The Weeping Safety Valve and the Isolating Gate Valve
A 150 hp, 150 psig firetube boiler at a Down East food plant carries 125 psig. Its 2½ in. safety valve has wept for two weeks, leaving a mineral trail down the discharge pipe. The plant's maintenance technician - not a licensed valve repair shop - proposes three things: (a) close the 3/4 in. gate valve he installed under the safety valve so the valve can be pulled without taking the boiler off line; (b) lap the disc and seat on a bench in the shop, then adjust the seating ring so it seats harder; (c) if that fails, raise the set pressure to 155 psig 'so it stops dribbling.' A spare valve in the warehouse is stamped 150 psig, 1,500 lb/hr - the same rating as the one in service. It has sat on a shelf for four years.
1. Evaluate the proposal to close the gate valve under the safety valve so the valve can be removed while the boiler stays in service.
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Not acceptable - remove the valve, not just close it.
- The code (ASME Section I for power boilers; Section IV for heating boilers) requires the safety valve to be connected directly to the boiler shell or drum with no intervening valve or other obstruction of any kind, and no valve in the discharge either. The reason is simple: the safety valve is the last line of defense against overpressure, and it must not be defeatable by one person turning a handwheel.
- A boiler sitting at 125 psig with a closed valve under its relief device still has a burner, a fuel train, and a control system fully capable of raising pressure. It has, in effect, zero overpressure protection. A stuck feedwater regulator plus a fired boiler is a classic route to a catastrophic failure.
- A valve 'left open' is a valve somebody will close. Since the gate valve serves no legitimate purpose, it has to come out, along with the spool piece, so the valve sits directly on its code-required nozzle.
- To actually remove the safety valve, the boiler must be taken off line, cooled, and depressurized. There is no legal or safe shortcut. The discharge piping must also be checked: the mineral trail means the valve has been passing steam, and the discharge and its drain holes may be scaled shut.
Also check the installation while you are in there: valve spindle vertical, full-size inlet and discharge piping, discharge terminated in a safe place away from walkways and not reduced, drain holes clear.
2. Evaluate lapping the seat in-house and either adjusting the seating ring or raising the set pressure to 155 psig.
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Reject both.
Raising the set pressure to 155 psig on a 150 psig MAWP boiler is a code violation, full stop. The safety valve setting must never exceed the maximum allowable working pressure of the boiler. You would be creating a situation where the boiler can be pressed above its design limit before the valve thinks about opening. The 150 psig stamp on the boiler does not become 155 because a valve weeps.
Setting, reseating, and adjusting a safety valve is not operator or general-maintenance work. It requires:
- a qualified valve shop with a calibrated test stand and a calibrated gauge,
- re-machining or replacing the disc and seat (bench lapping by untrained hands can wreck the disc geometry and change the valve's rated capacity),
- a new set-pressure tag and a test record.
A valve that has wept for two weeks has likely wire-drawn its seat and may have a warped disc; that is a repair, not an adjustment.
The correct sequence: take the boiler off line, remove the valve, install a properly set and tested replacement, and send the original out for repair and recertification. Do not 'fix' the symptom by lowering the operating pressure either - 125 psig operation under a 150 psig set pressure is normal and not the cause.
3. The warehouse spare: what must be true before it can be put into service on this boiler?
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Requirements to verify:
- Capacity at least equal to the valve being removed - 1,500 lb/hr equals 1,500 lb/hr, so this is acceptable. Capacity may be increased, never decreased.
- Set pressure at or below the boiler MAWP - 150 psig on a 150 psig boiler is permitted. Many plants intentionally set slightly lower; nothing above 150 is permissible.
- Correct service rating and ASME marking for saturated steam, with the capacity and set pressure stamped.
- Condition. Four years on a shelf means the spring, disc, and seat are unknown. A valve does not become 'pre-tested' by sitting in a warehouse. It must be set and tested on a stand immediately before installation - not installed on the strength of the stamp alone - and the set pressure, capacity, date, and tester recorded in the boiler log.
- Installation: direct connection, no valve, full-size inlet and discharge, drain holes clear, discharge piped to a safe location and not restricted, spindle vertical.
- Proof after restart: bring the boiler up and confirm the valve lifts at its set pressure, and check that pressure does not rise excessively above MAWP with the burner at maximum firing (the applicable accumulation limit depends on whether Section I or Section IV governs - verify against the code edition the state has adopted).
Finally, verify the capacity is adequate for this boiler's maximum evaporating rate, not merely that it matches the old valve. Two wrong valves are still wrong.