Boiler and Pressure Vessel Construction, Design, and Fittings
Construction fundamentals
Questions come from the ASME Boiler and Pressure Vessel Code: Section I (power boilers), Section IV (heating boilers), Section VIII (pressure vessels). Know the anatomy of a firetube boiler - shell, furnace or Morison furnace, tubesheet, tube holes, combustion chamber, crown sheet - and of a watertube boiler: steam drum, mud drum, generating tubes, waterwalls, superheater. Staybolts, ligaments and compensated openings carry one theme: every cut in a pressure part must be reinforced and every flat surface must be stayed.
Required fittings
Safety valve(s), pressure gauge with siphon, gauge glass and try-cocks, water column, low-water fuel cutoff, main steam stop valve, feedwater check valve plus stop valve, and a blowoff valve. Watch the order: the check valve prevents backflow on the feed line; the stop valve isolates. Nothing may be installed between the boiler and its safety valve, and no valve may block the safety valve discharge.
Traps
- Confusing a safety valve (steam, pop action) with a relief valve (liquid).
- Forgetting that the highest set pressure may not exceed MAWP, and that accumulation with all valves relieving is limited.
- Assuming one safety valve is always enough - Section I calls for two or more when bare tube heating surface exceeds 500 ft².
Rule of thumb
Set at MAWP; accumulation stays within 10%. On a 150 psig boiler, valves are set at 150 psig or lower, and the gauge may not pass 165 psig with every valve full open.