Cold Startup With Elevated Differential Expansion
A 150 MW steam turbine is being started cold after a 10-day outage. During the roll-off and low-speed warm-up, the operator sees the high-pressure rotor-to-casing differential expansion reach +4.2 mm against an alarm of +4.5 mm and a trip at +5.0 mm. The casing metal temperature is 120 °C and rising slowly; the rotor is heating faster than the casing. Main steam temperature is 320 °C at 40 bar, and the recommended steam-to-metal temperature margin is at least 50 °C. Vibration is normal. The operator is under pressure to reach synchronous speed before the end of the shift.
1. Explain the mechanism causing the differential expansion and whether continuing is acceptable.
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Differential expansion occurs because the rotor heats faster than the heavy casing during a cold start. The rotor is longer and has less thermal mass in the steam path, so it expands axially ahead of the casing. At +4.2 mm the machine is close to the +4.5 mm alarm and within 0.8 mm of the trip. Continuing to increase speed adds steam flow and heat input, which will drive the differential further.
Main steam at 320 °C against casing metal at 120 °C gives a 200 °C margin, well above the 50 °C minimum, so the steam is not the immediate problem; the problem is the rate of heating and the mismatch between rotor and casing. Continuing at the current rate is not acceptable without action, because the trip margin is small and an automatic trip at speed would be an uncontrolled event. The shift-end pressure is irrelevant to the mechanical limit.
2. What actions would you take to control differential expansion while progressing the start?
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I would hold speed and allow the casing to catch up, using the warm-up and soak period rather than pushing through. I would reduce or stabilize steam temperature and flow to slow rotor heating, confirm the steam admission and drain paths are correct, and verify that the casing drains are open so the casing heats evenly. I would check that the shaft grounding and rotor position measurements are valid and not drifting.
If differential expansion continues to rise toward the alarm, I would reduce speed or hold at the current point and consider shutting down steam admission to let temperatures equalize. I would not increase speed until the differential is stable and trending down. The goal is to reach synchronous speed with margin, not to meet a shift deadline.
3. What records and communications are required, and what would you tell the next shift?
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I would record the differential expansion trend, casing and rotor metal temperatures, steam temperature and pressure, speed, and vibration throughout the hold. I would document the decision to hold and the reason, and notify the shift supervisor and the dispatcher that the start is delayed. I would also note the alarm and trip setpoints and the margin at the worst point.
For the next shift, I would provide the trend, the current differential expansion, the hold point, and the criteria for resuming the start. I would flag that the casing needs to catch up and that the start curve may need review if the same pattern recurs. If the differential approached the trip, I would recommend a review of the warm-up procedure and the steam temperature control before the next cold start.