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The Pilot-operated safety valves are composed of a "pilot valve" and a "main valve", differently from spring-loaded safety valves, the pressure is monitored and adjusted by the pilot valve, not the main valve. Pilot-operated safety valves are widely used for large size valve and high-pressure applications, which need lager torque and high reliability.

the Principle of pilot operation:
Step 1: THE MAIN VALVE IS CLOSED
When P < 100% of Set Pressure, 
The feeding seat of the pilot is open and the main valve’s dome is pressurized.
 
Step 2: THE MAIN VALVE OPENS
When P =100% of Set Pressure, 
The drain seat of the pilot opens and drives the closing of the feeding seat.

Step 3: THE MAIN VALVE STAYS OPEN.
When 95% < P < 110% of Set Pressure, 
The drain seat of the pilot stays open and lets dome to the atmosphere.
 
Step 4: THE MAIN VALVE IS CLOSING.
When P = 95% of Set Pressure, 
The drain seat of the pilot recloses and drives the opening of the feeding seat.
 
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