Explosion Accident Analysis About Main Return Pipe Of Single Freezing Machine During Hot Ammonia Defrosting Process

Hot ammonia defrosting is currently in general use in cold storage and single freezing machine.Separate the compressor exhaust by oil filter,and then lead hot ammonia gas into the evaporator to melt frost on the outer surface of the evaporator,so as to achieve the aim of defrosting.Hot ammonia defrosting is reasonable and feasible,however,due to the incorrect operation,incomplete penetration of circular welds and low temperature brittleness,pipe explosion accidents are frequently happened,resulting in the leakage of ammonia,further causing great economic loss and harm.Therefore,for the sake of refrigeration enterprises’ stable operation,it has a very important practical significance to study on the cause of hot ammonia defrosting accidents in detail and put forward reasonable prevention measures.In this paper,by researching and analyzing explosion accident of hot ammonia defrosting,the system simulation study on structure form and bearing capacity of main return pipe and head which was chief destroyed was done.In the process of hot ammonia defrosting in ammonia refrigerating system,security assessment of hydraulic hammer on main return pipe and head in working condition of low-temperature was performed.A test was made of low temperature impact toughness of material and weld in main return pipe and head.The main causes and consequences of the accident were concluded.The measures for prevention of the accident were put forward from technology choosing,operation controlling,construction design and so on.The main research work and achievements of this paper are as follows:(1)Through the numerical simulation calculation,the limit load of three head structures of main return pipe were analysed and compared.The results showed that,the limit load of the elliptical head is much larger than that of the flat head,and the limit load of the inner flat head is larger than that of the outer welded flat head;the welding defect between main return pipe and head also has a certain effect on the limit load.It decreased obviously with the increase of the incomplete penetration depths,but the change was not significant with the increase of the incomplete penetration length.(2)Using computational fluid dynamics finite element analysis method to simulate hydraulic hammer and analyzed the influence of residual ammonia,inlet velocity acceleration of hot ammonia,lengths of main return pipe on the maximum pressure of the hydraulic hammer.The maximum pressure increased with the amounts of residual ammonia,showing two times curve proportional relationship.It also increased with inlet velocity acceleration of hot ammonia and the lengths of main return pipe,showing linear relationship.(3)The stress intensity of main return pipe under hydraulic hammer pressure was analyzed.When the lack of penetration depth was 1mm,the maximum stress intensity was located at the head;when the defect depth was 3mm,the weld joint between the pipe and head had the maximum stress intensity.The maximum stress intensity was proportional to the amounts of residual ammonia,inlet velocity acceleration of hot ammonia and lengths of main return pipe.(4)Through the low temperature impact test of 20 typical steel base metal and weld,it was showed that the impact absorbing energy decreased gradually with the decrease of temperature,the impact toughness got worse;for the transition temperature of low temperature brittleness,manual welding was higher than argon arc welding.