Safety Valve Test Results Conflict with Gauge Readings
During an annual inspection, a 10 bar gauge saturated-steam boiler has two spring-loaded safety valves set at 9.5 and 9.8 bar gauge. The boiler design pressure is 10 bar gauge. The valves are tested on the boiler with a calibrated test gauge, but the boiler's own gauge reads 0.3 bar higher than the test gauge at the same pressure. Valve A lifts at 9.2 bar and reseats at 8.5 bar. Valve B lifts at 9.7 bar and reseats at 9.4 bar. The test gauge calibration certificate expired two months ago. The inspector is due to sign the certificate tomorrow, and production wants the boiler back on line tonight. The safety valve nameplates are unreadable.
1. Which test results, if any, can be accepted, and what invalidates the test?
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No result should be accepted yet. The expired calibration certificate on the test gauge invalidates the reference pressure. The 0.3 bar discrepancy between the boiler gauge and the test gauge also shows that the boiler gauge cannot be used to judge lift pressure. Until a calibrated gauge is used, the recorded 9.2 and 9.7 bar values are not reliable enough to compare with the set pressures.
On the numbers as recorded, Valve A is the greater concern: it lifts 0.3 bar below its 9.5 bar set point and reseats 1.0 bar below it. A low lift pressure can cause nuisance opening and steam loss; a large blowdown can leave the valve simmering or failing to reseat tightly. Valve B is closer to its set point, but its reseat pressure is also below the set pressure and must be checked against the applicable tolerance. The unreadable nameplates mean the set pressures cannot be verified against the manufacturer's marking, and the valve spring condition and adjustment history are unknown. The correct judgement is to treat the test as inconclusive, not to sign on the basis of the boiler gauge.
2. What corrective sequence should be followed before the boiler is returned to service?
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First, isolate and lock out the boiler. Obtain a test gauge with a valid calibration certificate and confirm its range and accuracy are suitable for the set pressures. Re-test both valves with the calibrated gauge and record lift, reseat and blowdown. Check the boiler gauge separately and recalibrate or replace it. Verify the valve nameplate data from the manufacturer or the inspection record; if the set pressures cannot be established, the valves should be replaced or adjusted by a competent valve service.
Valve A should be removed for inspection and setting on a test stand. Check the seat, disc, spring and spindle; do not simply screw down the adjustment without checking the cause. Valve B should also be checked if its reseat pressure is outside tolerance. After reassembly, re-test on the boiler, fit new seals or lockwire as required, and record the final settings. The inspector should not sign until the calibrated results, nameplate data and adjustment records agree. Returning the boiler to service with a valve that lifts at 9.2 bar and reseats at 8.5 bar is not justified.
3. Explain the operating risk of running with Valve A as found.
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Valve A as found would open at about 9.2 bar, below its marked set pressure, and reseat at 8.5 bar. That means it may leak or simmer whenever the boiler operates near 9 bar. Continuous steam loss wastes fuel and water and can overload the feedwater system. More seriously, a valve that has opened may not reseat tightly because the disc and seat are contaminated or damaged, so steam can continue to pass even after pressure falls. The operator may then be tempted to isolate the valve or over-tighten it, which creates a much greater hazard.
A valve that lifts early can also mask the condition of the other valve: if Valve A is passing, the boiler may not reach the set pressure of Valve B during a test, giving a false impression of protection. If Valve A is isolated to stop leakage, the remaining valve may not have enough capacity. For a 10 bar design boiler, the safety valves are the final overpressure protection; the high-pressure trip and burner controls are not substitutes. The valve must be set and sealed correctly before operation.