Low-Fire CO and Flame Scanner Instability After Burner Tune
A 30,000 lb/hr packaged watertube boiler fires natural gas with parallel-positioning fuel/air linkage and O2 trim that has been left in manual for six months. After a contractor "tuned" the burner at high fire, operators report rumble and rising CO at low fire. At 3,000 lb/hr you measure 9.2% O2, 340°F stack temperature, 480 ppm CO, and a flame scanner signal fluctuating between 2.5 and 4.0 V (normal 4.5-5.0 V). The forced-draft fan inlet damper linkage has about 1/2 in. of lost motion, the furnace draft is -0.02 in. wc, and the low-fire air damper is set at 25% open. The boiler is on a winter load swing and cannot be taken offline for two weeks; the operator proposes raising gas pressure at the burner to stabilize the flame and leaving O2 trim in manual.
1. Diagnose the likely causes of the high CO, low scanner signal, and rumble at low fire. Which measurements support your diagnosis?
Read the worked answer
The combination points to unstable low-fire combustion caused by air/fuel mismatch and poor linkage repeatability, not simply a bad scanner. At 3,000 lb/hr the boiler shows 9.2% O2, which is high excess air for gas firing, yet CO is 480 ppm and rising. High excess air at low fire cools the flame and can quench CO oxidation, especially with a burner near its turndown limit. The 1/2 in. lost motion in the FD damper linkage means the air damper may not return to the same position each time; the 25% low-fire air setting is likely too far open for stable low-fire flame. The scanner signal fluctuating between 2.5 and 4.0 V is consistent with an unstable flame envelope, not a steady flame with a defective scanner. The furnace draft of -0.02 in. wc is weak and may allow products of combustion to linger or distort the flame. Rumble suggests oscillation between fuel-rich and air-rich pockets.
2. Explain why raising burner gas pressure could make the situation worse rather than better.
Read the worked answer
Raising gas pressure changes the fuel flow relationship at every firing rate, but the air linkage is already suspect and O2 trim is in manual. More fuel pressure can push the burner toward a fuel-rich condition at low fire, increasing CO and unburned hydrocarbons, lengthening the flame, and possibly causing flame impingement or furnace puffback. It can also move the burner outside the stable flame envelope and make the scanner signal less reliable. The root problem is repeatable air/fuel positioning and low-fire stability, not insufficient gas pressure. If gas pressure is raised without re-establishing the fuel/air curve, the burner may pass through a combustible mixture condition during load changes. The correct approach is to repair the linkage, establish the low-fire air position, and retune with a portable analyzer; gas pressure should be adjusted only within the manufacturer's specified range and only after the air side is stable.
3. Develop a safe two-week operating plan and the permanent correction. Include acceptance criteria.
Read the worked answer
For the interim, avoid sustained low-fire operation where CO and rumble occur. Raise the minimum firing rate if the load permits, or cycle the boiler with another unit so it does not sit at 3,000 lb/hr. Repair or temporarily secure the FD damper linkage so it cannot slip, and set the low-fire air damper to the burner manufacturer's recommended position. Place a portable CO analyzer on the stack and a CO monitor in the boiler room; establish an alarm and a written limit, and trip or reduce firing if CO rises further. Keep O2 trim in manual and tune by load blocks with a portable analyzer. Permanent correction: replace worn linkage, verify damper position feedback, retune the burner across its full range, place O2 trim back in automatic only after the curve is stable, and confirm scanner signal is steady above the manufacturer's minimum at all firing rates. Acceptance should include stable flame, CO below the plant limit, scanner signal at or above normal, and no rumble.