Accurately maintain a constant upstream pressure by discharging water as required.
Pressure sustaining valves are often used to protect system pipes, for example downstream of pumps to control the system flow rate, and on a tee off the mainline to act as a relief valve.
They are also frequently installed downstream of filter stations to provide adequate backflush pressure. In this application, they are often operated with a solenoid to form a pressure-sustaining-normally-open (PSNO) valve.
The Nelson pressure control pilot is famous for precision accuracy and responsiveness.