Long Term Operation of Safety Valves at Pressures Near the Set Pressure

Safety valves in nuclear power plants provide over pressure protection of pressurized systems. Accordingly, these valves are required to open quickly and stably (i.e., open, relieve pressure and close) during postulated transients to protect the integrity of the protected systems. Typically, postulated transients are classified as fast or slow. Fast transients have high system pressurization rates that proceed very quickly thereby requiring the safety valves to pop open. On the other hand, there are transients that proceed very slowly that are less challenging to the system but may initiate leakage across the installed safety valve seat. There is very limited knowledge on the impact of prolonged operation of safety valves during slow pressurization accident events. The integrity and functionality of these valves during such slow pressurization events are often in question. This paper examines analytically the behavior and the integrity of safety valves during slow pressurization transient events at pressures near the valve set pressure. This paper considers extended periods of valve simmering that may progress to valve cycling (popping fully open) during such events. To validate the analytical performance prediction, steam tests were performed with safety valves which confirmed that these valves can operate extensively under slow pressurization transient events while maintaining their capability to perform their intended design function.