Safety Valve That Will Not Seat After a Test
During a required safety valve pop test on a 250 psig steam boiler, the valve lifts at its set pressure but does not reseat cleanly; it continues to simmer and leak steam past the seat after pressure drops back to 220 psig. The valve was tested last year with no issue. The boiler has been on line continuously, and the water treatment log shows hardness excursions twice in the past month that were corrected. The valve is a direct-spring-loaded type with a lifting lever. The operator suggests gagging the valve or lifting the lever repeatedly to 'clear the seat.'
1. What are the likely causes of the valve failing to reseat, and why is the operator's suggestion risky?
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Likely causes:
- Debris or scale on the seat - the recent hardness excursions make carryover and deposit formation plausible; scale or dirt can hold the disc off its seat.
- Wire drawing or seat damage from the valve simmering during the test.
- Misalignment or a bent stem from the lifting lever being used improperly.
- Spring or guide binding preventing full reseating.
Gagging the valve is dangerous and prohibited as an operating fix - it defeats the overpressure protection and can lead to a boiler explosion if pressure continues to rise. Repeatedly lifting the lever to 'clear the seat' can damage the seat and disc further and is not a substitute for proper maintenance. The valve must be treated as a safety device, not adjusted in service.
2. What immediate and follow-up actions are required, and what does the code require about the boiler's status while the valve is out of service?
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Immediate actions:
- Reduce firing and hold pressure below the valve setpoint so the leaking valve is not the only protection; do not rely on a leaking valve.
- Do not gag the valve. If the valve cannot be made to seat, the boiler should be taken out of service or operated only under an approved alternate protection arrangement per the jurisdiction.
- Notify supervision and the inspection authority as required.
Follow-up:
- Remove and inspect the valve; the seat and disc should be machined or replaced by a qualified valve shop, and the valve reset and tested before return to service.
- Investigate the water treatment excursions - hardness carryover is the likely root cause and must be corrected.
- Verify the boiler has adequate relief capacity; a boiler must not be operated with its safety valve impaired. Most codes (e.g., ASME Boiler and Pressure Vessel Code and jurisdictional rules) require the boiler to be removed from service if the safety valve is not functional, and the valve must be tested and sealed by qualified personnel.