Diaphragm type pressure relieving/sustaining valves is installed in feeding water system's pipeline in high building,fire protection and other.When the line pressure is bey ond the valves's setting pressure,the valves automatically opens to relieve pressure to ensure the pipeline is safe,and at the same time it can be used as pressure sustaining valves to ensure the upstream pressure is enough.The valves can provide and unchanged setting pressure and once the pressure is over,the valves opens quickly andrelieve the over pressure.The valves can close stably and smoothly and prevent the pressure repercussions.