Adding Load to a Two-Boiler Plant: Capacity, Redundancy, and a Header Pressure Conflict
A food-processing plant runs two 300-hp firetube boilers, each rated 10,350 lb/hr at 212 °F feedwater, 125 psig MAWP, currently operating at 100 psig with 190 °F deaerated feedwater. The steam load is 14,000 lb/hr. A new process line will add 6,000 lb/hr of saturated steam at 100 psig, 24 hours a day, and plant policy requires that one boiler be able to carry the whole plant if the other trips. A vendor proposes adding a 400-hp firetube rated 150 psig with a low-NOx burner, piped into the existing 100 psig header. The deaerator, feed pumps, and softeners are all rated 25,000 lb/hr. Capital approval depends on your review.
1. Can the two existing boilers carry the new load? Show the capacity calculation and the resulting margin.
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Capacity must be corrected for feedwater temperature. The familiar 34.5 lb/hr per boiler horsepower is defined with 212 °F feed. At 190 °F the feed absorbs less heat per pound, so the same firing rate produces more steam:
10,350 × (1199.5 − 190) / (1199.5 − 212) = 10,350 × 1.022 ≈ 10,580 lb/hr per boiler, or 21,160 lb/hr for the pair.
Margin. Against a 20,000 lb/hr load that is only about 5.5 %, well below the 15–20 % normally held for pickup allowance, fouling, and control error. On a cold morning or with one boiler slightly fouled, the plant is at its limit.
Redundancy fails outright. One boiler at 10,580 lb/hr cannot carry 20,000 lb/hr. There is also no spare for the annual jurisdictional internal inspection, so any outage becomes a plant shutdown. The correct conclusion is that the addition is needed, but it must be sized for the redundancy requirement, not just for the incremental 6,000 lb/hr.
2. Evaluate the vendor's proposed 400-hp, 150 psig firetube tied into the existing 100 psig header. What problems do you raise?
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1. It does not satisfy redundancy. 400 hp ≈ 13,800 lb/hr at 212 °F, roughly 14,100 lb/hr at 190 °F - still short of the 20,000 lb/hr single-boiler requirement. To meet policy, the new unit must be at least ~20,000 lb/hr corrected, about 580–600 hp.
2. The feedwater system becomes the bottleneck. Connected capacity would rise to 21,160 + 14,100 ≈ 35,300 lb/hr against 25,000 lb/hr of deaerator, pump, and softener capacity. The DA and feed pumps would then limit the plant regardless of boiler size.
3. Header pressure conflict. The existing boilers have a 125 psig MAWP. A 150 psig boiler with its operating pressure control set near 140 psig would push the common header above the MAWP of the older units and lift their safety valves. Either hold the header at 125 psig or below with a pressure-reducing station, or give the new boiler a separate header. The new boiler's safety valves protect the new boiler only; they do not protect 125 psig equipment.
4. Design margin. A firetube at 150 psig is acceptable, but if the process might later need 250 psig, a watertube should be bought now - re-rating a firetube upward later is far more expensive than buying the right design once.
3. To defer the project, the plant proposes firing both existing boilers at 110 % of nameplate to cover the new load. Assess this.
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Firing above nameplate is not a free 10 %.
- Safety valves are sized to the boiler's rated evaporating capacity. If the boiler can generate more steam than the valves can relieve, the accumulation limit (6 % above the highest set pressure for Section I power boilers under normal full firing) can be exceeded during a fire, and the valves become the weak point rather than the protection.
- The burner listing and the air permit are tied to the rated input. Overfiring changes the combustion envelope, raises furnace heat release, and typically increases CO and NOx.
- Mechanical damage. Higher firing raises tube-sheet and furnace temperatures and accelerates fouling; the margin you are trying to borrow does not actually exist once fouling is accounted for.
The proposal should be rejected. The real options are to add correctly sized capacity, or to reduce the connected load until capacity is installed.