Construction and Design
Construction and Design
This domain tests pressure-part arrangement and the mechanical details that keep a boiler leak-tight and free to expand: drums, tubes, headers, furnaces, tube sheets, supports, refractory, casing, and access openings. Expect questions on materials, expansion, and why design details matter to operation.
Core rules and concepts
- Pressure parts are designed under codes such as ASME BPVC Section I for power boilers. Material specification, wall thickness, joint efficiency, and allowable stress all matter.
- Tubes connect drums and headers. Tube sheets and ligaments carry load around tube holes; wasted metal or cracks reduce strength.
- Boilers expand when heated. Sliding supports, hangers, expansion joints, and guided clearances allow movement without overstressing nozzles or piping.
- Refractory and insulation protect casing and reduce heat loss, but refractory must be dried slowly to avoid spalling or explosion from trapped moisture.
- Manholes, handholes, and inspection openings allow internal examination but must be correctly gasketed and tightened to avoid leaks.
Worked example
A carbon-steel superheater tube run is 40 ft long and heats from 100°F to 500°F. Use α ≈ 6.5 × 10⁻⁶ in/in/°F. Length is 480 in; expansion = 480 × 6.5 × 10⁻⁶ × 400 = about 1.25 in. If both ends were rigidly anchored with no allowance, that movement would load the tube and headers. The design must provide expansion loops, sliding supports, or clearances. The reasoning is to convert length to inches, multiply by the expansion coefficient and temperature rise, then check that the support arrangement permits the calculated movement.
Traps that cost marks
- Rigidly anchoring both ends of a hot tube run.
- Confusing tube-sheet ligament damage with ordinary surface corrosion.
- Ignoring differential expansion between hot parts and cooler casing.
- Skipping controlled refractory dryout after repairs.