Two Safety Valves, a Gate Valve, and a Request to Raise Operating Pressure
A 500-hp watertube boiler with a 150 psig MAWP supplies saturated steam at 100 psig. Its drum carries two safety valves. Valve A is set at 150 psig, stamped 10,000 lb/hr. Valve B is set at 160 psig, stamped 10,000 lb/hr at that pressure. A 4-inch gate valve is installed between the drum nozzle and Valve B so that B can be 'isolated during hydro tests.' The rated evaporating capacity is 17,250 lb/hr. A new press needs 140 psig, and management wants the operating pressure raised. Last week a mechanic reported that Valve A weeps slightly at 100 psig and asked whether he could draw the stem down with a gag during the night shift.
1. Evaluate the safety valve installation as it stands. Is it compliant and is the relieving capacity adequate?
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Set pressures. A required safety valve must be set at or below the MAWP, and the highest set pressure may not exceed the MAWP. Valve B at 160 psig on a 150 psig boiler is non-compliant: for the 10 psi band between 150 and 160 the drum is protected only by Valve A.
The gate valve is prohibited. Nothing that can be closed may sit between the boiler and a required safety valve - the relief path must be unobstructed at all times. 'Isolating it for hydro tests' is not an accepted reason. Remove the valve and its nozzle is to be a straight, free-flowing connection.
Capacity. Required relieving capacity = 500 × 34.5 = 17,250 lb/hr. With B reset to 150 psig, its capacity falls roughly in proportion to absolute pressure: 10,000 × 164.7 / 174.7 ≈ 9,430 lb/hr. Total ≈ 19,430 lb/hr, about 12 % above the required 17,250 lb/hr - adequate. For the final record, however, use the manufacturer's capacity stamped at the new set pressure; do not let a scaled estimate become the documentation.
Accumulation check. With both valves open at full firing, Section I limits the rise to 6 % above the highest set pressure (about 159 psig here). Confirm the figure against the edition of the code in force in your jurisdiction.
2. What has to change before the operating pressure can go from 100 to 140 psig, and what should the operating pressure actually be?
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The boiler is not being re-rated. 140 psig is below the 150 psig MAWP, so no NBIC re-rating is required and the nameplate stands.
But 140 psig is too close to the 150 psig set point. A safety valve needs a margin between operating pressure and set pressure, commonly at least 10 psi or about 10–15 %, or it will simmer and leak. At 140 psig operating you have only 10 psi of margin, and Valve A is already weeping at 100 psig - the problem will get worse.
Practical ceiling. With the set pressure capped at the 150 psig MAWP, the highest sensible operating pressure is about 130–135 psig. If 140 psig is a hard process requirement, the boiler must be re-rated above 150 psig through the National Board/engineering route with jurisdictional approval - the set pressure can never simply be raised above the MAWP.
Also verify before any change:
- Pressure gauge range (roughly 1.5–2× the operating pressure, so a 300 psi gauge).
- Operating pressure control set point and differential, high-pressure limit, and the burner's high-fire limit.
- Feedwater system and DA performance at the new pressure, and downstream PRVs and header ratings.
- Re-commission the burner at the new pressure, since steam capacity in lb/hr shifts slightly with pressure.
3. How do you answer the mechanic's request to gag Valve A?
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Refuse. A gag defeats the last line of protection on a pressure vessel. If firing continues and the gag is not removed, or if the valve does not reseat after the gag is released, the boiler can be overpressured with no relief. It also creates personal liability for whoever authorized it and can void insurance coverage.
Find the real cause of the weeping:
- Set pressure too close to operating pressure (here, 150 vs 100 should be fine, so look further).
- Dirt or scale on the seat from the original installation or from a hydro test.
- Thermal binding or a distorted disc/spindle, or a valve that was never re-certified after being set.
- Wrong valve type (relief rather than safety) or a bonnet drain problem.
Correct action: take the boiler down or use the approved isolation sequence, remove the valve, and send it to a shop holding the VR stamp for testing and repair. Valve A must be re-set and stamped before it is returned to service, and the as-found set pressure should be recorded to see whether it drifted.