Study On Accident Risk And Safety Distance Of Tank Farm Caused By Typical Natural Disasters

Major natural disasters will bring serious consequences to chemical storage tanks,and the shock wave,thermal radiation and fragments generated by the tank explosion may cause secondary accidents.Therefore,the study on accident risk and safety distance of tank farm caused by typical natural disasters is particularly important.Combined with the corresponding vulnerability models,event trees were established,the failure probability model of storage tanks under lightning,flood and earthquake was established,and the quantitative risk assessment system was further constructed.In the analysis of the failure of storage tanks caused by lightning,the different failure probabilities of hitting floating roof and tank body were considered respectively under the same event tree,and two types of storage tanks were considered,i.e.external floating roof tank and fixed roof tank.Through case analysis and calculation,it was proved that the quantitative risk assessment system was feasible and practical in engineering application.The results showed that the failure probability of external floating roof crude oil tank,fixed roof diesel tank,fixed roof LNG tank,fixed roof LPG tank and fixed roof liquid oxygen tank caused by lightning was 3.80×10-3,2.85×10-4,0.88×10-4,1.77×10-4 and 5.09×10-4,the failure probability of storage tank caused by earthquake was 8.50×10-3.Compared with the fixed roof tank,the external floating roof tank was more likely to fail under the action of lightning.The failure probability of atmospheric tank with high liquid level was small under the action of flood.The individual risks of external floating roof crude oil storage tanks under lightning and fixed roof diesel storage tanks under earthquake both reached the standard value of level two,and part of their social risks values were in the "unacceptable area",lightning and earthquake might cause domino effect in the tank farm.The hazard of the accident consequence was studied:shock wave overpressure,thermal radiation,explosion fragments.The LPG tank was selected as the research object,which is damaged by natural disasters,resulting in material leakage,the influence distance between overpressure shock wave and thermal radiation caused by different explosion accidents was calculated by accident consequence model.The results showed that the explosion energy produced by pressure vessel explosion was smaller,and the impact range of shock wave was the smallest,while the impact range of BLEVE was the largest due to the thermal radiation of fireball and overpressure shock wave,which was 205.30m from the explosion center.The fragments were divided into the primary fragments of the explosion tank itself and the tank accessories,and the secondary fragments formed by the surrounding facilities under the explosion load.The probability distribution models of the number,mass and velocity of the primary and secondary fragments were established,and the number of the primary fragments of the tank explosion obeys the discrete exponential distribution in the statistical interval[1,15].The finite element simulation software was used to simulate the process of the fragment impacting the vertical tank wall and the spherical tank pillar.Through calculation,it was found that the velocity of the primary fragments was basically distributed in the range of 90mm/ms～150mm/ms,while the velocity of the secondary fragments was in the range of 5mm/ms～20mm/ms and the tank wall was not affected under this speed,however,the velocity attenuation was obvious in high-speed impact,and the energy required to penetrate the tank wall was larger.The results showed that the diameter of the hole increased only 5%when the sphere accessory fragment impact velocity increased by 60%.The higher the impact velocity of the tank body fragment,the smaller the diameter of the hole.There was no instability and collapse phenomenon of different size spherical tank pillars under the impact of small explosion fragment.After the risk analysis of the tank farm,in the aspect of safety distance design,the storage tank was transfered from the ground to the underground.Different safety distance calculation methods were adopted.The results showed that:the method of reducing the diameter of the connecting hole with the ground could be used to reduce the diffusion range of the gas when the underground storage tank leaked.In this case,the initial kinetic energy of the destructive fragments generated by the explosion of the storage tank did not meet the kinetic energy consumed by breaking through the 30cm reinforced concrete slab,so it was considered that the roof on the top of the underground space could prevent the fragments from popping out.Because the fire distance was taken as the standard value,it was found that the impact range of thermal radiation of underground tank explosion was far lower than the standard value.when the overpressure shock wave and thermal radiation were calculated together,the safety distance of underground storage tank was close to the fire distance specified in the existing technical specifications.When LNG storage tank was set underground,which constituted a major hazard source,the social risk was "risk reduction area as far as possible",that is,the storage tank could be ensured in a safe state through corresponding monitoring and scientific methods.