Safety Valve Sizing After a Boiler Uprate

The engineer's argument is wrong on two counts.

First, safety valve capacity is not constant with set pressure. A safety valve's relieving capacity scales roughly with the absolute relieving pressure (for saturated steam, capacity is proportional to the absolute pressure times a compressibility factor near unity). A valve stamped 55,000 lb/h at 125 psig is stamped at a specific set pressure and a specific accumulation (typically 10% over set pressure for a single-valve installation, or the code-allowed accumulation). Raising the set pressure to 150 psig raises the relieving pressure and therefore the capacity - so the valve may well relieve more, not less, at the higher set pressure. But the stamp is only valid at the stamped pressure unless the valve is re-rated and re-stamped by the manufacturer or an authorized assembler. The engineer cannot simply assume the old stamp carries over.

Second, and more important, the boiler's maximum allowable working pressure change is a code event. Raising MAWP requires an ASME Code review of the pressure parts - the drum, tubes, and headers were designed for 125 psig, and the higher pressure reduces the margin on every pressure part. The safety valve capacity must be re-evaluated against the new MAWP and the new maximum firing rate, and the valves must be re-stamped for the new set pressure.

The correct answer: the existing valves may be adequate, but that must be proven by re-rating them at 150 psig and confirming the stamped capacity at the new set pressure exceeds the boiler's maximum steam-generating capacity, and the pressure parts must be reviewed for the higher MAWP. The 110,000 lb/h figure is not a valid capacity at 150 psig.

Set pressures. For a boiler with two safety valves, the code practice is to set the valves so that the highest set pressure does not exceed the MAWP and the lowest set pressure is close enough that the valves open in sequence without the drum pressure exceeding the accumulation limit. A common arrangement is one valve set at MAWP and the second set slightly lower (or both at MAWP with staggered set points). With a new MAWP of 150 psig, the valves would typically be set at 150 psig (or 150 and a few psi below) with the relieving pressure not exceeding 110% of MAWP (165 psig) for the accumulation case.

Accumulation. The code allows the boiler pressure to rise above MAWP during relief by a defined accumulation - typically 10% above MAWP for a single safety valve or the code-allowed value for multiple valves - but the pressure parts must be evaluated at that accumulated pressure, not just at MAWP. The 145 psig high-limit switch is below the new MAWP of 150 psig, which means the high-limit will trip the burner before the safety valves ever see set pressure. That is acceptable as a control, but it means the high-limit is now a primary protective device and must be tested and its set point reviewed against the new MAWP.

Other devices to review:

• High-limit pressure switch - set at 145 psig, below MAWP; confirm it is a manual-reset device and test it.
• Operating pressure controls - the modulating burner's normal operating band must be below the high-limit with margin.
• Low-water fuel cutoff - unaffected by pressure but must be tested as part of the MAWP change.
• Pressure gauge range - the gauge must read to at least 1.5× MAWP; a 0–200 psig gauge is adequate at 150 psig.
• Steam header and piping - the downstream piping and any relief discharge piping must be re-evaluated for the higher pressure and the new relief capacity.

The exam point: a set-pressure change is a code event, not an adjustment. The safety valves must be re-rated and re-stamped, the pressure parts reviewed, and every protective device re-verified against the new MAWP.