Boiler Construction, Design, and Code Requirements
Boiler construction questions test whether you can identify pressure parts and match the right code to the right boiler. Pressure parts include the shell, drums, headers, tubes, tube sheets, stayed surfaces such as the crown sheet, furnace and combustion chamber, and any appurtenance subject to pressure. Know the boundaries: ASME Section I governs power boilers above 15 psig steam or 160 psig water; Section IV governs heating boilers at or below 15 psig steam and 160 psig / 250 deg F water; Section VIII governs unfired pressure vessels; the NBIC governs in-service repair and alteration.
What to study
- Stayed versus unstayed surfaces, staybolt pitch, ligament efficiency, and how they set minimum plate thickness.
- Furnace and combustion chamber designs (dryback vs wetback, Scotch marine, fire-tube vs water-tube) and why wetback construction reduces refractory maintenance.
- Expansion: which end of the boiler is fixed, where sliding supports and expansion joints belong.
- Required access openings (manholes, handholes) for cleaning and internal inspection.
- Design pressure, maximum allowable working pressure (MAWP), corrosion allowance, and hydrostatic test requirements.
Traps
A heating boiler pushed above 15 psig steam stops being a heating boiler; the code classification changes. Setting a safety valve lower than the stamped MAWP does not license running above the stamp. Questions that offer to solve a nuisance-lifting safety valve by raising the setpoint are testing exactly this point.
Worked rule
The safety valve setting can never exceed the MAWP stamped on the nameplate, and the lowest-set valve governs the boiler's legal operating limit. If the nameplate reads 150 psig and an option proposes a 165 psig setting to stop nuisance lifting, the correct answer is re-rating by code calculation with jurisdiction approval, not a field adjustment.