Lucycn
A pressure relief valve respondes to the upstream pressure, to releive pressure above a set value, there are two types
Direct acting such as on your hot water heater which vent pressure flow over a curve, venting a small amount when the pressure is just above the setpoint and venting a larger quanity when the pressure is a significant value over the setpoint.
Pilot control, which the pilot opens and closes the mainvalve until the setpoint value is achieved. the pilot control adjusts the mainvalve until the setpoint is acheived maintaining a constant pressure upstream of the valve.
A pressure relief valve can also be used as a backpressure valve also known as pressure sustaining valve, in this application the pilot controlled is preffered.
A surge relief sometimes called a surge anticipator has a complex pilot system that monitors for a sudden drop in pressure which preceeds the water comming back and opens the valve before the high pressure wave returns to the source and the valve vents the pressure, the pilot system contains several componets and is not easy to troobleshoot or tune. The surge anticipator will also act directly as a pressure relief, which is the control function that returns the valve to the closed position after an event.
All pilot operated valves need strainers on the pilot system for proper operation, the strainer can be a y type or an in the flow type.
Hydrae
