Boiler Construction, Design, Fittings and Appurtenances
Design basis
ASME Section I governs power boiler construction: material specifications, allowable stress at temperature, joint efficiency (which rises with radiography), stayed versus unstayed surfaces, furnace and tube-sheet geometry, and minimum thickness. Heating boilers follow Section IV, which caps pressure and temperature. Every boiler carries a nameplate showing the maximum allowable working pressure (MAWP), capacity, heating surface and stamping - treat it as the controlling document for operating limits.
Required fittings
Every boiler needs safety valve capacity equal to or greater than the maximum steam the burner can generate at MAWP, with no shutoff valve between the valve and the shell and no gag left in place. Also required: a pressure gauge with siphon or pigtail, a water level gauge glass with try cocks, a low-water fuel cutoff, a blowoff valve or two-valve blowdown arrangement, feed and main steam stop valves, and where applicable a thermometer, high-limit control and vent.
Common traps
Watch for safety valves set above MAWP, discharge piping that is undersized or valved, plugged gauge glass connections, and pressure controls used in place of a safety valve. Another favorite: a burner replaced or uprated without re-evaluating safety valve capacity.
Worked rule of thumb
New construction is hydrostatically tested at 1.5 times MAWP, and the safety valve is set at or below MAWP. If the relief capacity is questioned, compare the valve's rated capacity in lb/hr steam against the burner's maximum firing rate.