Pilot-operated valves use line pressure or electronic/pneumatic signals to feed line fluid into selected ports for the desired control action. Special versions of the valve, using an integral power piston, are available for Electro-hydraulic/Servo control.
A line pressure surge above the nitrogen "set" pressure opens the valve immediately. Transient surge pressure is thus reduced as the extra surge volume is relieved. The plug "tracks" line pressure until equilibrium is re-established and the valve closes.