Boiler Construction, Fittings, and Appurtenances
What this domain tests
This domain checks that you know how firetube and watertube boilers are built, which fittings the code requires, and what each fitting does. Items often ask you to identify a fitting from its function, or to state a code limit on set pressure or location.
Construction basics
- Firetube boilers (HRT, firebox, Scotch marine) pass hot gas through tubes surrounded by water; watertube boilers do the reverse and are used for higher pressure and capacity.
- Critical stayed surfaces include the crown sheet, tube sheets, and flat plates; wasted or thinned stays are a rejection item.
- ASME Section I covers power boilers; Section IV covers heating boilers. Fittings must be rated for the boiler's maximum allowable working pressure (MAWP).
- A fusible plug is a low-water warning device on some firetube boilers, not a pressure relief device.
Required fittings and their jobs
- Safety valve or safety relief valve: the final overpressure protection; no valve may be placed between it and the boiler.
- Main steam stop valve, feedwater stop and check valves, and the bottom blowdown valve, which must be outside the boiler and of the quick-opening or two-valve slow-opening type.
- Water column and gauge glass, with try cocks as a backup level indication; the gauge glass has a top and bottom connection and must be blown down and tested.
- Pressure gauge with a siphon (to protect it from steam) and a three-way valve so it can be tested against a master gauge.
Worked example: setting two safety valves
A boiler has an MAWP of 150 psig and two safety valves. The lowest-set valve may not be set above the MAWP, so set it at 150 psig. The highest-set valve may not exceed the MAWP by more than 10%, so it can be set no higher than 165 psig - 160 psig is a common choice. With all valves at their set pressures, the boiler pressure must not rise more than 6% above MAWP (9 psi here, i.e. 159 psig) while the valves discharge the full rated capacity. Reasoning: the code protects two things at once - the lowest valve protects the vessel from ever exceeding MAWP, while the higher setting allows staged capacity without nuisance popping.
Traps that cost marks
- Placing a shutoff valve between the boiler and its safety valve.
- Confusing the water column with the gauge glass; the column slows level surges and holds the cutoff float.
- Forgetting that the siphon protects the gauge and that the three-way valve exists for testing.
- Treating the fusible plug as overpressure protection.