Pressure Relief Valve Testing: Methods & Requirements
Pressure relief valves protect equipment from overpressure. This guide covers the testing methods — bench testing, pop testing, and in-situ testing — and the regulatory requirements for relief valve testing.
Safety Critical
Pressure relief valves are the last line of defence against catastrophic pressure vessel failure. A failed relief valve can result in explosion, serious injury, or death. Never operate a pressure vessel with a blocked, isolated, or non-functional relief valve.
Types of Relief Valves
- Spring-loaded relief valve: A spring holds the valve disc against the seat. When system pressure exceeds the spring force, the valve opens. The most common type for general industrial service.
- Pilot-operated relief valve: A pilot valve senses system pressure and controls the main valve. More accurate than spring-loaded valves and can operate at higher pressures. Used in gas and liquid service.
- Safety valve: A specific type of relief valve for compressible fluids (steam, gas, air) that opens fully (pops) at the set pressure. Has a rapid full-lift characteristic.
Testing Methods
- Pop test (in-situ): The valve is tested while installed on the equipment. The system pressure is raised until the valve opens ('pops'). The opening pressure is compared to the set pressure. This is the most accurate test because it tests the valve under actual operating conditions, but it requires the system to be overpressurised.
- Bench test (off-line): The valve is removed and tested on a test stand. Air or water is used to pressurise the valve inlet until it opens. This is safer and more controlled than a pop test but doesn't account for the actual operating conditions (temperature, backpressure).
- In-situ testing device (Trevitest): A hydraulic lifting device assists the valve to open at operating pressure, without raising the system pressure. The assist force is measured and used to calculate the set pressure. Safer than a pop test but less accurate.
Testing Frequency
| Service | Recommended Interval |
|---|---|
| Steam boilers (safety valves) | Annually (AS/NZS 1200, AS 1277 |
| Air receivers | Every 2 years (AS 3873) |
| Pressure vessels (general) | Every 2–5 years, depending on service |
| Hazardous/toxic service | Annually or per risk assessment |
| Clean, low-pressure service | Every 5 years |
| After any overpressure event | Immediately — inspect for damage |
What to Check During Testing
- Set pressure — the valve must open within ±3% of the nameplate set pressure (±2 psi for pressures below 70 psi)
- Reseat pressure — the valve should reseat at no less than 90% of the set pressure (blowdown ≤ 10%)
- Simmer — the valve should not leak below 95% of set pressure
- Chatter — the valve should open smoothly without chattering (rapid open-close cycling)
- Seat tightness after reseat — the valve should not leak after re-closing (test to FCI 70-2 Class IV or better)
- Visible condition — check for corrosion, damaged threads, and missing components
Common Test Failures
- Valve opens below set pressure — weakened spring or damaged seat. Replace the spring or recondition the valve.
- Valve opens above set pressure — spring adjusted too high, seized spindle, or buildup on the disc preventing lifting. Clean and recalibrate.
- Valve doesn't reseat — damaged seat, debris on seat, or worn spring. Clean or recondition.
- Valve weeps below set pressure — damaged seat or disc. Recondition or replace.
- Valve chatters — excessive guide clearance or oversized valve. Consult the manufacturer.
Important: Only qualified, certified personnel should test and set pressure relief valves. In Australia, relief valve testing must be performed by a facility accredited to AS/NZS ISO 17025, and the test results must be recorded and retained for the life of the equipment.