What is the primary function of a pressure sustaining valve in an irrigation system?

• A. It prevents backflow into the water source when the pump stops.
• B. It protects system pipes downstream of the pump by maintaining pressure and relieving surges.
• C. It filters debris from water entering the mainline.
• D. It discharges air pockets during line filling.

Answer: (B)

A pressure sustaining valve is used to keep the pressure in the part of the system downstream of the pump at a desired minimum and to absorb pressure surges. By staying at its set point, it helps maintain steady flow to the sprinklers even as demand changes. When a surge or a rapid change in flow would push pressure above that set point, the valve opens just enough to relieve the excess, reducing water hammer and protecting pipes, fittings, and other components. So, the primary function is to stabilize downstream pressure and dampen pressure spikes.

It isn’t meant for preventing backflow into the source when the pump stops, which is the job of backflow prevention devices. It also isn’t a filter, which would remove debris, nor an air-release valve, which would vent trapped air during line filling.