Boiler Operation, Combustion, and Combustion Controls
Domain focus
This domain tests safe firing, furnace purge, light-off, flame supervision, fuel/air control, and trip logic. Class 1 questions often ask what must happen before fuel is admitted and what must happen when a flame or limit fails.
Core rules
- NFPA 85 provides the recognized logic for boiler combustion systems: pre-purge, ignition, flame detection, and master fuel trip (MFT).
- A furnace purge must move a defined volume of air through the furnace before light-off; purge credit is limited and must be documented.
- Flame scanners must prove flame at each burner or group as required; a failed scanner causes a fuel trip, not an operator bypass.
- Fuel/air ratio must be controlled through load changes; excess O2 too low creates CO and unburned fuel, too high wastes heat.
- The MFT must close fuel valves and trip burners on unsafe conditions such as low water, high pressure, flame failure, or loss of combustion air.
Worked example
Dry flue gas shows 3.0% O2. Estimate excess air: %EA = O2/(20.9 − O2) × 100 = 3.0/(17.9) × 100 = 16.8%. The reasoning: 20.9% is the O2 in air; the O2 left after combustion represents air beyond the stoichiometric requirement. This estimate assumes dry gas and no significant CO; a real test uses fuel analysis and corrected O2. If O2 rises to 5%, excess air becomes 31.4%, raising stack loss.
Traps
Treating a purge as a timer only; bypassing a flame scanner; lighting a burner without proving purge; using O2 alone when CO is high; and resetting an MFT without finding the cause.