Boiler Design, Construction, Materials and Pressure-Part Integrity
This domain tests how pressure parts are designed, fabricated, inspected and kept fit for service. It links ASME Section I rules to real damage: creep, fatigue, corrosion and weld defects. The exam expects you to choose the correct design basis and recognize the failure mechanism behind a tube or drum problem.
Pressure parts and materials
- Main pressure parts include drums, headers, waterwall tubes, superheater and reheater tubes, economizer tubes and attachment welds. Each has a design temperature and allowable stress from ASME Section II, Part D.
- Common specifications include SA-106 carbon steel pipe, SA-210 boiler tube, SA-213 alloy tube and SA-516 pressure-vessel plate. Alloy selection depends on temperature, steam oxidation and creep strength.
- Creep becomes significant at elevated temperature and is time-dependent; fatigue is cycle-dependent and often starts at welds, openings or attachments. Corrosion-fatigue and thermal fatigue are common at attemperator and header locations.
Design and integrity rules
- ASME Section I sets minimum wall thickness, opening reinforcement, ligament efficiency, staybolt rules and required safety accessories. Higher-strength materials do not remove the need to check temperature-limited allowable stress.
- Welding must follow qualified procedures and performance qualifications under Section IX. Post-weld heat treatment is required by material thickness, chemistry and code rules, not by convenience.
- Non-destructive examination may include radiography, ultrasonic, magnetic particle and dye penetrant. Inspection intervals and remaining-life assessments should reflect operating history, not only calendar age.
Worked example
Using the Section I tube wall form t = PD/(2SE + P) + 0.005D + e, let P = 1,500 psi, D = 3.0 in, S = 15,000 psi, E = 1.0 and e = 0.05 in. The pressure term = (1,500 x 3.0) / (2 x 15,000 + 1,500) = 4,500 / 31,500 = 0.143 in. Add 0.005 x 3.0 = 0.015 in and 0.05 in allowance to get 0.208 in. A minimum wall of 0.220 in would be selected. Using inside diameter or the wrong allowable stress would understate the required wall.
Traps that cost marks
Using Section VIII rules for a Section I boiler; taking allowable stress at the wrong metal temperature; ignoring corrosion allowance; assuming any weld of the same thickness needs the same heat treatment; and diagnosing a creep failure as fatigue without checking temperature and time at load. Also watch for caustic gouging and hydrogen damage in high-heat-flux areas.
Key points: design pressure, temperature and allowable stress must match the code; tube-wall formulas use outside diameter and code allowances; Section IX controls welding; PWHT depends on code and material; failure diagnosis requires operating history.