Intermittent Flame Failure After Scanner Replacement
A 600 hp firetube boiler fires natural gas through a forced-draft, modulating burner. After a contractor replaced the flame scanner and cleaned the burner, the boiler locks out on flame failure about one start in four. The flame signal at low fire is 1.8 VDC and jumps to 4.5 VDC at high fire; the scanner amplifier normally reads 4–6 VDC. The pilot lights with a visible blue flame, but the scanner sight tube is warm and the sight glass is slightly sooty. A portable combustion analyzer at high fire shows 3.0% O2, 18 ppm CO, and 9.5% CO2; stack temperature is 410°F. The burner management system is a microprocessor-based package with a 10-second trial for main flame and a 4-second pilot trial. The operator wants to increase the pilot trial time to stop nuisance trips. The plant needs steam by 07:00.
1. Evaluate the proposed fix of increasing the pilot trial time. What is the likely cause of the intermittent lockouts?
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Increasing pilot trial time is not a repair; it extends an unsafe condition and may violate the burner manufacturer's listed timing and the flame-safeguard standard. The marginal flame signal at low fire, 1.8 VDC versus a normal 4–6 VDC, points to a flame-detection problem: a sooty sight glass, wrong scanner angle, partially blocked sight tube, scanner cooling-air problem, moisture, a failing scanner or amplifier, or poor grounding and shielding.
The pilot may be stable visually, but the scanner may be seeing only part of the flame. The acceptable high-fire O2 and CO readings do not prove low-fire stability; low fire is often richer and the flame can lift or impinge. The intermittent pattern of one start in four suggests a marginal signal rather than a complete failure. The correct approach is to find and correct the detection problem, not to lengthen the trial time.
2. Outline a step-by-step diagnostic and correction plan, including what to verify before the next start.
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Lock out the burner. Read and record the BMS fault code and flame signal at the moment of trip. Clean the scanner sight glass and lens, and verify the sight tube is clear and aimed at the pilot and main flame as listed. Check scanner cooling air and temperature. Verify scanner supply voltage, cable shielding, and grounding; substitute a known-good scanner or amplifier if available.
Check pilot gas pressure, pilot orifice, ignition electrode gap, ignition transformer, and flame rod if used. Confirm the BMS pilot and main-flame trial times match the manufacturer's data. Perform a pilot turndown test and verify the scanner detects the smallest stable pilot. Then start and observe flame signal at low, mid, and high fire. Do not change trial times to mask the problem.
3. What combustion and safety checks should be repeated after repair, and what records should be kept?
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After repair, run combustion analysis at low, mid, and high fire: O2, CO, CO2, stack temperature, and smoke or NOx as applicable. Confirm CO is low and O2 is within the burner's range at each firing rate, not just at high fire. Verify flame signal is stable with margin at all rates and through modulation.
Prove the flame-failure response by closing the pilot gas valve or using the BMS test function, and confirm a lockout within the listed trial time. Record all readings, repairs, and test results. Repeat at least three consecutive successful starts before releasing the boiler. The records should show that the scanner sees a stable flame at every firing rate and that the safety shutdown still works.