Forced-Draft Fan Trip on a Balanced-Draft Boiler
A 250,000 lb/h balanced-draft industrial boiler is running at 85% load with a forced-draft fan and an induced-draft fan. The FD fan trips on an overcurrent relay at 14:20. The ID fan remains running. The burner management system does not initiate a master fuel trip because the FD fan status signal is wired to a auxiliary contact that chattered rather than a true run-status interlock. Furnace pressure swings to −3 in. w.c. and then toward +0.5 in. w.c. as the ID fan pulls against a furnace with no combustion air supply. The operator sees the FD fan ammeter at zero and the furnace pressure alarm annunciating.
1. Identify the interlock deficiency and the immediate hazard, and state the correct operator action.
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The interlock deficiency is that the FD fan run-status signal is taken from a chattering auxiliary contact rather than a proven-running interlock (current or airflow proof). The burner management system therefore never saw a loss of combustion air and did not trip the fuel. That is a code-level defect: NFPA 85 requires a master fuel trip on loss of combustion air, and a status contact that can chatter is not a proven signal.
The immediate hazard is a furnace explosion. With fuel still entering the furnace and no FD air, the ID fan is pulling air in through any opening it can find - burner registers, observation ports, the air preheater - and the fuel/air mixture in the furnace is uncontrolled. The pressure swing from −3 in. w.c. to +0.5 in. w.c. shows the furnace is alternating between air-starved and fuel-rich, which is exactly the condition that produces a delayed ignition.
Correct operator action:
- Trip the fuel immediately at the manual trip, do not wait for the BMS.
- Trip the ID fan after fuel is proven off, to stop pulling air through an uncontrolled path.
- Do not attempt to relight until the furnace has been purged per the BMS purge procedure and the FD fan interlock has been repaired and proven.
- Tag and lock out the FD fan and initiate a management-of-change review of the interlock wiring before restart.
The judgement call: the operator who sees the ammeter at zero and reaches for the FD fan reset instead of the fuel trip has misread the hazard. Loss of combustion air with fuel still on is a trip-first, diagnose-later event.
2. After the unit is secured, what purge and restart conditions must be met, and how would you verify the interlock repair?
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The furnace must be purged before any relight. The purge requirement is a proven volume exchange - typically five furnace volumes at the purge airflow, with all fuel valves proven closed, the igniter off, and the FD and ID fans both running and proven. The purge timer must be started only after airflow is proven, not when the fan starter is energized.
Restart conditions:
- FD fan proven running by airflow or current proof, not by an auxiliary contact.
- ID fan proven running and furnace pressure under control at the normal setpoint (typically −0.1 to −0.5 in. w.c.).
- All fuel valves proven closed and the fuel header leak test passed.
- BMS purge complete and the flame scanner proven seeing no flame before light-off.
- Drum level and feedwater in normal range before firing.
Verifying the interlock repair:
- Functional test - with the burner firing, simulate an FD fan trip and confirm the master fuel trip actuates within the required time and the fuel valves close.
- Check the proof device - confirm the run-status signal is now taken from a current switch or airflow switch, not a starter auxiliary contact.
- Document the test and update the interlock schedule and the BMS logic drawing.
The exam point: a purge is not a timer; it is a proven volume exchange with fuel proven off. And an interlock repair is not complete until it has been functionally tested under firing conditions.