Explosion concentration limits of flame propagation. Lower concentration limit of flame propagation. Rules for sampling gas for analysis
The theory of deflagration combustion does not impose restrictions on the possibility of reducing the rate of combustion propagation. However, experience shows that the value of the rate of propagation of combustion cannot be less than a certain critical value. Flame propagation in mixtures of fuel and oxidizer is possible only in a certain range of their concentrations. When igniting a mixture whose composition goes beyond these limits, persistent combustion does not occur.
For combustible mixtures, there are lower and upper concentration limits for flame propagation.
Lower concentration limit flame spread (NKPRP) - the lowest concentration of a combustible substance in a mixture with air, at which a stable, undamped spread of combustion is already possible.
Upper concentration limit flame spread (VKPRP) - the highest concentration of a combustible substance in a mixture with air, at which a stable, undamped spread of combustion is still possible.
The concentration limits of flame propagation (KPRP) is one of the most important characteristics of the explosiveness of combustible gases and vapors. The area of concentration of a combustible substance, which lies between the lower and upper KPRP, is characterized by the possibility of ignition and stable combustion of the mixture and is called the area explosive concentrations. If the concentration of a combustible substance goes beyond the concentration limits, the combustible mixture becomes explosive. So, if the concentration of a combustible substance is less than the lower KPP, then combustion is not possible at all. If the concentration of a combustible substance is greater than VKPRP, then diffusion combustion of such a gas mixture is possible when it enters the surrounding space and there is an ignition source.
The maximum rate of reaction and propagation of the flame front is observed at a stoichiometric ratio of components (fuel concentration equal to the stoichiometric φ gv = φ smk). When deviating from the stoichiometric ratio, the burning rate, and hence the rate of heat release, will decrease. So, at φ gv< φстм скорость тепловыделения уменьшается в результате нехватки горючего, и нагревании излишка окислителя, что приводит к дополнительным тепловым потерям. При φ гв >φ sm, the decrease in heat release occurs as a result of a lack of an oxidizer, and the cost of heating excess fuel that does not take part in chemical reaction. Thus, for gas-vapor mixtures, it is possible to distinguish both the minimum (lower) φ n and the maximum (upper) φ n fuel concentration, at which critical conditions for the propagation of the flame front occur.
Considering that the concentration limits of flame propagation can change when external conditions change, to ensure fire safety when working with combustible substances, not only concentration limits are determined, but also safe concentrations φ nb and φ wb, below or above which the mixture is guaranteed not to ignite. Safe concentrations can be calculated using the formulas:
φnb< 0,9(φн – 0,21), %
φvb ≥ 1.1(φv + 0.42), %
where φ n, φ in - NKPRP and VKPRP,%;
The location of the areas of possible fuel concentrations is shown in the figure.
The concentration limits of flame propagation can vary greatly with changing external conditions. Changes in the KPPP are explained in terms of the balance of heat release and heat transfer in the system. All the factors, the change of which will lead to an increase in heat release, will expand the KPP (reduce the lower KPP and increase the upper KPP). Factors that increase heat transfer will narrow the ICRP (increase the lower ICRP and decrease the upper ICRP). The greatest influence on the CPRP is exerted by:
The concentration of the oxidizing agent in the oxidizing environment (oxygen content in the air);
concentration of inert gases (phlegmatizers);
The temperature and pressure of the mixture;
The power of the ignition source;
BASIC TERMS AND CONCEPTS.
MPC (maximum permissible concentration) of harmful substances in the air of the working area are concentrations that, during daily work within 8 hours during the entire working time, cannot cause diseases or health abnormalities in the worker, detected by modern research methods directly in the process of work or more remote periods. And also the MPC of harmful substances should not adversely affect the health status of subsequent generations. Measured in mg/cu.m.
MPC of some substances (in mg/m3):
Petroleum hydrocarbons, kerosene, diesel fuel - 300
Gasoline - 100
Methane - 300
Ethyl alcohol - 1000
Methyl alcohol - 5
Carbon monoxide - 20
Ammonia (ammonia) - 20
Pure hydrogen sulfide - 10
Hydrogen sulfide mixed with oil hydrocarbons - 3
Mercury - 0.01
Benzene - 5
NKPR is the lower concentration limit of flame propagation. This is the lowest concentration of combustible gases and vapors at which an explosion is already possible when exposed to an ignition pulse. Measured in %V.
LEL of some substances (in % V):
Methane - 5.28
Oil hydrocarbons - 1.2
Gasoline - 0.7
Kerosene - 1.4
Hydrogen sulfide - 4.3
Carbon monoxide - 12.5
Mercury - 2.5
Ammonia - 15.5
Methyl alcohol - 6.7
VCPR – upper concentration limit of flame propagation. This is the highest concentration of combustible gases and vapors at which an explosion is still possible when exposed to an ignition pulse. Measured in %V.
VKPR of some substances (in % V):
Methane - 15.4
Oil hydrocarbons - 15.4
Gasoline - 5.16
Kerosene - 7.5
Hydrogen sulfide - 45.5
Carbon monoxide - 74
Mercury - 80
Ammonia - 28
Methyl alcohol - 34.7
DVK - pre-explosive concentration, is defined as 20% of LEL. (no explosion possible at this point)
PDVK - limiting explosive concentration, is defined as 5% of LEL. (no explosion possible at this point)
Relative density in air (d) shows how many times the vapors of a given substance are heavier or lighter than air vapors under normal conditions. The value is relative - there are no units of measurement.
Relative density in air of some substances:
Methane - 0.554
Oil hydrocarbons - 2.5
Gasoline - 3.27
Kerosene - 4.2
Hydrogen sulfide - 1.19
Carbon monoxide - 0.97
Ammonia - 0.59
Methyl alcohol - 1.11
Gas dangerous places - such places in the air of which there are or may suddenly appear toxic and vapors in concentrations exceeding the MPC.
Gas hazardous places are divided into three main groups.
IGroup – places where the oxygen content is below 18% V, and the content of toxic gases and vapors is more than 2% V. In this case, work is carried out only by gas rescuers, in insulating apparatus, or under their supervision according to special documents.
IIGroup– places where the oxygen content is less than 18-20%V, and pre-explosive concentrations of gases and vapors can be detected. In this case, the work is carried out according to work permits, with the exception of the formation of sparks, in appropriate protective equipment, under the supervision of gas rescue and fire supervision. Before carrying out work, an analysis of the gas-air environment (GVS) is carried out.
IIIGroup- places where the oxygen content is from 19% V, and the concentration of harmful vapors and gases may exceed the MPC. In this case, work is carried out in gas masks, or without them, but gas masks must be in good condition at the workplace. In the places of this group, it is necessary to analyze the hot water supply according to the schedule and the selection map.
Gas-hazardous work - all those jobs that are carried out in a gassed environment, or work during which gas can escape from gas pipelines, fittings, units and other equipment. Also, gas-hazardous work includes work that is performed in a confined space with an oxygen content in the air of less than 20% V. When performing gas hazardous work, the use of open flame is prohibited, it is also necessary to exclude sparking.
Examples of gas hazardous work:
Works related to inspection, cleaning, repair, depressurization of technological equipment, communications;
At removal of blockages, installation and removal of plugs on existing gas pipelines, as well as disconnection of units, equipment and individual units from gas pipelines;
Repair and inspection of wells, pumping out water and condensate from gas pipelines and condensate collectors;
Preparation for the technical examination of LPG tanks and cylinders and its implementation;
Excavation of soil in places of gas leaks until they are eliminated.
Hot work - production operations associated with the use of open fire, sparking and heating to temperatures that can cause ignition of materials and structures.
Hot work examples:
Electric welding, gas welding;
Electric cutting, gas cutting;
Application of explosive technologies;
Soldering work;
Educational cleaning;
Machining of metal with the release of sparks;
Heating of bitumen, pitches.
Currently, there is a NPA in terms of the NKPR:
Order of Rostekhnadzor dated December 26, 2012 N 777 "On approval Guides on safety for oil depots and warehouses of petroleum products"
DVK - pre-explosion concentration signaling sensors
NKPR - lower concentration limit of flame propagation
.10.26. tank farms storage of oil and light oil products are equipped with DVK,
triggered when the oil product vapor concentration reaches 20% of the LEL.
The number and order of placement of DVK signaling devices sensors are determined in the design documentation,
depending on the type of stored products, their storage conditions, the volume of single containers
tanks and the order of their placement in the warehouse (park).
(parks) from the inside at a height of 1.0 - 1.5 m from the planning mark of the earth's surface.
10.28. The distance between the sensors of the signaling devices is selected less than 2 ranges
sensor. With adjacent arrangement of groups of containers and tanks or individual tanks in
own embankment (fence) installation of signaling sensors along the adjacent (common for two
groups) embankment (fencing) is not required.
warehouse (park) located outside the embankment. Number of signaling sensors
is selected depending on the area occupied by the node, taking into account the allowable distance between
sensors not more than 20 m, but not less than two sensors. Sensors of signaling devices NKPR recommended
be located oppositely along the perimeter of the node site at a height of 0.5 - 1.0 m from the planning mark
A new RLA is INTRODUCED:
Order of Rostekhnadzor dated November 7, 2016 N 461 "On approval of the Federal rules and regulations in the field of industrial safety "Rules of industrial safety of warehouses of oil and oil products"
The beginning of the document -03.06.2017 .
Modified February 14, 2017 by KIP IPP2.2.27. On loading and unloading railway racks intended for loading and unloading oil and light oil products, gas contamination sensors must be installed in accordance with the requirements of regulatory legal acts in the field of industrial safety.
Draining and filling should automatically stop when the gas content of the air reaches more than 50% by volume of the lower concentration limit of flame propagation (hereinafter - NKPRP ).
The installation of gas sensors is justified in the project documentation in accordance with technical specifications devices specified in the passports of the manufacturer.
2.3.15. If the concentration of oil product vapors at the sites of loading and unloading stations and loading and unloading points exceeds more than 20% by volume of NKPRP, locks should be installed to stop loading and unloading operations and an alarm notifying about the prohibition of starting car engines.
2.8.15. In the premises of pumping stations, it is necessary to install means of automatic control of gas contamination according to NKPRP with a signal (light and sound) at the entrance to the pump room and to the operator room when the concentration of combustible gases and vapors of oil products reaches 20% by volume from NKPRP.
The distance from the gas sensor to the most distant point of possible leaks in the pump group should not exceed 4 m (horizontally). At least two gas sensors should be installed in the pump room.
The installation locations and the number of gas sensors are determined by the project documentation.
The emergency ventilation is switched on when combustible gases and vapors of oil products reach 50% by volume of the NKPRP.
3.5.8. For ventilation systems should provide for:
automatic switching on of emergency ventilation when the concentration of combustible gases and vapors of oil products in the room reaches 50% by volume of the NKPRP;
3.6.3. Pumping stations of industrial sewage, buried more than 0.5 m, must be equipped with gas sensors with a signal output to the control room control panel. In case of gas contamination pumping station 50% by volume of the LCVRP, emergency ventilation should be turned on.
3.1.10. All measuring instruments are subject to verification.
2.1 Natural gas - a product extracted from the bowels of the earth, consists of methane (96 - 99%), hydrocarbons (ethane, butane, propane, etc.), nitrogen, oxygen, carbon dioxide, water vapor, helium. IvTETS-3 receives natural gas as fuel through a gas pipeline from Tyumen.
The specific gravity of natural gas is 0.76 kg / m 3, the specific heat of combustion is 8000 - 10000 kcal / m 3 (32 - 41 MJ / m 3), the combustion temperature is 2080 ° C, the ignition temperature is 750 ° C.
Combustible natural gas, according to the toxicological characteristics, belongs to substances of the 4th hazard class ("low-hazardous") in accordance with GOST 12.1.044-84.
2.2 The maximum permissible concentration (MPC) of natural gas hydrocarbons in the air of the working area is 300 mg / m 3 in terms of carbon, the MPC of hydrogen sulfide in the air of the working area is 10 mg / m 3, hydrogen sulfide mixed with hydrocarbons C 1 - C 5 - 3 mg / m 3.
2.3 The safety regulations for the operation of gas facilities determine the following hazardous properties of gaseous fuel:
a/ lack of smell and color
b/ the ability of gas to form flammable and explosive mixtures with air
c/ asphyxiating ability of the gas.
2.4 Permissible concentration of gas in the air of the working area, in the gas pipeline when performing gas hazardous work - no more than 20% of the lower concentration limit of flame propagation (LCPR):
3 Rules for sampling gas for analysis
3.1 Smoking and the use of open flames in gas hazardous places, when checking the gas contamination of industrial premises, is strictly prohibited.
3.2 The shoes of workers who measure gas contamination and are in gas hazardous places should not have metal horseshoes and nails.
3.3 When performing gas hazardous work, use explosion-proof portable lamps with a voltage of 12 volts
3.4 Before performing the analysis, it is necessary to inspect the gas analyzer. Measuring instruments with an expired verification period or damage are not allowed to be used.
3.5 Before entering the hydraulic fracturing room, it is necessary: to make sure that the emergency signal lamp "GASED" at the entrance to the hydraulic fracturing room is not lit. The signal lamp turns on when the concentration of methane in the air of the hydraulic fracturing rooms reaches 20% or more of the lower concentration limit of flame propagation, i.e. equal or higher vol. one%.
3.6 Gas sampling in the premises (in the GRP) is carried out by a portable gas analyzer from the upper zone of the premises in the most poorly ventilated areas, because natural gas is lighter than air.
Actions in case of gas contamination are specified in point 6.
3.7 When taking air samples from the well, approach it from the windward side, making sure that there is no smell of gas nearby. One side of the well cover should be lifted with a special hook by 5 - 8 cm, a wooden gasket should be placed under the cover for the time of sampling. Sampling is carried out using a hose lowered to a depth of 20 - 30 cm and connected to a portable gas analyzer, or into a gas pipette.
If gas is detected in the well, it is ventilated for 15 minutes. and repeat the analysis.
3.8 It is not allowed to descend into wells and other underground structures for sampling.
3.9 In the air of the working area, the content of natural gas should not exceed 20% of the lower concentration limit of flame propagation (1% for methane); The oxygen concentration must be at least 20% by volume.
For all harmful substances currently known, the maximum concentration is established at which there is no harmful effect on the human body (GOST 12.1.005-88), this concentration is called maximum permissible concentration (MAC).
MPC- this is the concentration that, during daily (except weekends) work for 8 hours or for another duration, but not more than 40 hours per week, during the entire working experience cannot cause diseases or deviations in the state of health detected by modern research methods in in the process of work or in the remote periods of life of the present and subsequent generations.
MPC is of great importance for the prevention of poisoning and diseases. The lower the MPC, the more serious requirements should be imposed on measures to protect workers.
Depending on the MPC values and a number of other indicators, the degree of exposure to harmful substances on the human body is determined.
Combustible gases and vapors of flammable liquids are capable of forming explosive mixtures in a mixture with atmospheric oxygen.
The lowest concentration of combustible vapors and gases at which an explosion is already possible is called lower concentration limit of flame propagation NKPR(LEC is the minimum fuel content in the mixture "combustible substance - oxidizing environment", at which flame propagation through the mixture is possible at any distance from the ignition source).
The highest concentration of combustible vapors and gases at which an explosion is still possible is called upper concentration limit of flame propagation VKPR(VKPR is the maximum content of fuel in the mixture "combustible substance - oxidizing environment", at which flame propagation through the mixture is possible at any distance from the ignition source).
The concentration from LEL to VKPR is called the explosive range. At a concentration below the LEL or above the LEL, an explosion does not occur, in the first case due to the low content of vapors or gases, in the second - due to insufficient oxygen content.
Each substance has its own LEL and VKPR values, i.e., each substance has its own explosive range.
Oil is a complex (multicomponent) substance, and the composition of various oils differs from each other, therefore, the range of explosiveness for different oils is different, as evidenced by the data in Table 3, which indicates the LEL for various oils. Therefore, in order not to introduce confusion in this matter, a single (averaged) explosive range has been adopted for all oils (see Table 4).
In order to ensure explosion and fire safety, the maximum permissible explosion-proof concentration of PDVK is established for all substances, it is 5% of the value of the lower concentration limit of flame propagation. PDVK has great importance when assessing the degree of risk during different kind works related to the release of combustible vapors and gases.