A pressure reducing and sustaining valve maintains constant set pressure downstream and a minimum set pressure upstream of the valve. The valve is controlled by two pilots, one reducing, the other sustaining, which can be set to the desired pressure. The pilots sense the downstream and upstream pressure causing the valve to gradually close or open to maintain the desired pressure. Pressure reducing and sustaining valves can be remotely operated with an addition of a solenoid or a hydraulic relay.

A pressure reducing and sustaining valve is used to protect pipelines or consumers from excess pressure downstream and to maintain a minimum upstream pressure. Typically it is used as a main valve sustaining pressure on the pump during pipeline fill up and then reducing the line pressure once the pipeline is full.