Cold Startup Exceeds Drum Differential Limits
A 25 t/h water-tube boiler at 4.0 MPa is being started cold after an annual inspection. Drum wall thickness is 90 mm; initial drum metal temperature is 30°C. The operating procedure requires a drum metal temperature rise below 1.5 K/min and a drum differential below 50 K. The operator fires at 40% rate to meet a promised steam time. After 30 minutes, drum pressure is 0.6 MPa, the upper drum metal is 190°C, the lower drum metal is 120°C, and the gauge glass is at +60 mm. The superheater outlet drain is closed, and the economizer bypass is open. The boiler has a low-water trip and a high-pressure trip, but the drum differential alarm is inhibited during startup.
1. Interpret the drum temperatures and pressure, and identify the main risk.
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At 0.6 MPa gauge, the saturation temperature is about 159°C. The upper drum metal at 190°C is hotter than saturation, while the lower drum metal at 120°C is well below it. The differential is 190 − 120 = 70 K, which exceeds the 50 K limit. The average metal temperature rise is also high: the drum has moved from 30°C to an average of about 155°C in 30 minutes, roughly 4 K/min, far above the 1.5 K/min procedure limit.
The main risk is differential thermal expansion and high thermal stress in the drum shell and ligaments. The upper half, in contact with steam, heats and expands faster than the lower half, which is cooled by water and by any cold feedwater entering. Thick drum walls make the through-thickness gradient worse. Continued fast firing can lead to drum cracking, tube-to-drum weld damage or leakage. The high water level at +60 mm also reduces steam space and increases carryover risk, and the inhibited differential alarm removes the automatic warning that should have prompted action.
2. What immediate actions should the operator take, and which actions should be avoided?
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Cut firing back immediately to reduce the pressure rise and the heat input to the drum. Do not open the superheater drain abruptly if it is closed and the superheater is hot; follow the procedure for draining and cooling. Maintain drum level at the normal midpoint, not at +60 mm; reduce feedwater flow smoothly and avoid slugging cold water into the drum. Open the drum vent only if the procedure calls for it at this pressure; do not vent a pressurized drum without authorization. Increase monitoring: read upper and lower drum metal temperatures, saturation temperature, pressure ramp and level every few minutes.
Avoid the common mistakes of increasing feedwater flow to control temperature, dumping the boiler completely, or continuing to fire because steam is promised. If the differential continues to rise after firing is reduced, or if pressure cannot be held, shut down the burner and allow controlled cooling. The drum differential alarm should be restored as soon as possible; an inhibited alarm during startup is acceptable only with a documented alternative monitoring routine.
3. What should be corrected in the startup procedure before the next cold start?
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The procedure should include a defined pressure-temperature ramp, not just a firing rate. For this drum, 1.5 K/min average metal rise and a 50 K differential limit imply a much slower pressure rise than 0.6 MPa in 30 minutes. The operator should have a table of allowable pressure versus time and a requirement to log upper and lower drum metal temperatures. The differential alarm should be active, with a temporary lower alarm limit during startup rather than being inhibited.
The procedure should also require the superheater drain and vent lineup to be verified before firing, specify when the economizer bypass is opened or closed, and set a maximum drum level deviation. The promised steam time should be scheduled from the achievable ramp, not the other way around. After this event, inspect the drum for cracking and check the drum-to-tube welds before returning the boiler to service. A cold start after an inspection is exactly when the drum is most vulnerable, so the startup curve and alarm configuration are safety controls, not administrative details.