Safety Valve Setting and Steam Test After a Superheater Repair
A water-tube boiler with a design pressure of 25 bar has two safety valves: one on the drum and one on the superheater outlet. After repair of the superheater header, the boiler is to have a steam test. The attendant notes that the drum valve is set at 25 bar, the superheater valve at 24 bar, and that the superheater valve was gagged during the hydrostatic test and has not been released. He proposes to raise steam to 25 bar, check that both valves lift, and leave the superheater valve gagged until the boiler is on load because it leaks slightly.
1. Evaluate the valve settings and the proposal to leave the superheater valve gagged.
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A gagged safety valve is a defeated safety device and must not be left in that state when the boiler is in service. The superheater valve protects the superheater against overpressure when the main steam stop valve is closed, so it must be free and correctly set before steam is raised.
The settings should follow the code and the insurer's requirement. Commonly the drum valve is set at or below design pressure and the superheater valve at a slightly lower pressure so it lifts first, protecting the superheater. A 24 bar superheater valve and a 25 bar drum valve are plausible, but the actual values must be confirmed from the valve data and the code, and the total relieving capacity must meet or exceed the boiler rating. The leaking valve should be repaired or replaced, not gagged.
2. Describe how you would conduct the steam test and what you would check.
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Raise steam slowly with the superheater drain open, and confirm that the drum and superheater valves are free and correctly set before pressure approaches the set point. Check the pressure gauge against a calibrated master gauge. As pressure rises, note the lift pressure and the blowdown or reseat pressure of each valve, and confirm that the superheater valve lifts first. Check for leakage at the repaired header and for correct operation of the drains and non-return valves.
Record the lift and reseat pressures, the gauge readings, and the witness signature. Any valve that fails to lift, lifts outside tolerance, or leaks after reseating must be corrected and the test repeated.
3. What records and authorisations are needed before the boiler returns to service?
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The steam-test certificate or report should record the design pressure, the set pressures, the lift and reseat results, the relieving capacity, the gauge calibration, and the inspector's signature. The repair records for the superheater header, including welder and material details, should be filed. The gag must be formally removed and its removal recorded.
Return to service must be authorised by the competent person, normally the insurer's inspector, not by the attendant. The boiler log should show the test, the removal of the gag, and the authority to fire.