Installed in the pipe-rise buildings, fire water supply systems and other water supply systems, water supply pipe when the pressure exceeds the set pressure relief valve, pressure relief valves automatically open fast relief to protect the safety of the pipeline, but also for holding pressure valve to protect the main valve upstream water pressure. Can accurately maintain constant pressure on the security settings, once overpressure relief valve quickly open and timely relief. Main closing smooth and reliable, eliminating pressure aftermath.