A Pressure Sustaining Valve (PSV) is used to maintain a set pressure at a specific point in the pipe network. The valve can be in one of three states:

• Partially opened (i.e., active) to maintain its pressure setting on its upstream side when the downstream pressure is below this value.
• Fully open if the downstream pressure is above the setting.
• Closed if the pressure on the downstream side exceeds that on the upstream side (i.e., reverse flow is not allowed).