CO Rise After Burner Linkage Overhaul on a 600-hp Gas Boiler
After a burner linkage overhaul on a 600-hp, natural-gas watertube boiler, the low-fire combustion readings are O2 1.2%, CO 850 ppm, and stack temperature 410°F. The flame scanner signal fluctuates between 2.0 and 3.0 V, and the burner has tripped on flame failure three times in two shifts. High fire reads O2 3.1% and CO 35 ppm. The burner technician wants to raise the low-fire O2 setpoint to 5% and install a jumper on the flame scanner to 'get through the week.' The boiler is supplying a food-processing line that cannot tolerate another trip. The breeching draft at low fire is -0.10 in. w.c.
1. Interpret the O2/CO/flame-signal data and state immediate operator actions.
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The low-fire O2 of 1.2% is too low for stable clean combustion, and the CO of 850 ppm confirms incomplete combustion. High-fire readings are much better, so the problem is concentrated at low fire, likely in the linkage adjustment, fuel pressure, air damper position, or flame pattern. The fluctuating flame signal and repeated flame failures point to an unstable flame at low fire, not simply a bad scanner. Immediate actions are to stop the burner, purge the furnace, and not restart until the low-fire combustion is corrected. The boiler should be taken off line or kept at a stable higher fire only if the burner can be proven safe across the required range. The operator should record the readings, notify the supervisor, and have the combustion technician recheck the low-fire cam, air/fuel linkage, gas pressure, and scanner sighting.
2. The chief wants to raise low-fire O2 to 5% and jumper the flame scanner to get through the week. Assess that plan.
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Raising O2 may reduce CO, but it does not correct the underlying cause and may increase stack loss and instability. More importantly, jumpering the flame scanner defeats a safety device. The scanner is there to prove flame and trip the fuel valve if flame is lost. A jumper can allow fuel to enter an unignited furnace, creating an explosion hazard. The repeated flame failures indicate that the burner is not reliably established at low fire; bypassing the scanner would hide that condition rather than solve it. The correct decision is to keep the boiler out of service or restrict it to a proven stable firing range while the burner is readjusted and the scanner is checked for sighting, cooling, contamination, and signal strength. Production pressure does not justify defeating a flame safeguard.
3. Outline the combustion-side corrective work and the verification tests.
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The technician should start from the burner manufacturer's setup data and verify the low-fire cam and linkage positions, gas pressure at the burner, air damper position, and furnace draft. The scanner should be cleaned, correctly aimed at the flame, and checked for adequate signal with a stable flame. The burner should be purged and started at low fire, then checked with a calibrated combustion analyzer for O2, CO, and stack temperature. The firing rate should be increased in steps to high fire and back down to low fire to confirm repeatability and acceptable CO at every point. The flame signal should be stable and above the manufacturer's minimum. The low-water cutoff, flame safeguard, and other interlocks should be tested after the work. The results should be recorded, and the boiler should not be returned to normal service until the low-fire CO is reduced to a safe level and the flame failures stop.