Boilers and Boiler Fittings: Construction, Ratings, and Protective Accessories
What this domain tests
The Boilers and Boiler Fittings domain checks whether you can identify boiler types and their internal circulation paths, apply the ASME Section I rules that govern allowable working pressure and fittings, and recognise which protective device performs which function. Questions tend to be device-identification and code-rule items rather than deep heat-transfer theory.
Core rules and concepts
- Allowable working pressure is set by the weakest pressure part and the material's allowable stress at design temperature; stamping on the shell or heads is the authoritative source.
- Safety valves protect against overpressure. A boiler needs at least one, sized to discharge all steam the boiler can generate without pressure rising more than 6% above the set pressure. The highest-set valve is the controlling valve; lower-set valves are easing valves and must not be gagged.
- Low-water fuel cutoff protects against overheating from loss of water. It is a level device, not a pressure device, and on steam boilers it must be tested by blowing down the float chamber while firing, or by a slow-drain test on some designs.
- Water column and gauge glass: the gauge glass must have a visible connection to the water column, with try-cocks if fitted. Blowdown of the gauge glass confirms both connections clear.
- Attemperation (desuperheating) controls final steam temperature; spray attemperators inject feedwater between superheater stages.
- Circulation: natural circulation relies on the density difference between the steam-water mixture in the risers and water in the downcomers. Forced circulation uses a pump.
Worked example
A firetube boiler is stamped for 1034 kPa (150 psi) MAWP. The safety valve is set at 1034 kPa and a second valve is set at 965 kPa. What is the maximum permissible pressure rise during a full-capacity fire test?
Reasoning: ASME Section I allows the pressure to rise to 6% above the highest set pressure, not the lowest. 1034 x 1.06 = 1096 kPa (159 psi). If the test shows 1100 kPa, the valve is undersized or obstructed and must be corrected before the boiler is accepted.
Traps that cost marks
- Confusing the low-water cutoff (level protection) with the low-water alarm (warning only).
- Assuming the lowest-set safety valve governs the 6% allowance; it is the highest-set valve.
- Forgetting that a gauge glass blowdown proves the water and steam connections, not the water column's own blowdown line.
- Mixing up firetube and watertube circulation: in a firetube boiler, hot gases pass through tubes surrounded by water; the reverse is true for a watertube boiler.