Proposed Fix for a Firetube Boiler Overpressure and Classification Question
A laundry has a 1970s steel firetube boiler: 200 hp, 15 psig design, currently operating at 12 psig steam. The burner is gas-fired and rated 8,400,000 Btu/hr input. The safety valve is set at 15 psig with a capacity of 6,900 lb/hr, and the boiler feedwater is 210 °F. A new flatwork ironer will add 2,500 lb/hr of steam at 12 psig. The owner wants to raise the operating pressure to 15 psig to improve drying and asks you, the contract operator, to evaluate the change and determine whether the existing safety valve capacity is sufficient for the added load.
1. Determine the current steam capacity and whether the added load can be met at the existing firing rate.
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Capacity at the given firing rate. 200 hp × 34.5 lb/hr per hp = 6,900 lb/hr at 212 °F feedwater. The boiler feedwater is 210 °F, so the correction is small: 6,900 × (1199.5 − 210) / (1199.5 − 212) = 6,900 × 1.002 ≈ 6,910 lb/hr. The new load adds 2,500 lb/hr to the existing load, which is not stated; assume the current load is already at or near capacity. The added 2,500 lb/hr would require about 72 additional horsepower - roughly 2,500 × 34.5 ≈ 86,000 Btu/hr of firing capacity beyond the existing 8.4 MMBtu/hr input. With an 80 % combustion efficiency, the 8.4 MMBtu/hr input produces about 6.7 MMBtu/hr of useful heat, which is roughly 6,900 lb/hr of steam at 12 psig. Adding 2,500 lb/hr therefore pushes the boiler beyond its rated capacity, and the firing rate cannot support it. The owner must either add boiler capacity or reduce the connected load.
2. Evaluate the safety valve capacity if the operating pressure is raised to 15 psig and the load is increased.
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Safety valve capacity must match or exceed the boiler's maximum evaporating capacity. The valve is set at 15 psig and stamped for 6,900 lb/hr. If the boiler is fired at its rated input, it can generate about 6,900 lb/hr at 212 °F feed, or about 6,910 lb/hr at 210 °F. That is roughly equal to the valve capacity, leaving no margin. Raising the operating pressure to 15 psig is not permitted unless the valve is re-set or replaced to a higher set pressure, but the valve's set pressure cannot exceed the boiler's MAWP of 15 psig. Since the valve is already at the MAWP, the only compliant options are: reduce the maximum firing rate so the boiler cannot exceed 6,900 lb/hr, or replace the safety valve with one of higher capacity at the same set pressure. Simply raising the operating pressure does not change the valve's set pressure, but it reduces the margin between operating pressure and set pressure, which can cause the valve to weep. Also verify the accumulation limit: with the valve fully open, the pressure must not rise more than 10 % above the set pressure for a Section IV heating boiler, i.e. about 16.5 psig. The valve capacity must be adequate to hold that limit at maximum firing.
3. The owner proposes to change the burner to a 10 MMBtu/hr input model to cover the new load. What design and code issues must be addressed before making that change?
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1. Boiler design pressure and MAWP. The boiler is stamped at 15 psig. Increasing firing rate does not change the MAWP, but the higher heat input raises the maximum evaporating capacity beyond the safety valve's capacity. The safety valve must be re-evaluated and possibly replaced to handle the new maximum steaming rate at the stamped MAWP.
2. Furnace and tube-sheet heat release. A firetube boiler has a limited furnace volume and heat-transfer surface. Raising the firing rate from 8.4 to 10 MMBtu/hr increases the heat flux and the furnace exit temperature, which can shorten tube life, cause tube-sheet cracking, and increase thermal stress. The manufacturer should be consulted to confirm the boiler is rated for the higher input.
3. Burner listing and combustion air. The new burner must be listed for the application and sized for the available gas pressure and combustion air. The boiler room ventilation must be checked for the higher air demand, and the stack and breeching must handle the increased flue gas volume.
4. Controls and safety devices. The operating pressure control, high-pressure limit, low-water cutoff, and flame safeguard must all be compatible with the new firing rate. The burner management system must be re-commissioned and tested.
5. Code and jurisdictional approval. Any change to the firing rate or safety valve capacity requires notification to the jurisdictional authority and may require a new inspection. The authorized inspector should review the change before the boiler is returned to service.