Boiler Construction, Fittings, and Accessories
What this domain tests
Boiler Construction, Fittings, and Accessories asks you to identify boiler parts, explain how each fitting protects the boiler, and separate a fitting from an accessory. At Class 5 the questions are largely descriptive and vocabulary-driven, so precise names matter more than design calculations.
Construction basics
- Firetube: hot gases pass through tubes surrounded by water (HRT, Scotch marine, vertical). Watertube: water inside tubes, fire outside; used for higher pressure and capacity.
- Pressure parts: shell or drums, tubesheets, furnace, mud drum, staybolts, handhole and manhole plates and gaskets.
- Setting: refractory protects the casing from radiant heat; insulation limits shell heat loss to the room.
Fittings versus accessories
- Fittings mount on the boiler for safety and operation: safety valve(s), pressure gauge with siphon, water gauge glass and try cocks, low-water fuel cutoff, feedwater check and stop valves, main steam stop, bottom and surface blowdown valves.
- Accessories are auxiliary equipment: economizer, air preheater, superheater, sootblower, feedwater regulator, draft fans.
Worked example
A firetube boiler has 1,800 ft2 of heating surface. What is the minimum number of safety valves? ASME Section I requires at least one safety valve and two or more when heating surface exceeds 500 ft2. Since 1,800 is greater than 500, two valves are the minimum, sized so that together they discharge all the steam the boiler can generate at Maximum Allowable Working Pressure. The set pressure is stamped on the valve and must not exceed the boiler MAWP.
Traps that cost marks
- Calling the safety valve an accessory. It is a fitting and the last line of defence, never a control valve.
- Forgetting the siphon on the pressure gauge, which keeps live steam off the Bourdon tube.
- Reversing gauge glass connections: the lower connection must have a clear, unrestricted path to the water.