The concept of NKPR, VKPR and PDVK, their numerical values for oil vapors. Lower concentration limit of flame spread What is the upper limit of flame spread
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 of 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 the lack of an oxidizing agent, and the cost of heating excess fuel that does not take part in the 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;
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.
Gas, tasteless, colorless, odorless. Air density 0.554. It burns well, with an almost colorless flame. Self-ignition temperature 537°C. Explosive limit 4.4 - 17%. MPC in the air of the working area is 7000 mg/m3. It has no poisonous properties. Headache is a symptom of suffocation at 80% methane and 20% oxygen. The danger of methane is that with a strong increase in the content of methane, the oxygen content decreases. The danger of poisoning is reduced by the fact that methane is lighter than air, and when an unconscious person falls, he enters an atmosphere richer in oxygen. Methane is a suffocating gas, so after bringing the victim to consciousness (if the victim has lost consciousness), it is necessary to inhale 100% oxygen. Give the victim 15-20 drops of valerian, rub the body of the victim. Filtering gas masks from methane do not exist.
Ticket number 2
1. Define the term "Lower explosive limit (LEL) (lower concentration limit of flame propagation - LEL)". The minimum concentration of combustible gas in air at which an explosion of a mixture of combustible gas and air occurs. When the gas concentration is below the LEL, no reaction occurs.
2. Control of the air environment at gas transportation facilities.
4.1. Before commissioning a pipeline for the transport of natural gas, it is necessary to displace air from the pipeline with gas at a pressure of not more than 0.1 MPa (1 kgf / cm 2) at the place of its supply, in compliance with safety measures. The displacement of air by gas can be considered complete when the oxygen content in the gas leaving the gas pipeline is no more than 1% according to the gas analyzer readings.
The analysis of residual oxygen in the pipe when purging the repaired section should be carried out with a specialized device that simultaneously analyzes the content of oxygen (low concentrations) and combustible gas (from 0 to 100% by volume fraction).
The use of individual gas analyzers designed to ensure the safety of personnel in these cases is unacceptable, as it leads to failure of the sensors.
The equipment used must:
Have an explosion-proof design;
Have a sampling probe for sampling from the pipe;
Have a built-in expense booster;
Have a lower operating temperature limit of minus 30 ° С;
Have automatic calibration (adjustment) of zero;
Have a display for simultaneous display of measured concentrations;
Ensure registration of measurement results.
4.2. The tightness of equipment, pipelines, welded, detachable joints and seals is controlled using leak detectors in explosion-proof design, with the function of protecting the sensor from overloads.
The use of individual gas analyzers for these purposes is unacceptable, since these gas analyzers do not display leaks with a concentration of less than 0.1% LEL.
4.3. Control of gas contamination in wells, including water supply and sewerage, underground premises and closed channels located at industrial sites, is carried out according to the schedule at least once a quarter, and in the first year of their operation - at least once a month, as well as every times immediately before the commencement of work in the indicated places. Gas control should be carried out using remote sampling by portable (individual) gas analyzers with a connected manual or built-in motorized sampling pump.
4.4. The control of leaks and gas contamination along underground gas pipelines is carried out using leak detectors similar to those used in the control of equipment tightness.
4.5. Along with the control of the air environment for gas content with stationary devices, it is necessary to continuously monitor (while in the danger zone) the air environment with portable gas analyzers:
In rooms where gases and liquids containing harmful substances are pumped;
In rooms where the release and accumulation of harmful substances is possible, and on outdoor installations in places of their possible release and accumulation;
In rooms where there are no sources of emission, but it is possible for harmful substances to enter from the outside;
In places where service personnel are permanently located, where there is no need to install stationary gas detectors;
During emergency work in a gassed area - continuously.
After the liquidation of the emergency, it is necessary to additionally analyze the air in places where harmful substances can accumulate.
4.7. In places of gas leakage and in areas of gas contamination of the atmosphere, a sign “Caution! Gas".
Yellow
black color
4.8. Start-up and operation of equipment and installations of gas transportation facilities with a switched off or faulty monitoring and alarm system for the content of combustible gases in the air is not allowed.
4.9. The operability of the automatic alarm system and automatic switching on of emergency ventilation is controlled by the operational (duty) personnel upon acceptance of the shift.
Information about the operation of the automatic gas detection system, about the failure of sensors and related measuring channels and automatic signaling channels, about equipment shutdowns carried out by the automatic gas detection system, is received by the operational (duty) personnel, who informs the head of the facility (service, section) with entry in the operating log.
The operation of automatic indoor air gas detection systems shall be tested in accordance with the manufacturer's instructions.
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 a working disease or health abnormalities 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.
The range of values of the graph of the dependence of the KPRP in the "combustible gas - oxidizer" system, corresponding to the ability of the mixture to ignite, forms the ignition region.
The following factors influence the values of NKPRP and VKPRP:
- Properties of reacting substances;
- Pressure (usually an increase in pressure does not affect the LKPR, but the VKPR can increase greatly);
- Temperature (an increase in temperature expands the CRRP by increasing the activation energy);
- Non-flammable additives - phlegmatizers;
The unit of CPRP can be expressed in volume percent or in g/m³.
The introduction of a phlegmatizer into the mixture lowers the value of VKPRP almost in proportion to its concentration up to the point of phlegmatization, where the upper and lower limits coincide. At the same time, NKPP rises slightly. To assess the ability to ignite the "Fuel + Oxidizer + Phlegmatizer" systems, they build the so-called. fire triangle - a diagram where each vertex of the triangle corresponds to one hundred percent content of one of the substances, decreasing to the opposite side. Inside the triangle, the area of \u200b\u200bignition of the system is distinguished. In the fire triangle, a line of minimum oxygen concentration (MCC) is marked, corresponding to such a value of the oxidizer content in the system, below which the mixture does not ignite. Evaluation and control of the ICC is important for systems operating under vacuum, where leakage through the leakage of process equipment of atmospheric air is possible.
With regard to liquid media, the temperature limits of flame propagation (TPRP) are also applicable - such temperatures of the liquid and its vapors in the oxidizer medium at which its saturated vapors form concentrations corresponding to the CPRP.
KPRP is determined by calculation or found experimentally.
It is used when categorizing rooms and buildings according to explosion and fire hazard, for analyzing the risk of an accident and assessing possible damage, when developing measures to prevent fires and explosions in process equipment.
see also
Links
Wikimedia Foundation. 2010 .
See what "" is in other dictionaries:
lower concentration limit of flame propagation- LEL The concentration of a combustible gas or vapor in air below which no explosive gas atmosphere is formed. [GOST R IEC 60050 426 2006] Topics explosion protection Synonyms NKPR EN LELlower explosive limit ...
lower concentration limit of flame propagation- 3.1.6 lower explosive limit (LEL); LEL, %: Volume fraction of combustible gas or vapor in air, below which an explosive gas atmosphere is not formed. Source …
lower concentration limit of flame propagation (ignition) (LEL)- 2.10.1 lower flame propagation (ignition) concentration limit (LEL) minimum content of combustible gas or vapor in air at which flame propagation through the mixture is possible at any distance from the source. Source: GOST ... ... Dictionary-reference book of terms of normative and technical documentation
lower concentration limit of flame propagation (LEL)- 2.1.6 lower concentration limit of flame propagation (NKPR): According to GOST 12.1.044. Source … Dictionary-reference book of terms of normative and technical documentation
lower concentration limit of flame propagation, LEL 3.12 lower explosive limit LEL Dictionary-reference book of terms of normative and technical documentation
lower concentration limit of flame propagation NKPR Electrotechnical dictionary
LEL (lower concentration limit of flame propagation)- 3.37 NKPR (lower concentration limit of flame propagation): According to GOST 12.1.044. Source … Dictionary-reference book of terms of normative and technical documentation
LEL lower concentration limit of flame propagation- lower explosive limit, LEL The concentration of a combustible gas or vapor in air below which no explosive gas atmosphere is formed ... Electrotechnical dictionary
lower (upper) concentration limit of flame propagation- The minimum (maximum) content of a combustible substance in a homogeneous mixture with an oxidizing environment, at which flame propagation through the mixture is possible at any distance from the ignition source. [GOST 12.1.044 89] Fire safety topics ... Technical Translator's Handbook
lower concentration limit of propagation (LCPR) of a flame (ignition)- 3.5 lower concentration limit of propagation (LEL) of a flame (ignition): The minimum content of a combustible substance in a homogeneous mixture with an oxidizing environment (LEL, % vol.), at which flame propagation through the mixture is possible to any ... ... Dictionary-reference book of terms of normative and technical documentation