Analysis on Fracture Reason of Pneumatic Actuator Bolt in Steam Trap Valve of High Pressure Heating System

In the process of shutdown maintenance of a nuclear power unit, it was found that five of six pneumatic actuator bolts in steam trap valve of high pressure heating system had been breakdown. A comprehensive analysis which was based on chemical composition analysis, mechanical properties testing, metallographic examination and microstructure observation, as well as actual operating information, was made on the breakdown of the pneumatic actuator bolts. The results showed that working pressure exceeding nominal pressure in value cylinder induced that the bolts were in long-term tension-tension alternating stresses state, causing the bolts fatigue rupture. In the meanwhile, several measures for the failure control were proposed.