How to Set a Relief Valve

Setting a pressure relief valve (PRV/PSV) establishes the pressure at which the valve opens to protect equipment from overpressure. The set pressure must equal or be less than the maximum allowable working pressure (MAWP) per ASME Section VIII and ASME B31.3.

Definition of Key Terms

Term	Definition
Set pressure	The inlet pressure at which the valve starts to open (pop) under service conditions
Overpressure	The pressure increase above set pressure during full valve discharge (typically 10% for non-fire cases)
Accumulation	The pressure increase above MAWP allowed during a relief event
Blowdown	The pressure difference between set pressure and reseat pressure (when the valve closes)
CDTP (Cold Differential Test Pressure)	The inlet pressure at which the valve is set on the test bench, adjusted for backpressure and temperature
Reseat pressure	The pressure at which the valve closes after relieving

Set Pressure Procedure (Bench Pop Test)

1. Review the data sheet: Obtain the valve data sheet specifying set pressure, overpressure allowance, service medium, operating temperature, and backpressure.
2. Calculate CDTP: Adjust the set pressure for any constant backpressure and temperature effects: CDTP = Set Pressure - Backpressure Correction + Temperature Correction.
3. Mount on test bench: Install the relief valve on a calibrated test bench with a regulated air or nitrogen supply (for safety valves) or hydraulic supply (for relief valves).
4. Increase pressure gradually: Slowly raise the inlet pressure at a rate of approximately 2-3 psi/second (15-20 kPa/second) until the valve pops open. Record the pop pressure.
5. Verify set pressure: Compare the pop pressure to the CDTP. The tolerance per ASME PTC 25 (Pressure Relief Devices) is:
   • +/- 2 psi for set pressures up to 70 psi (4.8 bar)
   • +/- 3% for set pressures above 70 psi (4.8 bar)
6. Adjust the spring: If the pop pressure is outside tolerance, adjust the adjusting screw (spring compression) to increase or decrease the set pressure. Tighten the lock nut after adjustment.
7. Repeat pop test: Perform a minimum of two additional pop tests to verify repeatability. All three pops must be within tolerance.
8. Set blowdown: Adjust the blowdown ring (nozzle ring or guide ring) to achieve the specified blowdown percentage (typically 7-10% of set pressure for gas service).
9. Seal the adjustments: Apply lead seals or security wire to the adjusting screw, lock nut, and cap to prevent unauthorized tampering.
10. Document and tag: Record all test data on a valve test report. Attach a metal nameplate or tag showing the set pressure, test date, and test bench certificate number.

ASME Requirements for Set Pressure

Application	Maximum Set Pressure	Overpressure Allowance
Single relief device protecting a vessel	100% of MAWP	10% above MAWP
Multiple relief devices (supplemental)	105% of MAWP	16% above MAWP
Fire case (external fire exposure)	110% of MAWP	21% above MAWP
Non-fire case (blocked outlet, thermal expansion)	100% of MAWP	10% above MAWP
Piping systems (ASME B31.3)	Design pressure of the piping	10% above design pressure (or 50 psi, whichever is greater)

Common Mistake

Never adjust a relief valve set pressure in the field without performing a bench pop test. Field adjustments without testing can result in a set pressure outside tolerance. ASME and API 520/521 require all adjustments be verified by a certified facility.

Key Points

• Set pressure testing must be performed by qualified personnel certified to ASME Section VIII and National Board NB-65.
• The test medium is air or nitrogen for safety valves and water or oil for relief valves, unless otherwise specified.
• All relief valve test records must be maintained as part of the plant’s mechanical integrity documentation, reviewed during piping inspections.

Read the full guide to hydrostatic testing