Experimental Study of LNG Transportation Tank Leakage, Pool Fire Suppression

Transcription

1 Experimental Study of LNG Transportation Tank Leakage, Pool Fire Suppression Liu X. 1, *, Ji C. 1, Jiang S. 2 1 Tianjin Fire Research Institute of MPS, Key Laboratory of Building Fire Protection Engineering and Technology, Weijin, Tianjin, China 2 Changzhou University, Changzhou, Jiangsu, China *Corresponding author liuxuanya@tfri.com.cn ABSTRACT As an efficient clean energy, LNG (Liquefied Natural Gas) has been widely applied in many fields. Large-scale LNG vehicle transportation is becoming more and more frequent, and due to accidents involving LNG transportation tanks, tank leakage, pool fire and explosion accidents are also increasing. Large-scale LNG tanker leaks, as well as pool fire and explosion experiments are carried out in a 5 m 3 m 1.5 m deep experimental pool. LNG tanker leakage, LNG discharge and gasification due to different conditions of loading pressure, and LNG pool combustion, gas cloud explosion accident evolution and chain disaster accident probability and its influencing factors are analysed in the paper. The fire fighting suppression effectiveness are also analysed by using the powder gun and high foam in the large scale LNG pool fire. On the basis of the experiments, the LNG leakage, explosion and pool fire disasters accident evolution mechanism, including fire emergency disposal tactics and LNG tank fire effective suppression strategy are proposed. The results of the experiments are also of great significance and value to improve the efficiency of LNG tanker accident emergency disposal. KEYWORDS: LNG transportation tank, pool fire, real scale experiments, suppression effectiveness. INTRODUCTION With the implementation of a national clean energy strategy in China, large-scale liquefied natural gas (LNG) storage, transportation and application is more and more extensive. Large LNG transportation vehicles and fuel cars have also been gradually popularized. LNG tanker trucks are the road transport vehicle, which can have long, complicated road conditions and operation characteristics in China. During transportation, if LNG tanker traffic accidents result in LNG leakage, it may produce pool fire. The resulting flame radiation can increase the LNG tanker pressure and produce a which is easy to produce physical explosion. The LNG tank truck fire, explosion disasters and accidents caused by the disposal of leakage are becoming more common [1]. In 2012, there have been more than 200 cases of LNG leakage, fire and explosion accidents caused by the traffic accidents. About 82.8% of the tanker transportation of dangerous goods accident was caused by traffic accidents [2]. Relevant domestic research institutions have carried out large amounts of research to analyze factors that influence LNG tanker spills, fires, and explosions. They have recommended technical measures to prevent these accidents. In Europe, the United States and other developed countries where LNG has been in used much earlier, a large number of numerical simulation and experimental research work were carried out that focused on the formation of the liquid pool, gasification, and the diffusion process of LNG leakage under different conditions [3]. Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8), pp Edited by Chao J., Liu N. A., Molkov V., Sunderland P., Tamanini F. and Torero J. Published by USTC Press ISBN: DOI: /c.sklfs.8thISFEH

2 Part II Fire Current research are mainly focused on LNG leakage on water surfaces, gasification and diffusion, fire accidents of LNG ship transportation and LNG pool fire behavior. These studies have developed the mechanism of LNG fire prevention and security concept of LNG application. While there are strategies to prevent accidents, little research has been conducted in China. In order to improve the LNG tank truck accidents disposal efficiency and technology capability, LNG tank truck leakage flow diffusion, gasification rate influencing factors and LNG cryogenic pool fire combustion kinetics are analyzed using the simulation calculation. Full-scale experiments are also conducted that provide details of the LNG tank truck accidents disposal technical strategy and methods [4]. FIRE RISK ANALYSIS OF LNG TANK TRUCK ACCIDENTS A small amount of LNG tank truck leakage can easily be gasified and form into gas cloud in local area. As the amount of LNG increases, the external environment cannot provide enough heat to gasify the LNG and a liquid pool can form. During the above process, an ignition source can ignite a gaseous explosion and a pool fire, which is controlled with high expansion foam or a dry powder extinguishing agent. However, the inhibition efficiency and disposal strategy have not been verified effectively through experiments and it is not clear which disposal scheme for an LNG spill is the best. It is also of importance to carry out an experimental study on the inhibition effect of an LNG pool fire. LNG tank truck leakage risk points are mainly concentrated in the transportation and gasification process where the loading and unloading valve, connecting line, loading and unloading pipe, valve, emergency cut-off valve, pressure gauge and other parts of LNG transportation tank lorry can cause leakage due to accidents or aging in the process of filling, drainage, storage. According to LNG tank truck leakage accident reasons and the possibilities of fire and explosion accident consequence, a model of BowTie to analyze the detail prevention measures is put forward as shown in Fig. 1. Figure 1. BowTie model of prevention measures for LNG leakage accidents. The main prevention measures to prevent the onboard LNG leakage include: LNG storage equipment and pipeline safety protection, leak emergency safety device, onboard LNG equipment condition monitoring and accident warning device, driving operators operating safety, driver safety condition 221

3 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) monitoring and so on. The main control measures to prevent fire or explosion accident after onboard LNG leakage accident are including: leakage diffusion ventilation, enclosed space, open device area, the use of dry powder or high foam fire extinguishing and explosion suppression, and adopts the security isolation protection, leakage protection vehicle equipment, accident area to prevent fire and isolation protection measures, etc. From the risk model of the LNG fire accidents prevention measures for LNG transportation tank lorry, it is important to improve the tank truck intrinsic safety performance, establish efficient measures of early warning of prevention before the accident and emergency safety treatments according to the accidents. In order to dispose the accidents efficiently, it is important to understand the accidents consquence severity. According to the LNG tank truck accidents scene, the accident consequences influence of LNG pool fire, evolution process of pool fire accident and LNG diffusion distribution are analyzed by using CFD simulation software KFX in numerical simulation [5, 6]. KFX software of Computit company based on CFD technology can simulate turbulent flow and fire with good visibility and accuracy. A 60 m 2 pool fire is simulated for evaluating LNG tank leakage, flame radiation intensity and scope.the wind speed set to 5 m/s. Fig. 2 is the result of three-dimensional image of LNG gasification diffusion effects of LNG tanker leakage forming nearly 60 m 2 liquid pool under 5 m/s wind speed conditions. Respectively the pool fire radiation influence scope and LNG pool fire flame plume three-dimensional image of LNG liquid pool fire of 60 m 2 is shown as Fig. 3 [7]. Figure 2. Three-dimensional image of LNG gasification diffusion effects. Figure 3. LNG pool fire heat radiation influence scope. It can be seen from the diagram that, under the effect of 5 m/s wind speed, the leakage of LNG gasification liquid pool of flammable gas in the direction of the wind can be spread to the scope of 222

4 Part II Fire the nearly 100 m level of scope. Within the scope of 5 m the area from the leaked LNG liquid pool fire, the maximum radiation heat can reach to kw/m 2. While the distance within the scope of 30 m region from the pool fire, the maximum radiation heat can reach up to kw/m 2. As is shown in Fig. 4, the 60 m 2 pool fire flame height can be up to 22 m around, even under the effect of 5 m/s wind speed. According to CFD numerical simulation analysis results of LNG leakage diffusion and pool fire combustion characteristic, the LNG tank truck leakage diffusion and pool fire radiation influence scopes are significant. And it is easy to cause peripheral and adjacent vehicles, building fire and explosion chain disaster. Figure 4. LNG pool fire flame height. EXPERIMENT SCHEME AND EQUIPMENT The large scale LNG tanker leaks, as well as the pool fire and explosion simulation experiments are carried out in the 5 m 3 m 1.5 m deep experimental pool. Ignition rod and high foam generator is put at the side of concrete liquid pool. Powder monitor extinguishing systems set 20 m from the liquid pool. LNG tanker stay 50 m away connected with liquid pool by a pipeline. Test device arranged on the peripheral and internal stent in the liquid pool with the experimenter and acquisition device terminal settings in the position of the 35 m away. The layout and details as shown in Fig. 5. For deep understanding on a large scale LNG pool fires burning and explosion characteristics, to explore the technical measures of LNG tank fire control saves, further analysis comparing different inhibition technology for LNG pool fire, rapid gasification deflagration fire suppression and fire extinguishing efficiency, inspection powder, high foam control large-scale LNG pool fire and inhibition efficiency of the gas cloud deflagration, for rational selection of disposal of such fire and explosion accident of the fire fighting technology and method, improve the fire brigade combat ability, and provide a scientific basis for the revision of the relevant national norms. Experiment based on the real scale of fire, explosion accident simulation, the main simulation scenario is: LNG transportation by accident, resulting in a large number of leakage, leakage of LNG flow to the low-lying place to assemble a liquid pool, gasification LNG gas cloud deflagration fire formation as well as the large pool fire. Experiment chooses high foam extinguishing system, portable powder extinguishing system for fire suppression and fire extinguishing experiment. Specific research contents are as follows: (1) LNG pool fire and the characteristics of gas cloud deflagration fire research; (2) the LNG massive pool fire disposal technology and personnel safety protection methods. (3) LNG pool fire suppression technology and its inhibition efficiency analysis. 223

5 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) Figure 5. Experimental site. 1.4 tonnes of LNG is used and suppression methods included high expansion foam, dry powder extinguishing agent, and water mist isolation. Experimental conditions are shown in Table 1. Table 1. Experiment condition Suppression method Parameter of apparatus Supply intensity High expansion foam Foam expansion: L/s Dry powder Powder fire monitor: PF20 20 kg/s Water mist Sprayer: ZSTW L/(min m 2 ) Test equipment include CCD cameras, high-speed photography, infrared thermal imaging, thermocouple, and heat flow meter. Radiation heat flux often has severe consequences and is, therefore, necessary to measure for understanding the hazard of an LNG pool fire. Two kinds of heat flow meter (0-50 and kw/m 2 ) are for used for testing. As is shown in Fig. 2, four radiation heat flow meters are installed at a height of 1.5 m and 2, 4, 6, and 10 m from the concrete tank. Figure 6. Heat flow meter set locations. 224

6 Part II Fire According to the LNG combustion situation, 0.5 mm diameter K type sheathed thermocouples are used because it has the minimum interference. PT100 type thermal resistance is used to measure low temperatures. There are a total of 22 thermocouples on the scaffold, which is shown in Fig. 7. Figure 7. Thermocouple set. EXPERIMENTAL RESULTS After successfully forming stable pool fires of LNG, suppression measure including dry powder extinguishing agent, high expansion foam and water mist is to carry out step by step. Fire-fighting capability is evaluated with comparing the flame height, shape, change of the parameters such as temperature, radiation in the process of fire. Baseline test results Pool fire is formed after LNG importing and ignition. Temperature, radiation intensity and flame height are collected for comparing the suppression effect. Fig. 8 shows the flame height in the stable combustion.the max temperatures at the same time are recorded and equation of the relation between the flame temperature and the highest position is obtained by polynomial fitting. As is shown in Eq. (1), when H is about 1.8 m, the flame temperature increases to K: T= H H H 3, (1) where H is height and T is temperature. Figure 8. Height of flames of LNG pool fire. 225

7 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) Effect of dry powder on LNG pool fire As shown in the photograph and infrared thermal images in Fig. 9, the flame is suppressed, package and cut off in an instant of powder spraying, but the internal high temperature point has not disappeared. Liquid pool rapidly burning again when dry powder supply is stopped. Figure 9. Photograph and heat maps of dry powder. Fig. 10 shows the temperature with the different height in the center of liquid pool, which the max temperature is o C in the height of 2.65 m. Temperature, flame height and high temperature range reduced when powder begin spraying. Figure 10. Thermocouple data in the pool. Temperatures on the wall of pool rise again even when the dry powder is still spraying because it is difficult for the powder to research the base of the flame. 226

8 Part II Fire Effect of high expansion foam on LNG pool fire The flame becomes violent when high expansion foam is sprayed because the water strengthens LNG vaporization, as can be seen in Fig. 11. There are higher flames in the early stages. Figure 11. Figure and heat maps of high expansion foam. We measure and count the height of flames and compared with the stable combustion. Fig. 12 shows the flame height when high expansion foam start to spray. Although there are higher flames in the early stages of high expansion spraying, the average flame height reduced greatly from 11.6 to 4.7 m. Figure 12. Height of flames of LNG pool fire. At the same time, the intensity of radiation reduced. In Fig. 13, the area 10 m away from the fire is safe under the high foam cover, but high expansion foam is still unable to put out the LNG pool fire. 227

9 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) Separation effect of water mist on LNG pool fire As is known to all, water will accelerate LNG gasification and make the fire uncontrollable, which makes water an ineffective method to suppress LNG pool fires. However, water mist can have very good protection effect in LNG pool fire, which is also an LNG tank protection measure. Radiant heat measurement used to verify the protection of the water mist. Fig. 14 shows thermal radiation dropped from 7-8 kw/m 2 to 2 kw/m 2, which is under the threshold of thermal radiation tolerance by humans. It is shown that water mist can effectively reduce the pool fire burning thermal impact on the surrounding environment and can effectively control the LNG tanker pool fire with powder and high foam application. Figure 13. Radiation heat flow with the change of distance. Figure 14. Radiation heat flow with water mist separation. CONCLUSION In the paper, the LNG tank truck leakage risk and evolution mechanism are analyzed. The accidents disposal strategies are also put forward. According to accident characteristics the efficient disposal measures to control the hazard is very important to prevent the accident develop further. The experiments mainly study the suppression method efficiency of LNG pool fire. Dry powder extinguish system, high foam extinguish system and water mist system are compared and analysed by recording 228

10 Part II Fire different phenomena, infrared thermal images, temperature, radiant heat. The main research conclusion is as follows: (1) It plays an important role to control the disasters that prevent leakage of LNG tank trucks, and prevent pool fire heat radiation effect on the tank trucks. (2) Flame is suppressed, package and cut off in an instant of powder spraying, but the internal high temperature point has not disappeared. Liquid pool rapidly burning again when dry powder supply is stopped. But temperature, flame height and high temperature range reduced when powder begin spraying. (3) There are higher flames in the early stages of high expansion foam spraying, but the average flame height reduced greatly from 11.6 to 4.7 m. There has to be a balance between bubbles producing and disappearing. The intensity of radiation reduced when high expansion foam covering the LNG pool fire but the fire was not extinguished. (4) LNG pool fire cannot use water to suppress, but the water mist can have very good protection effect. ACKNOWLEDGEMENT This paper is supported by Science and Technology Strengthen Police Special Funds of MPS (2014GABJC047). REFERENCES 1. Brezoniek, M. MVE Seeks to Make Economic Case for LNG in Heavy Duty Trucks: New Tanks, Fuel Stations Aim to Make Fuel Transparent to Diesel, Diesel Progress Engines and Drives, 62(2): 48-51, Burger, U., and Owren, G. Development Potentials for Small Mobile Storage Tanks with Vacuum Powder Insulations, International Journal of Hydrogen Energy, 23(4): , Ponchaut, N. F., Kytomaa, H. K., Morrison, D. R., and Chernovsky, M. K. Modeling the Vapor Source Term Associated with the Spill of LNG into a Sump or Impoundment Area, Journal of Loss Prevention in the Process Industries, 24(6): , Ji, C., Liu, X. Y., Xu, X. Y., Yu, N. H., and Zhu, H. Y. A Review of LNG Applied and Security Researches, Advanced Materials Research, : , Kim, B. K., Ng, D., Mentzer, R. A., and Mannan, M. S. Key Parametric Analysis on Designing an Effective Forced Mitigation System for LNG Spill Emergency, Journal of Loss Prevention in the Process Industries, 26(6): , Gavelli, F., Bullister, E., and Kytomaa, H. Application of CFD (Fluent) to LNG Spills into Geometrically Complex Environments, Journal of Hazardous Materials, 159(1): , Jujuly, M. M., Rahman, A., Ahmed, S., and Khan, F. LNG Pool Fire Simulation for Domino Effect Analysis, Reliability Engineering and System Safety, 143: 19-29,