Cause of Fracture on EH Oil Pipe of High Pressure Steam Regulating Valve in a Power Plant

The EH oil (fire resistant oil) pipe of the high pressure steam regulating valve in a power plant fractured and caused unplanned downtime of the unit. The causes of fracture on EH oil pipe were analyzed by means of macro observation, metallographic examination and microhardness test. The results show that the loose connection terminal of No. 4 high pressure steam regulating valve VP (valve position) card caused the fluctuation of high pressure steam regulating valve, causing the vibration of EH oil pipe branch pipe, and forming large alternating stress at the stress concentration area of the fillet weld between EH oil pipe tee and branch pipe. The cyclic alternating stress caused the transformation of normal austenite structure into deformed martensite, which led to the increase of hardness and brittleness. At the same time, the existence of design defects of tee and grinding linear defects made EH oil pipe brittle fracture finally.