Boiler Construction, Components, and Fittings
Core concepts
Know the pressure parts and their functions. The shell is the cylindrical body; the drum (steam drum and mud drum in water-tube units) separates steam from water and provides blowdown points; tubes form the heating surface; the furnace or firebox contains combustion; tube sheets and headers support and connect tubes; staybolts brace flat surfaces against pressure. The steam drum internals include the steam separator, baffles, and the dry pipe, which gathers steam away from the water surface to reduce carryover.
Know the fittings required by code. Safety valves (at least one, sized for the maximum firing rate), the main steam stop valve, feedwater check and stop valves, water column and gauge glass, low-water fuel cutoff, pressure gauge with siphon, blowdown valves (surface and bottom), and the nameplate. Understand the water column: it dampens gauge glass turbulence and provides a mounting point for the low-water cutoff; its connections must not be obstructed. Know that the gauge glass shows the true water level only if its connections are clear and the top and bottom fittings are open.
Traps
Questions often test whether a check valve is required on the feedwater line (yes, before the stop valve, so backflow cannot empty the boiler), the purpose of the siphon on a pressure gauge (protect the Bourne tube from steam temperature), and whether the safety valve may be used as a shutoff (never). Another trap: identifying which valve is opened first during blowdown and which is closed first.
Worked example
Rule of thumb: during a bottom blowdown, open the valve nearest the boiler first, then the downstream valve; close the downstream valve first, then the one nearest the boiler, so the line is never left pressurized against the boiler.