Failure analysis of pressure guiding pipe fracture of BOG compressor in a LNG terminal

Objective The aim is to identify the causes of the fracture in the pressure guiding pipe of the boil-off gas (BOG) compressor, proposing recommendations for preventing such failure issues, and avoiding the occurrence of similar accidents. Methods Conventional physical and chemical property tests are conducted to examine the performance of the fractured failure samples of pressure guiding pipe of BOG compressor in a LNG terminal. Macro and micro morphologies of the fracture are analyzed by using macro observation, optical microscopy and scanning electron microscopy. Results The chemical composition and hardness of the pipe and sleeve meet the standard requirements, the microstructure have not significant abnormal. The circumferential fillet weld exhibites quality defects such as shrinkage porosity at the arc termination point, undercutting, weld reinforcement and burn-through. Combining with characteristics of the macro and micro fracture morphology, the fracture conforms characteristics of fatigue fracture, which are mainly divided into crack source, the slow propagation area of fatigue crack and the instantaneous fracture area. The crack sources are located at the weld toe of the fillet weld and the burn-through defect. Conclusion The causes of the fracture failure are that the foundation vibration caused by the compressor operation leads to pipe vibration. Additionally, welding defects such as burn-through and undercutting exists near the circumferential fillet weld. The edge of the burn-through defect forms crack sources due to stress concentration. Under the combining action of stress concentration and vibration, the cracks propagate until complete fracture occurrs.