Burner Instability After Gas Regulator Replacement
A 25 MMBtu/h natural-gas burner on a 30,000 lb/h watertube boiler is unstable after a fuel pressure regulator replacement. At high fire, O2 reads 1.2%, CO 900 ppm, stack temperature 510°F, and the flame scanner signal fluctuates. The gas supply pressure is 7 psig at the regulator outlet but drops to 4.5 psig when the burner goes to high fire; the nameplate requires 5-8 psig. The burner linkage was adjusted to look better and the forced-draft fan inlet damper is partly closed. The boiler is at 120 psig and the plant wants to keep it online because the process cannot stop.
1. Diagnose the instability from the O2, CO, gas pressure, and scanner data. What is the most likely combustion problem?
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High CO at 900 ppm with low O2 at 1.2% means the burner is air-starved or the fuel/air ratio is badly matched; CO is not just an efficiency number, it is a sign of incomplete combustion. The gas pressure dropping from 7 psig to 4.5 psig at high fire is below the burner's 5-8 psig requirement, so the regulator or gas train cannot hold capacity. That causes the flame to change shape and the scanner signal to fluctuate. Closing the forced-draft fan inlet damper further reduces air and makes the CO worse. The linkage was adjusted by eye, so the fuel/air cam is no longer coordinated. The combination-low O2, high CO, low gas pressure, unstable flame-is not a tuning nuisance; it is an unsafe firing condition. Do not try to correct it by opening the gas valve or trimming the linkage at high fire.
2. What immediate operating action is appropriate while the process needs steam?
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Reduce firing rate or transfer load to another boiler; do not keep the burner at high fire with a fluctuating flame and 900 ppm CO. If the flame scanner signal is unstable, the burner management system may trip on flame failure-do not bypass it. Check and restore the gas supply pressure to the burner nameplate range, verify the regulator is not undersized or icing, and confirm the forced-draft fan damper and combustion air path are open. Make a limited combustion test at low fire only; if CO remains high or the flame is unstable, shut the burner down. The process need does not justify operating outside the burner's proven envelope. Notify the plant and use the standby boiler or curtail load. Keep the boiler on manual only within safe limits and with a watch.
3. Outline the permanent corrective actions and the combustion targets you would verify.
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Correct the gas train first: verify regulator capacity, lock-up, and pressure drop at maximum firing rate; replace or resize if it cannot hold 5-8 psig at the burner. Restore the fan inlet damper and linkage to the manufacturer's settings, then have a qualified technician set fuel/air on a combustion analyzer across the firing range. For natural gas, a typical target is about 2-3% O2 with CO well below 100 ppm, but the burner manufacturer's curve controls. Verify stable flame scanner signal, acceptable stack temperature, and no CO rise at low fire. Check draft and furnace pressure. Document the settings and re-test after any linkage change. Also confirm the burner management system and gas pressure switches are in service. The boiler should not be returned to automatic high fire until the pressure and combustion tests are stable.