Safety Valve Gagging and Capacity Shortfall on a 600 psig Drum
A 600 psig, 150,000 lb/h steam drum is protected by two spring-loaded safety valves. The nameplate MAWP is 675 psig. Valve A is set at 650 psig and has a certified capacity of 95,000 lb/h; Valve B is set at 675 psig and has a certified capacity of 95,000 lb/h. The superheater outlet has one safety valve set at 660 psig with a certified capacity of 30,000 lb/h. During a hydrostatic test six months ago, Valve A was gagged; the gag was removed, but the broken seal was not replaced and the lever is now wired shut. The boiler normally operates at 640 psig. To reduce nuisance lift, operations proposes to remove Valve A, leave Valve B at 675 psig, and rely on the superheater valve plus Valve B. The jurisdiction has adopted ASME Section I and requires capacity certification and seals.
1. Evaluate the proposed removal against code requirements and the boiler's required relief capacity.
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The proposal is not acceptable. The boiler's maximum evaporative capacity is 150,000 lb/h, but the remaining drum valve capacity would be only 95,000 lb/h. Adding the superheater valve's 30,000 lb/h gives 125,000 lb/h, still less than the boiler's required capacity, and the superheater valve protects the superheater rather than substituting for drum overpressure protection. Removing Valve A would also eliminate redundancy and leave the drum with a valve that has been gagged and wired, which is not a normal operating condition. A code-compliant evaluation must account for all certified capacities, the drum and superheater protections separately, and the jurisdiction's seal and test requirements. The unit cannot be returned to service on this basis.
2. What immediate actions are required regarding Valve A, and what tests must be performed before it is returned to service?
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Immediately remove the wire and gag, inspect the valve for stem or seat damage, and send it to a qualified shop if it has been damaged or has no valid test record. Reinstall it at the correct set pressure of 650 psig, verify its certified capacity, and perform a set-pressure test using steam or an approved test fixture. Check the discharge piping, drain, and support conditions, replace the seal, and record the test and seal number. Do not return the boiler to service with the valve gagged, wired, or unsealed. After restoration, confirm the total installed relief capacity against the boiler maximum evaporative capacity.
3. If the plant wants to reduce nuisance lifting, what legitimate options exist?
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Investigate the cause of nuisance lifting: normal operating pressure too close to the set pressure, valve chatter, thermal binding, drum-level swings, or stressed discharge piping. Legitimate options include lowering normal operating pressure, repairing or replacing the valve with a code-compliant valve of the same set pressure, improving valve maintenance, adding a controlled start-up vent or warm-up procedure, or using a pilot-operated safety valve where allowed by the code and jurisdiction. Any change must preserve the required certified capacity and keep set pressures within the MAWP limits. Never raise set pressure above the MAWP, remove a required valve, or leave a valve gagged to solve a nuisance-lift problem.