Boiler Construction, Design, and Fittings
What this domain tests
This domain tests how boilers are built to code and how required fittings protect them. Expect questions on ASME code sections, fire-tube versus water-tube construction, and the function and placement of safety valves, water columns, gauge glasses, and low-water cutoffs.
Code basis and construction
- ASME Boiler and Pressure Vessel Code Section I covers power boilers above 15 psig steam or above 160 psig water at 250°F; Section IV covers heating boilers; Section VIII covers pressure vessels.
- Minnesota adopts the ASME code through Rules chapter 5225. Look for the “S” stamp on power boilers and “H” stamp on heating boilers.
- Fire-tube boilers pass hot gas through tubes surrounded by water; water-tube boilers pass water through tubes surrounded by hot gas. Water-tube designs respond faster and are common at higher pressures.
Required fittings
- Safety valve: set at or below the maximum allowable working pressure (MAWP); no shutoff valve may be installed between the boiler and the safety valve. Discharge must be piped to a safe location.
- Water column and gauge glass: show the true water level. Try cocks are a backup. Blow the glass down regularly.
- Low-water fuel cutoff: cuts off fuel on a low-water condition; it is not merely an alarm.
- Pressure gauge with siphon; feedwater check valve and stop valve; bottom blowdown valve.
Worked example
A boiler is rated 12,000 lb steam/hr. Each safety valve is rated 7,000 lb/hr. Required relieving capacity must be at least the maximum evaporation. One valve gives 7,000 lb/hr, which is insufficient; two valves give 14,000 lb/hr, which is sufficient. If MAWP is 150 psig, the set pressure cannot exceed 150 psig. Reasoning: code capacity is based on maximum evaporation, not normal load.
Traps
- Confusing MAWP with hydrostatic test pressure, which is typically 1.5 times MAWP.
- Believing a valve between the boiler and safety valve is acceptable.
- Leaving gauge glass isolation valves closed during operation.
- Mixing boiler horsepower with pounds of steam per hour: 1 boiler hp equals 34.5 lb steam/hr at 212°F.