The concept of NKPR, VKPR and PDVK, their numerical values ​​for oil vapors. Chow uts "new perspectives" Rules for sampling gas for analysis

The lower (upper) concentration limit of flame propagation is the minimum (maximum) concentration of fuel in the oxidizer that can be ignited from a high-energy source with subsequent spread of combustion to the entire mixture.

Calculation formulas

The lower concentration limit of flame propagation φ n is determined by the limiting heat of combustion. It has been established that 1 m 3 of various gas-air mixtures at the NKPR emits during combustion a constant average amount of heat - 1830 kJ, called the limiting heat of combustion. Hence,

if we take the average value of Q pr. equal to 1830 kJ / m 3, then φ n 6 will be equal to

(2.1.2)

where Q n - lower calorific value of a combustible substance, kJ / m 3.

The lower and upper flame CRC can be determined by the approximation formula

(2.1.3)

where n - stoichiometric coefficient at oxygen in the chemical reaction equation; a and b are empirical constants, the values ​​of which are given in Table. 2.1.1

Table 2.1.1.

The concentration limits of flame propagation for vapors of liquid and solid substances can be calculated if the temperature limits are known.

(2.1.4)

where R not) is the saturated vapor pressure of a substance at a temperature corresponding to

lower (upper) limit of flame propagation, Pa;

p O- ambient pressure, Pa.

Saturated vapor pressure can be determined from the Antoine equation or from Table. 13 applications

(2.1.5)

where A, B, C- Antoine constants (Table 7 of the appendix);

t - temperature, 0 C, (temperature limits)

To calculate the concentration limits of flame propagation of mixtures of combustible gases, the Le Chatelier rule is used

(2.1.6)

where
lower (upper) CRC of the flame of the mixture of gases, % vol.;

- lower (upper) limit of flame propagation i-ro combustible gas%, vol.;

- mole fraction i-ro of combustible gas in the mixture.

It should be kept in mind that ∑μ i =1, i.e. the concentration of combustible components of the gas mixture is taken as 100%.

If the concentration limits of flame propagation are known at a temperature T 1, then at a temperature T 2. they are calculated according to the formulas

, (2.1.7)

, (2.1.8)

where
,
- the lower concentration limit of flame propagation, respectively, at temperatures

T 2 . and T 1 ;
and
- the upper concentration limit of flame propagation, respectively, at temperatures T 1 and T 2 ;

T G- combustion temperature of the mixture.

Approximately when determining the LEL of the flame T G take 1550 K, when determining the VKPR of the flame -1100K.

When the gas-air mixture is diluted with inert gases (N 2 , CO 2 H 2 O vapors, etc.), the ignition region narrows: the upper limit decreases, and the lower one increases. The concentration of an inert gas (phlegmatizer), at which the lower and upper limits of flame propagation are closed, is called the minimum phlegmatizing concentration φ f . Oxygen content Such a system is called the minimum explosive oxygen content of MWCS. Some oxygen content below the MVSC is called safe.
.

The calculation of these parameters is carried out according to the formulas

(2.1.9)

(2.1.10)

(2.1.11)

where
- standard heat of formation of fuel, J/mol;

, ,- constants depending on the type of chemical element in the fuel molecule and the type of phlegmatizer, table. 14 applications;

- the number of atoms of the i-th element (structural group) in the fuel molecule.

Example 1. Based on the maximum heat of combustion, determine the lower concentration limit of ignition of butane in air.

Solution. For calculation according to the formula (2.1.1) in table. 15 applications we find the lowest calorific value of a substance 2882.3 kJ / mol. This value must be converted to another dimension - kJ / m 3:

kJ / m 3

Using formula (2.1.1), we determine the lower concentration limit of flame propagation (LCPR)

According to the table 13 applications find that the experimental value
- 1.9%. The relative calculation error, therefore, was

.

Example 2. Determine the concentration limits for the propagation of an ethylene flame in air.

The calculation of the CRC of the flame is carried out according to the approximation formula. Determine the value of the stoichiometric coefficient for oxygen

C 3 H 4 + 3O 2 \u003d 2CO 2 + 2H 2 O

In this way, n = 3, then

Let us determine the relative calculation error. According to the table 13 applications, the experimental limit values ​​are 3.0-32.0:

Consequently, when calculating the LEL for ethylene, the result is overestimated by 8%, and when calculating the LEL, it is underestimated by 40%.

Example 3. Let us determine the concentration limits for the propagation of a flame of saturated methanol vapor in air, if it is known that its temperature limits are 280 - 312 K. Atmospheric pressure is normal.

For calculation by formula (2.1.4), it is necessary to determine the saturated vapor pressure corresponding to the lower (7°C) and upper (39 o C) limits of flame propagation.

According to the Antoine equation (2.1.5), we find the saturated vapor pressure, using the data in Table 7 of the Appendix.

P H \u003d 45.7 mm Hg \u003d 45.7 133.2 \u003d 6092.8 Pa

P H \u003d 250 mm Hg \u003d 250 133.2 \u003d 33300 Pa

According to the formula (2.1.3), we determine the LEL

Example 4. Determine the concentration limits of the flame propagation of a gas mixture consisting of 40% propane, 50% butane and 10% propylene.

To calculate the CRC of the flame of a mixture of gases according to the Le Chatelier rule (2.1.6), it is necessary to determine the CRC of the flame of individual combustible substances, the calculation methods of which are discussed above.

C 3 H 8 -2.1÷9.5%; C 3 H 6 -2.2÷10.3%; C 4 H 10 -1.9÷9.1%

Example 5. What is the minimum amount of diethyl ether, kg, capable of creating an explosive concentration upon evaporation in a container with a volume of 350 m 3.

The concentration will be explosive if φ n =φ pg where ( φ pg- concentration of vapors of combustible substance). Calculation (see examples 1-3 of this section) or according to the table. 5 applications we find the LEL flame of diethyl ether. It is equal to 1.7%.

Let us determine the volume of diethyl ether vapor required to create this concentration in a volume of 350 m 3

m 3

Thus, to create an LEL of diethyl ether with a volume of 350 m 3, it is necessary to introduce 5.95 m 3 of its vapor. Taking into account that 1 kmol (74 kg) of steam, reduced to normal conditions, occupies a volume equal to 22.4 m 1, we find the amount of diethyl ether

kg

Example 6. Determine whether it is possible to form an explosive concentration in a volume of 50 m 3 during the evaporation of 1 kg of hexane, if the ambient temperature is 300 K.

Obviously, the vapor-air mixture will be explosive if φ n ≤φ pg ≤φ v- At 300 K, we find the volume of hexane vapor resulting from the evaporation of 5 kg of a substance, taking into account that when 1 kmole (86 kg) of hexane evaporates at 273 K, the volume of the vapor phase will be 22.4 m 3

m 3

Hexane vapor concentration in room with a volume of 50m 3, therefore, will be equal to

Having determined the concentration limits for the propagation of a hexane flame in air (1.2-7.5%), we establish from tables or calculations that the resulting mixture is explosive.

Example 7. Determine whether an explosive concentration of saturated vapors is formed above the surface of a tank containing 60% diethyl ether (DE) and 40% ethanol (ES) at a temperature of 245 K?

The vapor concentration will be explosive if φ cm n ≤φ cm np ≤φ cm v (φ cm np- concentration of saturated vapors of a mixture of liquids).

Obviously, as a result of different volatility of substances, the composition of the gas phase will differ from the composition of the condensed phase. The content of components in the gas phase according to the known composition of the liquid will be determined according to Raoult's law for ideal solutions of liquids.

1. Determine the molar composition of the liquid phase

,

where
- mole fraction of i-th substance;

- weight fraction of the i-th substance;

- molecular weight of the i-th substance; ( M DE =74, M ES =46)

2. By equation (2.1.5), using the values ​​of Table 12 of the Appendix. We find the pressure of saturated ether and ethyl alcohol at a temperature of 19 ° C (245 K)

R DE\u003d 70.39 mm Hg \u003d 382.6 Pa

R ES\u003d 2.87 mm Hg \u003d 382.6 Pa

3. According to Raoult's law, the partial pressure of saturated vapors of the i-th liquid over the mixture is equal to the product of the saturated vapor pressure over a pure liquid and its mole fraction in the liquid phase, i.e.

R DE(steam) \u003d 9384.4 0.479 \u003d 4495.1 Pa;

R ES(steam)\u003d 382.6 0.521 \u003d 199.3 Pa.

4. Taking the sum of the partial pressures of saturated vapors of diethyl ether and ethyl alcohol equal to 100%, we determine

a) the concentration of vapors in the air

b) molar composition of the gas phase (Raoult-Duartier law)

5. Having determined by calculation or by reference data (Table 16 of the appendix) the CRC of the flame of individual substances (diethyl ether 1.7 ÷ 59%, ethyl alcohol 3.6 ÷ 19%). according to the Le Chagelier rule, we calculate the CRC of the vapor phase flame

6. Comparing the concentration of the vapor-air mixture obtained in paragraph 4a with the concentration limits of flame propagation (1.7-46.1%), we conclude that at 245 K above this liquid phase an explosive concentration of saturated vapors is formed in the air.

According to Table 15 of the application, we find the heat of formation of acetone 248.1·10 3 J/mol. From the chemical formula of acetone (C3H 6 O) it follows that T With = 3, T n = 6, T O = 1. The values ​​of the remaining parameters required for the calculation according to formula (2.8) are selected from Table. 11 for carbon dioxide

Consequently, when the oxygen concentration in a four-component system consisting of acetone, carbon dioxide, nitrogen and oxygen vapors decreases to 8.6%, the mixture becomes explosion-proof. When the oxygen content is equal to 10,7% this mixture will be explosive. According to reference data (handbook "Fire Hazard of Substances and Materials Used in the Chemical Industry". - M, Chemistry, 1979), the MVSC of an acetone-air mixture when diluted with carbon dioxide is 14.9%. Let us determine the relative calculation error

Thus, the results of the calculation of MWCS are underestimated by 28%.

Assignment for independent work

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.

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.

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 ignition area.

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 oxidant content in the system, below which the mixture does not ignite. Evaluation and control of ICC is important for systems operating under vacuum, where it is possible to suck through the leaks of the process equipment of atmospheric air.

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

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See what "NKPR" is in other dictionaries:

NKPR- National Confederation of Industrial Workers Union of Trade Unions Brazil, organization NKPR lower concentration limit of flame propagation Source: http://www.ecopribor.ru/pechat/signal03b.htm ... Dictionary of abbreviations and abbreviations

NKPR- National Confederation of Industrial Workers... Dictionary of abbreviations of the Russian language

LEL (lower concentration limit of flame propagation)- 3.37 NKPR (lower concentration limit of flame propagation): According to GOST 12.1.044. A source …

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 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

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.