Safety Valve Set Pressure Versus Stamped MAWP
A 200 hp Scotch marine firetube boiler is stamped MAWP 150 psig steam. It has two spring-loaded safety valves: No. 1 set at 125 psig and No. 2 set at 150 psig. The plant operates at 135 psig and wants to raise header pressure to 145 psig for a new process. The chief operator proposes to adjust No. 1 from 125 to 145 psig, leave No. 2 at 150 psig, and change the operating setpoint. The boiler nameplate lists a maximum steaming capacity of 6,900 lb/hr. The safety valve data sheets show certified steam capacity at the old set pressures, but no capacity at 145 psig. The boiler has a single pressure gauge with a 0-300 psig range, a test connection, and a non-return main steam valve. The Minnesota inspector will witness the final setting. The plant cannot afford a long outage, but it also cannot operate above the stamped limit.
1. What is the maximum allowable set pressure for either safety valve, and is the proposed 145/150 psig arrangement acceptable?
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The stamped MAWP is the ceiling: neither safety valve may be set above 150 psig. On that narrow point, 145 and 150 psig are within the stamp. The arrangement is still poor practice and should not be accepted as proposed. Operating at 145 psig with a valve set at 145 psig leaves no margin for normal pressure swings; the valve will simmer, leak, and deteriorate. A common operating margin is at least 5 percent below the lowest set pressure, so a 145 psig header needs the lowest valve set near 152 psig, which exceeds MAWP. Therefore the plant cannot both operate at 145 psig and retain a practical margin with this boiler. The choices are to keep the header lower, reduce the process demand, or replace the boiler with a higher-MAAP unit. Changing only the setpoint is not a code-compliant fix.
2. What relieving-capacity verification is required before the pressure is raised, and what data are missing?
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The total certified relieving capacity of all safety valves, at their new set pressures, must equal or exceed the boiler's maximum steaming capacity. The nameplate says 6,900 lb/hr, so the valves must relieve at least that much. Capacity is not a single fixed number: it depends on set pressure, orifice area, and the certified coefficient. The old data sheets at 125 and 150 psig do not prove capacity at 145 psig. The operator needs manufacturer's certified steam capacity tables or curves for each valve at the proposed set pressure, including the accumulation basis used for the code calculation. If the combined capacity at 145/150 psig is below 6,900 lb/hr, the proposal fails even though the set pressures are below MAWP. The inspector should see the capacity calculation, not just the gauge on the valve.
3. What should be checked on the pressure gauge and what records should the inspector witness?
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A 0-300 psig gauge is acceptable in range because 145 psig falls near mid-scale, where readability is best, but it must be calibrated and have a working test connection. The inspector should witness the final set pressure with a calibrated test gauge, confirm the valve reseats, and verify that the valve is sealed or tagged to prevent unauthorized adjustment. The operator should record the boiler MAWP, each valve's set pressure, orifice size, certified capacity, total capacity, and the calculation showing capacity exceeds 6,900 lb/hr. The operating permit and boiler log should be updated to the approved pressure. If the capacity or margin cannot be demonstrated, the boiler should remain at its existing pressure until engineering and inspection approval are complete.