Cold Start of a Watertube Boiler with a Rising Drum Differential
A 500 hp watertube boiler, rated 250 psig, is being returned to service after a two-week outage for economizer tube replacement. It has a superheater, a deaerator, and a balanced-draft furnace. Outdoor air is 40°F; the upper drum metal is 55°F, and the refractory is 60°F. The site procedure limits drum metal heating to 100°F/hr and the difference between top and bottom drum metal to 50°F. The superheater outlet valve is closed; the superheater drains are open; the economizer has a recirculation line. The operator plans to light the burner at 25% firing and reach 100 psig in two hours, then open the main steam stop. After 30 minutes at low fire, the bottom drum metal is 35°F hotter than the top and rising at about 150°F/hr. The stack draft is −0.15 in. wc, and the flame scanner signal is stable. A steam line drain is open, but the line is still cold.
1. List the sequence you would follow from cold standby to first firing, including purging and superheater/economizer protection.
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Before lighting off, confirm the boiler is filled to normal level, all air vents and superheater drains are open, the economizer recirculation is running, and the furnace and breeching have been inspected. Purge the furnace and gas passages per the manufacturer-typically at least five volume changes-and prove draft and air flow.
Light the pilot, prove its flame, then establish main flame at minimum fire. Hold low fire and monitor drum metal temperatures, expansion, water level, and stack temperature. Keep superheater drains open until steam flow through the superheater is established. Warm the main steam line through its drains before opening the stop valve.
The planned two-hour rise to 100 psig must be checked against the 100°F/hr limit; cold metal and refractory may require a slower schedule. Do not open the main steam stop on a cold, undrained line.
2. At 30 minutes the bottom drum metal is 35°F hotter than the top and rising at 150°F/hr. What do you do and why?
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A 35°F top-to-bottom difference is still inside the 50°F limit, but a 150°F/hr rise exceeds the 100°F/hr procedure limit, and the trend is wrong: the bottom is hotter than the top. That suggests stratification, poor circulation, or a thermocouple or installation error. I would reduce firing immediately, hold at minimum fire, and verify the drum thermocouples and water level.
Do not open the main steam stop or increase load. If the differential continues to climb or reaches 50°F, shut down and investigate before proceeding. I would not use bottom blowdown to chase the reading; on a watertube boiler, blowdown changes level and circulation and can worsen thermal stress unless the manufacturer’s procedure calls for it. The startup should resume only when the heating rate and differential are back within limits and stable.
3. The steam line is cold and the operator hears banging when the header valve is cracked. Explain the correct warm-up and drain procedure.
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Banging in a cold steam line is water hammer from condensate. The line must be drained and warmed slowly before the stop valve is opened. Keep all line drains and superheater drains open; crack the stop valve just enough to admit a small amount of steam, let the line heat gradually, and keep the drains open until the line is hot and dry.
If banging continues, close the valve, let the line drain, and inspect for low points, failed traps, or undrained sections. Do not slam the valve open to “blow through” the water; that can drive condensate slugs into the header or turbine. After the line is warm, open the stop valve slowly while watching drum level and pressure. Record the warm-up time and drain status.