Upsized Burner, Undersized Relief Capacity on a 400 hp Firetube
A 400 hp Scotch-marine firetube boiler at a food plant is stamped for 150 psig with a rated capacity of 13,800 lb/hr saturated steam. Last spring the original 400 hp gas burner was replaced with a 500 hp low-NOx burner whose maximum input is 16,800 MBH - roughly 16,500 lb/hr at this plant's feedwater temperature. The boiler carries two 2-inch ASME Section I safety valves, each stamped 6,900 lb/hr at 150 psig. During the retrofit the contractor gagged one valve because it had been weeping at the seat, and never returned to replace it. Both valves discharge into a common 3-inch header that also receives the deaerator vent; each valve outlet is bushed down from 2 inch to 1¼ inch, and there is no drip pan at either valve. The plant engineer wants to fire the new burner at full rate tonight because the evaporator upstairs is short of steam. The boiler inspector is not scheduled to visit for three weeks.
1. Determine whether the boiler's pressure-relief capacity is adequate for the new burner. Show the arithmetic and name the code basis for your conclusion.
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Start from the principle in ASME Section I (the safety-valve capacity rules, PG-67 through PG-70): the total rated relieving capacity of the safety valves, at a set pressure no higher than the boiler MAWP, must be at least the maximum evaporating capacity the boiler can produce - which is now set by the burner's maximum input, not by the old nameplate.
Arithmetic: 16,800 MBH ÷ ~1,015 Btu/lb (saturated steam at 150 psig minus feedwater enthalpy) ≈ 16,500 lb/hr required. With both valves in service the stamping totals 13,800 lb/hr - already about 2,700 lb/hr short. With one valve gagged, only 6,900 lb/hr is available, a shortfall of roughly 9,600 lb/hr, or 58% of the requirement.
Two independent conclusions follow. First, the boiler must not be fired at the new burner's rated input; the valve set cannot relieve it, so an overpressure event would lift the remaining valve and continue raising pressure. Second, the deficiency cannot be corrected by simply ungagging the old valve - the pair is still undersized for the new firing rate. Adding valve capacity, reducing and documenting a permanent maximum firing rate, or returning to a burner matched to the boiler all require engineering review and inspector involvement before the boiler is fired again.
2. Evaluate the safety-valve discharge piping arrangement and list every deficiency you can identify, explaining why each matters.
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The discharge arrangement fails on several counts, and each one creates a real hazard rather than a paperwork issue:
- Outlet reduction. ASME Section I requires the discharge pipe to be at least the full size of the valve outlet, and it must not be smaller anywhere downstream. Bushing 2 inch down to 1¼ inch restricts flow and can cause the valve to chatter or fail to relieve capacity.
- Common header shared with the deaerator vent. Safety-valve discharges must run independently to a safe point of discharge, with no connection to any other piping. A deaerator vent line can carry pressure, condensate, or steam back to the valve and can also be blocked; either condition defeats the valve.
- No drip pan. A drip pan pocket and drain at the valve outlet collects condensate that would otherwise sit on the disc and cause corrosion or water hammer on lift.
- No visible safe termination. Discharge must terminate where discharge cannot injure personnel, and the pipe must not be threaded at the outlet.
Any one of these would be a deficiency on an inspection report; together they mean the remaining valve cannot be credited at its stamped capacity in the installed condition.
3. The plant engineer proposes to run at half firing rate until the inspector arrives. What conditions would have to be met for even temporary operation, and what must be done before normal firing resumes?
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Half firing rate is not automatically acceptable, because the code requirement is tied to the maximum input the burner can actually achieve, not to where the operator intends to leave the dial. Temporary operation is defensible only if all of the following are true: the maximum firing rate is physically and permanently limited (for example, by a certified orifice or fuel-train change, or a locked and documented control limit) so the boiler cannot evaporate more than the 6,900 lb/hr the remaining valve can relieve; the limit is documented, sealed, and accepted by the inspector or the authority having jurisdiction; the gag is removed and the weeping valve is either repaired or replaced; and the discharge piping is corrected to full size, independent, with drip pans and safe termination.
Before normal firing resumes, the owner needs a capacity calculation stamped by an engineer or accepted by the inspector, additional valve capacity installed and tagged, the new burner's input verified against the boiler MAWP, the boiler certificate updated to reflect the new firing rate, and operators briefed on the new limit. Running tonight at the new burner's full rate, or at any rate the relief capacity cannot handle, is the one option that is not available.