Pilot operated valves are larger (2" to 8") in size but functionally equivalent to direct acting valves with the exception that there are minimum pressure drop considerations for proper operation.
The minimum pressure drop varies for each valve but ranges from 3-1/2 PSID to 9 PSID. It is important to consult the Valve Data Sheet or Drawing for the exact minimum pressure drop for the particular valve in question.
Changes in the pressure or temperature of the actuation fluid operate a small internal pilot valve, which controls the position of the main valve by modulating the flow of water out of internal diaphragm chambers and removing the closing pressure acting on the diaphragm.
This variable closing force balances against any opening force provided by the counterbalance spring or pressure acting under a smaller diaphragm, along with water flow or pressure acting against the main valve plug. The surface area of the diaphragms are large enough to turn small differential pressures into useful actuation forces.