Crack Failure Analysis And Application Of Pipings For Liquid Petroleum Gas Storage And Filling Station

The storage and filling station of liquid petroleum gas (LPG) is used in storage and filling LPG into mobile pressure vessel such as cylinder and tank. Since the pipeline is complex, vibrating will arise when LPG pump and compressor is getting started. Since the intermittent operation of the filling process, the pipe is under the fatigue load, which is likely to induce cracking failure and cause the leakage of LPG. and thus impedes the normal operation of storage and filling station. The cracking of pipe tee is investigated in this paper to give certain guiding rules to the design, installation, operation and management of storage and filling station.The cracked pipe is located in the welded tee of same diameter on the outlet position of pump. The cracking is vertical to the weld joint and is located in the lowest weld joint of tee. Based on the structure of pipe, the location of pipe tee and the operational performance of the storage and filling station, the cause of cracked pipe is investigated and studied as follows:(1) The through-wall crack of tee is macroscopically and microscopically inspected. The chemical constituents and microstructure of material is also studied. The chemical constituents of the pipe are qualified to the standard requirement. However, the grain in the heat affected zone of weld is coarser than base metal, and the needle widmanstatten structure appears. In the view of macroscopic feature of cracking fractograph, the fracture is pyramid suppression and shows fatigue stripe, which shows a typical fatigue crack. (2)Finite Element Method is used in the analyzing of modal and static of the pipe. The natural frequency and vibration mode of pipe is obtained. The lowest natural frequency of pipe (13.8HZ) is close to the natural frequency of pump (13HZ), which may cause resonance, especially in the position of pipe tee, where the vibration effect is comparatively obvious.(3) The stress of tee is studied by the finite element analytical software ANSYS by building the model interaction of internal pressure and the vibration. The stress concentration in the welding of tee is serious, and the alternating stress which coursed by vibration reached 423MPa, which caused the fatigue life is only 25,000 times.Some optimal modification is put forward in this paper to enhance the pipe service life. Firstly, the section sizes of the main pipe should change to decrease the stress concentration in the joint of the tee. Secondly, the alternating stress intensity should be reduced to lower the fatigue limit intensity of pipe by installing brackets at both sides of the tee, which limits the amplitude and reduces the stress, thus to meet the requirement of infinite fatigue life theoretically. At the same time, the natural frequency of pipe will increase from 13.8HZ to 15.7HZ, which avoids the natural frequency of pump and reduces the risk of resonance probability. The pipe of pump does not show leakage under the 6 months of running after transformation, which indicates the optimal modification is effective.This paper suggests that the holistic tee is used, for the crack in weld tee and the effect of welding is occurred, such as welding residual stress, metallurgical structure and welding defect.