Cold Start After a 14-Month Layup
A 60,000 lb/hr, 350 psig gas-fired watertube boiler was laid up dry for 14 months with the burner and controls left in place. It is now being started for winter. On light-off the forced-draft fan runs and the purge timer is set for 30 seconds; the pilot lights and the main flame ignites, but the flame scanner signal is 2.0 V against a 4.5 V baseline and the flame safeguard trips on 'no flame' about four seconds after main flame ignition. Stack oxygen at low fire reads 11% with CO at 500 ppm. The operator proposes to jumper the scanner, lengthen the purge to 60 seconds, and go straight to high fire to see whether the problem clears. The low-water fuel cutoff has not been tested since 2023, and no combustion test has been run since the layup.
1. Evaluate the operator's proposal. What is acceptable and what must not happen?
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Nothing in the proposal is acceptable as written except the instinct to lengthen the purge. Jumpering a flame scanner is never permissible: the scanner is the only device standing between a fuel-rich furnace and an explosion, and a jumpered scanner defeats the flame safeguard's entire purpose. Going straight to high fire to 'see if it clears' is also wrong, because an unstable low fire will be worse at high fire, and the boiler has not had its interlocks proven since layup.
A 30-second purge on a 60,000 lb/hr unit is likely too short to give the required number of furnace and convection-pass air changes; that should be calculated and set per the burner manufacturer's instructions and NFPA 85, not guessed at 60 seconds. The low scanner signal deserves diagnosis, not bypass: check the sighting tube for moisture or soot after a 14-month layup, clean the lens, verify cooling air, confirm the scanner is the correct type for the fuel, check flame position and gas pressure at the burner, and confirm the scanner's amplifier and setpoints are correct. The 11% oxygen and 500 ppm CO say the burner is running with far too much excess air and poor mixing - a tuning issue to be corrected before the unit is pushed.
2. List the pre-start checks this boiler actually needs after 14 months dry, and explain why each matters.
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After a long dry layup the boiler should be treated as a new installation. Internally, verify the drums and tube surfaces are clean and free of desiccant residue, tools, and debris, and confirm all manhole and handhole gaskets are new and correctly torqued. Externally, check for corrosion under insulation, settling, and support damage, and confirm the safety valves are in date, untagged, and piped correctly.
On the fuel and controls side: leak-test the gas train, verify all interlock switches (low gas pressure, high gas pressure, low air, high stack temperature) by actually tripping them, prove the purge timer and its airflow switch, and confirm the flame safeguard is doing a proper trial for ignition and lockout. The low-water fuel cutoff must be tested - with the burner firing, by evaporation or slow-drain method per the manufacturer - because a float chamber that sat idle for 14 months is exactly where a stuck or sludge-bound mechanism hides. Exercise the feedwater and main steam stop valves and verify the gauge glass is clear and valved correctly.
Finally, fill with deaerated water at the correct temperature, vent the superheater if fitted, and confirm the drum level instrumentation agrees with the glass before any burner is started.
3. Once the flame proves and the boiler is running, how would you bring it up and tune it without damaging the unit?
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Bring it up slowly and in stages. Hold low fire until the flame signal is stable and the furnace has warmed; a watertube that has been cold for 14 months should be warmed at a controlled rate - commonly on the order of 50 to 100 °F per hour for the water and metal, within the manufacturer's limits - with the drum vents open until steam appears, then closed. Watch drum level and differential expansion, and avoid rapid load swings or repeated bottom blows.
Tune the burner with a combustion analyzer at low fire first: reduce excess air to the manufacturer's range (typically around 3 to 5% oxygen dry for natural gas) while keeping CO low - the current 11% O2 and 500 ppm CO indicate both too much air and poor mixing. Establish a stable low fire, then step through mid and high fire, recording O2, CO, stack temperature, and flame signal at each point, then recheck low fire to confirm the linkage or curve is repeatable. Confirm the scanner signal at every firing rate, not just the one that happened to prove.
Only after the burner is tuned and all interlocks have been proven should the unit be handed over to automatic control, and the inspector should see the completed test records before the boiler carries winter load.