Cold Startup of a Watertube Boiler in Winter
A 500 hp packaged watertube boiler, 250 psig design, has been shut down for annual inspection for two weeks in January. The boiler room is 35°F and outdoor air is 5°F. The boiler has a superheater, an economizer, and a low-water fuel cutoff with a float chamber. Before startup, the operator finds the drum water level at the bottom of the gauge glass, the superheater drain valves closed, the economizer bypass closed, and the gas train purge fan not yet run. The plant needs steam in four hours. The manufacturer's manual limits pressure raise to 100 psi/hour and drum metal temperature differential to 100°F. The deaerator is at 20 psig and 250°F. The main steam header is at 180 psig and has been kept warm. The operator wants to light the burner at 50 percent firing rate to save time.
1. List the checks and preparatory steps that must be completed before the first light-off.
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The boiler must be filled to the normal water level with properly treated, deaerated water, and the gauge glass blown down and refilled so the level is proven. Because the room is near freezing, verify that the low-water cutoff float chamber, gauge glass piping, and any exposed sensing lines are not frozen; a frozen float chamber can give a false normal reading. Open the superheater drains so steam can flow through the superheater during warm-up, and place the economizer in service with flow through it or use the bypass according to the manufacturer's cold-start procedure. Purge the gas train and furnace for the full required time before trial for ignition. Check the burner management system, flame scanner, fuel pressure, and all safety interlocks. Confirm the drum vents are open until steam appears. Do not light off at 50 percent firing: a cold watertube boiler needs a low firing rate and controlled warm-up to limit differential expansion.
2. What warm-up sequence and firing limits should be used, and what symptoms indicate the limit is being exceeded?
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Start at the minimum stable firing rate, usually 10 to 25 percent, and hold it while the drum and headers warm gradually. Use the manufacturer's pressure-raising limit of 100 psi/hour, but the tighter control is drum metal temperature differential, which must stay below 100°F. Monitor drum shell temperature at top and bottom, superheater metal temperature, and expansion indicators. A rapid rise in drum differential, popping or groaning from expansion joints, steam from a drum vent that stops too soon, or water level surging indicate warm-up is too fast. Bottom blowdown can be used briefly to promote circulation and equalize drum temperature, but only after the water level is stable. Do not rush to 50 percent firing to meet the four-hour schedule; a thermal shock or tube-joint leak will cost far more time than a controlled warm-up.
3. What must be done before and during paralleling with the 180 psig steam header?
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Before paralleling, the boiler pressure should be brought slightly above header pressure, and the main steam stop should be opened slowly to warm the line and equalize pressure without a sudden surge. Drain the main steam line and superheater drains before admitting steam, and verify that the non-return valve is free. During paralleling, watch drum level closely because steam demand can swell the level, and watch steam temperature and superheater metal temperature. After the boiler is on line, place the feedwater control in automatic only after level is stable, and keep the low-water cutoff and flame safeguards in service. The boiler should not be left unattended during the initial load increase; the operator should stay until pressure, level, and flame are stable and the boiler is sharing load predictably.