Boiler Accessories and Safety Devices
This domain asks you to explain how accessories support combustion and steam generation while safety devices prevent unsafe conditions. Know each accessory's function: feed pumps move water against boiler pressure; economizers recover heat from flue gas to preheat feedwater; air preheaters warm combustion air; draft fans supply or exhaust gas; sootblowers clean heat-transfer surfaces; steam traps remove condensate without losing steam.
Safety devices include low-water fuel cutouts, high-pressure and high-temperature limits, flame scanners, fuel pressure switches, and relief valves. Understand the low-water cutout logic: it must interrupt burner firing when the water level falls below the lowest safe level, and it should be tested regularly-often by blowdown-to confirm it trips. Know that a flame scanner must prove flame before and during firing; loss of flame signal shuts off fuel. High-pressure limits should be set below the safety valve setting so the burner stops before the relief valve lifts.
Traps: assuming a low-water cutout opens the feed valve (it does not-it stops firing); confusing a safety valve with a relief valve; thinking an economizer adds steam capacity rather than recovering heat; and forgetting that a sootblower can cause a sudden draft or steam surge if used incorrectly. Watch for questions that ask which device acts first-the control limit, not the relief valve, should respond first.
Rule of thumb: control devices act before protective devices; protective devices act before the safety valve.