ANALYSIS OF CRACK AND RESIDUAL STRESS OF LIQUEFIED PETROLEUM GAS SPHERICAL TANK

There were a large number of cracks found in liquefied petroleum gas (LPG) spherical tank after servicing for four years,the wall thickness of the tank is 42mm,and the volume is 1 500m3. Besides,cracks were repeatly found during local patch welding in the process of repairing,finally recovering the spherical tank by re-welding and heat treatment to the equatorial welded zone with half year. Residual stress was measured roundly by X-ray. The results show that the residual stress is as high as 0.72σs at the prior welded equatorial zone. It indicates that the result of heat treatment during producing is not very well. After local repairing and heat treatment,the residual stress reaches or approaches the yield stress. So the local heat treatment didn′t make the potential result. Taking re-welding and heat treatment to the equatorial zone,and testing the same place for tow times,the residual stress reduce greatly,the average reduce reach about 50%,so the result of final heat treatment is better.