Boiler Operation, Combustion, and Controls
What this domain tests
Boiler Operation, Combustion, and Controls covers the start-up and shutdown sequence, furnace purge and light-off, drum level control, combustion control arrangements, and the interlocks that trip the burner. Expect questions on order of operations and on why each step exists.
Sequence and control basics
- Before light-off: confirm normal water level, open the boiler vent if filling, close all drains, then purge the furnace and gas passages with air for the manufacturer's fixed purge time to clear any combustible gas.
- Light-off: igniter first, prove the flame with the scanner inside the trial-for-ignition period, then open the main fuel valve.
- Firing modes: on-off, high-low, or fully modulating; positioning systems include single-point jackshaft, parallel positioning, and metering with oxygen trim.
- Drum level control: single element (level only), two element (level plus steam flow), three element (level, steam flow, and feedwater flow).
- Trips and interlocks: low water, high pressure, flame failure, fan failure, high stack temperature, low atomizing steam or oil pressure.
Worked example
A flue gas analyser reads 3.0% oxygen. Approximate excess air = O2 / (20.9 - O2) x 100 = 3.0 / 17.9 x 100 = 16.8%. Combustion air is therefore about 1.17 times theoretical. Because 3% O2 corresponds to roughly 10-17% excess air, a normal target for a tuned gas burner, the reading indicates the unit is close to tune, and no major air adjustment is justified on oxygen alone.
Traps that cost marks
- Shortening the purge to save time. This is the classic cause of a furnace explosion.
- Bypassing or defeating a flame scanner or low-water trip during testing.
- Confusing high-low firing with modulating control.
- Treating O2 alone as an efficiency figure while ignoring stack temperature.