Boiler Accessories and Instrumentation
This domain tests the equipment that supports the boiler and the instruments operators use to judge water level, pressure, temperature, flow, and flame. Expect accessory function, installation details, calibration, and interpretation of readings.
Accessories versus fittings
Accessories include feed pumps, economizers, air preheaters, superheaters, deaerators, and sootblowers. Fittings include safety valves, gauge glasses, pressure gauges, and water columns. Accessories improve efficiency or supply the boiler; fittings protect it.
Instrumentation
- Drum level may be indicated by a differential-pressure transmitter, conductivity probe, or capacitance probe. The gauge glass remains the independent local check.
- A DP level transmitter uses a reference leg and a variable leg; density changes affect the reading.
- Pressure gauges need a siphon or pigtail on steam service to protect the element from temperature.
- Flame scanners must discriminate flame from hot refractory; O2 analyzers must be leak-free and calibrated.
Worked example: A DP level transmitter is calibrated for water specific gravity 1.00, but the boiler water is 1.05. At the same actual level, the measured differential pressure is 5% higher. At 40 inches actual level, the transmitter indicates about 42 inches. The indicated level is high, while the true level is lower, creating a low-water risk. Correct by calibrating for actual density or applying density compensation, then verify against the gauge glass.
Traps: zeroing a DP transmitter while the drum is pressurized; blocked impulse lines; relying on one level device; a flame scanner sighted on hot refractory; calibration without a traceable standard.