How can fire fighters determine the safe rescue distance in "Firefighting Technology" petrochemical fires?

Diesel, petrol and other petrochemical products generate high heat and radiation when burned. Even if they wear protective clothing and thermal protective clothing , the firefighters’ own safety will still be greatly threatened when extinguishing fuel tank fires. Yuan Jing et al. calculated the safety distance of ground personnel under the condition of no wind during the fire of crude oil storage tanks through the heat radiation empirical model; Zhuang Lei et al. provided the safety distance of ground personnel and the destruction of adjacent tanks during the fire of tanks through empirical models and numerical simulations. Time; Zhang Fang calculated the relationship between the thermal radiation intensity and the secondary burn time when the gasoline tank was burned through an empirical model. However, in actual situations, when using an overhead vehicle to extinguish an operation, the radiant heat received is different due to the different heights. On the basis of previous research, the author constructed a FDS simulation model based on a small-scale oil pool fire combustion test, and verified the parameters of the FDS model using an oil pool fire test. Combined with the specifications, the firefighters used the two types of fire service clothing often worn by firemen. The radiation conditions were analyzed and the safety critical distances of tank fires at different heights near the ground were proposed to provide reference for fire fighting operations.

1 Small-scale oil pool fire test

1.1 Test device

A small-scale model test was used to study the combustion process of the oil pool fire and its effect on the surrounding radiation. The test system is mainly composed of a test system and a fire source. The test system mainly includes thermocouple temperature measurement system, thermal radiation meter, thermal imager, and camera. The diameter of the oil pool used in the fire source is 5.00m, the wall thickness is 5mm, and the depth of the oil pool is 1.50m. In order to prevent the oil pan from melting and destroying, the lower part of the test was set with a water cushion layer, which was 1m deep. The upper test fuel was 100L diesel, and 15L gasoline was used for ignition. The temperature and thermal radiation measurement points are arranged around the fire source. The measurement point arrangement is shown in Figure 1, and the blank test photo is shown in Figure 2.

1.2 Test observations

The test environment temperature is 5°C and there is no wind. The thermocouple temperature measurement system, the thermal imager and the thermal radiation meter data recording sequence were adjusted to the same time, and the pilot data was recorded after ignition of the pilot gasoline. The test combustion process lasted for 276 seconds, about 4.5 minutes, in which the highest temperature occurred at the 81s, and the measuring point 802 was at 901.9°C. Figure 3 shows the characteristic temperature of each temperature measurement section. The measurement points in the oil pool reflect the temperature in the flame zone. Figure 4 shows the heat radiation intensity in the test.

Figure 1 Test device layout

Figure 2 blank test photo

Fig. 3 Trend of characteristic temperature of temperature measuring section

Figure 4 Thermal radiation intensity at 2m from the center of the oil pan at 10.5m height

2 Construction of a small-scale simulation model

2.1 Analog parameter setting

For the oil pool fire model, the common simulation software based on CFD is FLUENT, FDS, etc. In the absence of wind, the results of FDS are in good agreement with the empirical formula, so FDS 5.0 is used to simulate the test.

In order to reduce the impact of environmental factors, the model's calculation scenario is set to a rectangular parallelepiped of 30, 30, and 15 m in length, width, and height. The oil pool is located in the center. The simulated ambient temperature is 5°C and the wind speed is 0m/s. The size of the oil pool is a circular oil pool with a diameter of 5m , which is the same as the test. In order to compare temperature and thermal radiation, data measurement points were set in the simulation model at the same position as the test.

2.2 Analysis of Simulation Results

Fig. 5 is a comparison chart of the average temperature near the peak temperature of the oil pool in the simulation and test points.

Figure 5 Comparison of Peak Temperatures at Test Points in Tests and Simulations

It can be seen that, except for the measurement point 706, the simulation values ​​of the remaining 92% measurement points and the test values ​​have a good degree of fitting, and the maximum error of the measurement points 802 and 803 that directly reflect the flame temperature is only 16.3%. This shows that the simulated temperature field can basically reflect the field conditions of the test.

The time interval of ±30 s at the peak temperature of each measurement point was selected to compare the trend of temperature field variation between the simulation and the oil pool fire test. Select 802, 803 measuring points that can represent the flame temperature of the oil pool and compare it with the simulated temperature change trend, as shown in Figure 6. Relatively large fluctuations in the measured value of the simulated data relative to the test. Among them, the measuring point 803 is located in the center of the flame zone, below the tank wall, is less affected by the air flow, better reflects the flame temperature, and the fitting degree is relatively higher; and the measuring point 802 is disturbed by the on-site breeze, the measuring point temperature and simulation There is a difference, but the trend of the temperature field basically conforms to the actual measurement trend in the test.

Figure 6 Comparison of temperature changes at measuring points 802 and 803

The heat radiation intensity is another parameter that describes the development of fire. The heat radiation meter is arranged at 10.5m from the center of the oil pool in the test. Figure 7 shows the comparison of simulated and experimental thermal radiation data near the peak temperature of the oil pool fire.

The comparison period is within 30 s of the maximum time of the test heat radiation reading. Since the recording interval of the test heat radiation meter reading is 3 s, and the recording interval during simulation is only 0.2 s, the simulation records the change of the heat radiation value in more detail, so the simulated value fluctuates greatly. However, the overall change trend of the value of thermal radiation in the simulation is similar to the change trend of the experimental value, indicating that this model can be used to better describe the thermal radiation field in the experiment.

Figure 7 Comparison of thermal radiation data

3 Research on fire extinguishing distance of oil storage tanks

3.1 Full-scale simulation test

When an oil pool fire occurs, the greatest threat to the firefighters' personal safety is the heat radiation intensity of the environment because the intensity of heat radiation that people can bear is low. However, the test of large-scale tank fires is difficult to carry out. Based on the results of the above-mentioned simulation analysis, actual fire extinguishing operations are guided by analyzing the distribution of heat radiation fields obtained by FDS simulation of real-size tank fires.

According to the literature, the maximum diameter of a successful oil tank for full surface fire suppression is 35m. According to the common tank size, a full-scale test model was built with an inner floating roof tank with an inner diameter of 40.50m and a tank height of 15.85m. The model was set to a windless environment with a temperature of 20°C. The model of the tank was simplified to a diameter of 40m and a height of 15m. The liquid in the tank was diesel and the top of the tank caught fire. The thermal radiation data of different heights in the actual size tank fire model obtained by FDS simulation is shown in FIG. 8 .

Figure 8 Thermal radiation values ​​of a full-scale tank fire model

According to the full-scale simulation results, below the height of the tank, the radiation is not the closer the distance, but with the increase of the distance, the thermal radiation gradually reaches its maximum, and then it gradually decays. This is because in the vicinity of the tank, the flame cannot irradiate the lower area of ​​the tank due to the shielding of the tank wall. However, as the distance increases, the shielding effect gradually disappears, the ground reflection effect increases, and the heat radiation and gas temperature increase. The rate of increase in thermal radiation intensity with distance at a near-atmospheric height is approximately 0.163 (kW/m 2 )/m. The 40-70m tank shielding weakened and gradually reached a maximum value. This area is called a dangerous area; then as the distance increases, the gas temperature decreases and the ground reflection decreases, thermal radiation decays, and the near-surface height thermal wave 0, which can be As a fire extinguishing area. From the perspective of thermal radiation of different heights, in the danger zone, the lower the height, the lower the thermal radiation; but in the fire extinguishing area, the higher the height is, the smaller the radiation is, and the faster the rate of decline of thermal radiation is, indicating that there is a large amount of radiation at this time. In part due to ground reflections, using a climbing car to lift firefighters high will help reduce the radiation exposure.

3.2 Insulation Clothing < Endure Limit Calculation

In the fight against fire, the protective clothing for firefighters can be broadly divided into two types, thermal protective clothing and fire protective clothing. The protective performance of fire protective clothing against heat radiation is relatively poor.

For thermal protective clothing, Article 6.3 of GA 634-2006 “Insulation Protective Clothing for Fire Fighters” regulates the thermal protection performance of the insulation worn by firefighters and requires the overall thermal protection value (TPP). Should not be less than 35.0. The thermal protection performance value given in the specification is calculated as TPP=F×T (F is the heat flow rate to the garment surface, which is specified in GA 634-2006 as 2cal/(s•cm2); T is for wearing human skin. Level 2 burn time, s). From this calculation, the overall thermal protection performance of the fire insulation clothing should be greater than or equal to 35.0cal/cm2, about 1465(kW•s)/m2. Taking the time of combat rotation in the fire field as 240s, the heat radiation value that the firefighter can bear when wearing a thermal insulation suit with a TPP of 35 is 1465/240≈6.10kW/m2. In this heat radiation intensity exposure 240s, human skin reaches grade 2 burns, threatening firefighters personal safety, 6.10kW/m2 can be used as the maximum value of heat radiation can withstand.

For fire protective clothing, the heat protection capability TPP value in GA 10-2002 "Firefighters Protective Clothing for Fire Protection" shall not be less than 28.0. With the insulation clothing, the maximum heat radiation value that can be endured when wearing protective clothing is 4.88 kW/m2.

Based on the above analysis, it can be concluded that: If the rotating combat duration is 240s, when wearing protective clothing and the ambient heat radiation intensity is greater than 4.88kW/m2, or when wearing thermal protective clothing and the heat radiation intensity is greater than 6.10kW/m2 Firefighters have personal danger. The distance from the tank's edge is the critical safety distance.

3.3 Analysis of safety rescue distance

Based on the simulation results and the resistance of the combat suits, it can be seen that when the height is 1.7m, the thermal radiation value is greater than 4.88kW/m2 from the tank wall 68m, ie, the safety distance is 68m. According to the pot wall point source calculation formula, the thermal radiation intensity at the height of 1.7m at 68m is 4.30kW/m2, and the relative error with the FDS model data is 11.8%. At the same time, this value is also similar to the fire fighting fire fighting position (2 D from the tank wall). Therefore, when a firefighter is less than 68m away from the tank and the combat time exceeds 240s, there is a high risk of burns due to heat radiation from the combustion of diesel fuel in the tank and even the risk of death.

Combined with the tolerance of the insulation suit, the thermal radiation intensity in the interval is within the range of 0 to 100 m from the tank wall because the thermal radiation intensity is approximately a single peak curve and the peak value is less than 6.10 kW/m2. The maximum value of heat radiation that can be withstood when wearing thermal protective clothing; if the ambient heat radiation intensity at 68m away from the tank wall is 4.88kW/m2 as the heat radiation intensity of the heat insulation clothes, the combat rotation time can be increased. To 300.2s. In this simulation, 68m is considered to be the critical safety distance when the fire is extinguished on the ground.

When the ground level is at a safe distance, it is difficult to inject from the ground to the combustion surface without the foam losing its buoyancy. According to the simulation results, the rate of reduction of thermal radiation intensity at a higher point is faster and the critical distance to safety is closer, and it can be considered that the fire extinguishing can be performed by using a ladder truck. Due to the destruction of the heat radiation intensity of the equipment is 25kW/m2, it is higher than the maximum heat radiation intensity that the heat insulation clothing can withstand. Therefore, if the ambient heat radiation intensity is less than 6.10 kW/m2 at the height of the ladder, the safety of personnel equipment can be guaranteed. When wearing thermal protective clothing, if you use a ladder to lift the height up to 10m and the safety distance is less than 68m when the ground is high, you can find it by looking at the 10m height curve in Figure 8. When the ambient heat radiation intensity reached 4.88kW/m2 , it was shortened to 14m from the oil tank wall 54m. If the standard of 6.10 kW/m2 when wearing thermal protective clothing is used as the maximum heat radiation intensity that can be tolerated, the safety distance can be shortened to 50 m. And through the use of multiple wheels, the fire extinguishing agent can be sprayed continuously and the fire extinguishing efficiency can be improved.

4 Conclusion

(1) The numerical model established by FDS 5.0 can simulate the small-scale test well. 92% of the 12 test points in the test set can be fitted well with the test data and the simulated heat radiation. The trend and value of the data can better reflect the development of diesel pool fire in the test.

(2) Combining with the current fire service industry standard calculations, two types of heat radiation criteria for safety distances of fire fighting clothing are proposed. When the duration of the rotating combat is 240s, when firefighters wear protective clothing and the ambient heat radiation intensity is greater than 4.88kW/m2, or when they wear thermal protective clothing and the heat radiation intensity is greater than 6.10kW/m2, the firefighters will have personal injuries. Danger.

(3) For a diesel tank with a diameter of 40m and a height of 15m, the fire extinguisher protective clothing has a safety distance of 68m when the fire is extinguished at the ground level, and the resistance time for wearing fire extinguishers at this distance is 300s; if a ladder truck is used, When the firefighters extinguish fire at a height of 10m while wearing fire protective clothing, the critical distance can be reduced to 50m.

Because the simulation results under windless conditions are presented, firefighters in fire fighting in windy conditions generally choose to rescue in the upwind direction, which is conducive to the reduction of temperature and heat radiation, and the safety critical distance may be further reduced.

This article comes from "Fire Command and Rescue" and was reorganized and edited by China Rescue Equipment Network.