The correct answers are A and D because they describe the fundamental operating principle of a pilot-operated relief valve (PORV) , as recognized in CCPS guidance on pressure relief systems within Mechanical Integrity.
A pilot-operated relief valve uses a pilot (small control valve or regulator) to control the pressure acting on the main valve. Under normal conditions, system pressure is applied to keep the main valve tightly closed. When the system pressure reaches the set pressure , the pilot valve opens, which vents or releases pressure from above (or behind) the main valve seat . This creates a pressure imbalance that allows the main valve to open rapidly—this corresponds directly to D .
Additionally, A is correct because pilot-operated valves are designed to maintain set pressure accuracy even in the presence of backpressure , unlike conventional spring-loaded relief valves, which can be significantly affected by backpressure. This makes PORVs advantageous in systems with variable or high backpressure.
Options B and C are incorrect. Diaphragm rupture is a failure mode, not a normal operating mechanism, and manual opening is not how pilot-operated relief valves function during automatic overpressure protection.
CCPS emphasizes understanding device operating principles to ensure correct application, reliability, and proper maintenance of pressure relief systems.
