found in the reference tables. The explosion temperature of gas mixtures at constant volume is calculated by the same method as the combustion temperature of a mixture at constant pressure.
Explosion pressure is found from the explosion temperature. The pressure during the explosion of a gas-air mixture in a closed volume depends on the temperature of the explosion and the ratio of the number of molecules of combustion products to the number of molecules in the explosive mixture. During the explosion of a gas-air mixture, the pressure usually does not exceed 1.0 MPa, if the initial pressure of the mixture was normal. When the air in the explosive mixture is replaced by oxygen, the pressure of the explosion increases sharply, since the combustion temperature increases.
During the explosion of even a stoichiometric gas-air mixture, a significant amount of heat is spent on heating the nitrogen in the mixture, so the explosion temperature of such mixtures is much lower than the explosion temperature of mixtures with oxygen. Thus, the explosion pressure of a stoichiometric mixture of methane, ethylene, acetone, and methyl ether
ra with oxygen is 1.5 - 1.9 MPa, and their stoichiometric mixtures with air is 1.0 MPa.
The maximum explosion pressure is used in calculations of the explosion resistance of equipment, as well as in the calculations of safety valves, explosive membranes and shells of explosion-proof electrical equipment.
The explosion pressure P vzr (in MPa) of gas-air mixtures is calculated by the formula
,
where Р 0 is the initial pressure of the explosive mixture, MPa; T 0 and T vzr - the initial temperature of the explosive mixture and the temperature of the explosion, K; is the number of molecules of gases of combustion products after the explosion; is the number of gas molecules in the mixture before the explosion.
Example 4.1 . Calculate the pressure at the explosion of a mixture of ethyl alcohol vapor and air.
.
P 0 \u003d 0.1 MPa; T vzr = 2933 K; T 0 \u003d 273 + 27 \u003d 300 K; \u003d 2 + 3 + 11.28 \u003d 16.28 mol; \u003d 1 + 3 + 11.28 \u003d 15.28 mol.