High CO and Low O2 After a Gas Regulator Change
A 500 hp natural-gas firetube rated 150 psig and 21 MMBtu/hr high fire serves a milk-drying plant. Three months ago the burner was tuned to 3.0% O2, 60 ppm CO, and 380 deg F stack temperature at high fire. Last week the gas utility replaced an upstream regulator during a curtailment. Since then the flame scanner signal has fluctuated between 8% and 15%, the burner rumbles at high fire, and a portable analyzer shows 2.2% O2, 1,300 ppm CO, and 460 deg F stack at high fire; breeching draft is -0.08 in w.c. and gas pressure at the burner train is 4.5 in w.c. against a design of 7.0 in w.c. The boiler is staying on line because the plant needs steam, and the operator has been slowly opening the air damper to 'lean it out.' The plant has no CO monitors in the boiler room and keeps the door propped open.
1. Interpret the readings. What is the most likely explanation and what is the immediate risk?
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The readings point to a fuel/air mismatch, not a simple need for more air. O2 at 2.2% is low, meaning very little excess air, while CO at 1,300 ppm shows incomplete combustion; a gas flame with low excess air can still make CO if the burner is not mixing properly. Gas pressure at 4.5 in w.c. against a 7.0 in w.c. design means the burner is not receiving its rated input and the supply is unstable after the regulator change, so the linkage and gas valve may be operating off their calibrated curve. Stack temperature up 80 deg F is consistent with poor combustion and possible afterburning in the breeching. The rumbling and fluctuating flame signal suggest an unstable flame shape. The immediate risks are carbon monoxide in the boiler room and a flame failure that lets unburned gas accumulate; a propped door is not ventilation, and there are no CO monitors.
2. What immediate actions do you take?
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Take the boiler off automatic and reduce to low fire; if the flame is unstable or CO is present in the room, shut it down and ventilate. Keep people out of the boiler room until a calibrated CO monitor shows safe levels, and put a personal CO monitor on the operator. Do not continue opening the air damper as a tuning shortcut; it can cool the flame, increase CO, or cause flame loss. Notify the gas utility that its regulator change altered burner supply pressure and request verification of delivered pressure and regulator capacity. Have a qualified burner technician verify the gas train, orifice pressure, linkage, and scanner sighting. Do not leave the boiler on line in automatic with an 8 to 15% flame signal; a low, fluctuating signal increases the chance of a fuel trip and a subsequent unsafe lightoff attempt.
3. What is the correct repair and verification plan?
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Repair and verify in sequence: restore design gas pressure at the burner train or re-rate the burner for the available pressure, and do not adjust the linkage to compensate for low gas pressure. Inspect the linkage, setscrews, and damper for slippage, and check the diffuser and the flame scanner lens and cooling. Then perform a full combustion setup at low, mid, and high fire with a calibrated analyzer, confirming O2, CO, CO2, and stack temperature at each point; typical targets for this type of gas burner are roughly 3 to 5% O2 with CO below about 100 ppm, but the manufacturer's curve governs. Verify flame signal strength and stability, test flame-failure response, and confirm draft at the breeching. Record before-and-after readings and recheck after 24 hours of operation.