Safety Valve Simmering / Leaking Below Set Pressure
A safety valve that leaks continuously at operating pressure is 'simmering' - a critical issue that wastes product, accelerates seat erosion, and can cause the valve to be damaged by the time it needs to relieve. Any safety valve simmer must be investigated and corrected immediately.
Symptoms
Root Causes
Operating pressure too close to set pressure
API 520 and ASME Section VIII require that the maximum allowable operating pressure (MAOP) not exceed 90% of the safety valve set pressure. If operating at 92–98% of set, the valve will start to simmer. This is a system design issue, not a valve fault.
Set pressure has drifted down
The spring in a safety valve loses preload over time, particularly on steam valves subject to thermal cycling. The set pressure may have drifted below the original stamped set pressure.
Seat damage from previous over-pressure event
If the valve has lifted previously, debris or erosion may have prevented it from reseating fully. Even microscopic particles on the seat face prevent bubble-tight shutoff below set pressure.
Back pressure on discharge
Built-up back pressure on the discharge side (blocked discharge pipe, flooded collection header) prevents the valve from reseating after a lift.
Tampering with the adjustment screw
Unauthorized adjustment of the set pressure adjustment cap nut (where accessible) can lower the set pressure below the operating pressure.
Safety Precautions
- ASME Section VIII Section UG-135: safety valve maintenance must be performed by a National Board-certified repair shop (NB-R stamp) for code vessels
- Do not operate a vessel above MAOP with a simmering safety valve
- Discharge piping must be rated for full valve capacity and temperature
- Wear face shield and heat-resistant gloves when testing hot service valves
Tools Required
- Calibrated pressure gauges (traceable to NIST)
- Safety valve test bench
- Lapping plate and compound
- Torque wrench
Supplies Needed
- Replacement disc and nozzle seat (if damaged)
- New body gasket/O-ring
- Safety valve spring (if set pressure drift requires new spring range)
Step-by-Step Repair Guide
- 1
Immediately verify operating pressure vs. set pressure
Read the operating pressure from the vessel pressure transmitter or gauge. Read the valve set pressure from the nameplate stamped on the valve body (required by ASME and API 526). If operating pressure exceeds 90% of set pressure, the system must be reduced in pressure before the valve can be properly evaluated. Inform the process supervisor - this may require a process rate reduction.
Never blank-flange or mechanically hold-down a simmering safety valve without formally reducing the vessel pressure. This conceals a safety system malfunction.
- 2
Check discharge piping for back pressure
Inspect the valve discharge line. A full or blocked discharge pipe creates back pressure that prevents the valve from reseating. Check for: closed discharge isolation valve (lock-open required per OSHA PSM), flooded closed drain system, blocked vent pipe (bird nests, ice, closed cap). Remove any obstruction and verify free discharge.
- 3
Test set pressure by simmering test or lift test
After verifying 90% operating-to-set margin and clear discharge, check if simmer stops. If it does not, the valve must be removed from service for bench testing. On non-critical services with a spare connection, install a spare certified valve and remove the faulty valve for shop testing. Bench-test the set pressure per API 527 and ASME Section VIII using a set pressure test stand with calibrated pressure gauges.
Never adjust a safety valve set pressure in the field without proper authorization, a calibrated test stand, and an ASME/NB-certified shop or inspector. Field adjustment of safety valves is a regulatory violation in most jurisdictions.
- 4
Inspect disc and seat for debris or damage
In the shop, disassemble the valve per the manufacturer's repair procedure. Inspect the disc and nozzle seat for: embedded particles (scale, weld spatter, condensate deposits), erosion channels from previous simmer, corrosion pits. Clean the seat with lint-free cloth and isopropyl alcohol. Polish the disc face and nozzle seat using the lapping procedure defined in the valve's repair manual. Reassemble and set on the test stand.
- 5
Set pressure calibration and certification
Adjust the spring compression to achieve the certified set pressure (within +/-3% per API 527 for spring-loaded valves, within +/-0.5% for pilot-operated types). Test for ANSI FCI Class IV seat tightness at 90% of set pressure. Stamp the valve with the new test date and seal the adjustment cap. Reinstall with a new gasket and return to service. Update the safety valve inspection register.
When to Replace Instead of Repair
Replace the safety valve when: the nozzle seat is integral (non-replaceable) and damage exceeds lapping depth, spring is damaged or fatigued beyond reuse, body is corroded below minimum wall thickness, or the valve has exceeded its recommended service interval (typically 1–5 years depending on service as defined in API 576).
Related Products
Key Terms Explained
Unfamiliar with any terms used in this guide? Each links to a full engineering definition.
Quick Reference
- Difficulty
- Complex
- Est. Time
- 4–8 hours including shop test
- Steps
- 5
- Category
- Safety Valves
Steps
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