Thrust Bearing Distress on an Extraction Steam Turbine
A 35 MW condensing-extraction steam turbine is rated 900 psig, 900°F at the throttle and exhausts at 2.5 in Hg absolute. It supplies 180,000 lb/h extraction to a feedwater heater at 140 psig. At steady 28 MW, the thrust bearing temperature rises from 180°F to 235°F in 20 minutes; the differential-expansion indicator moves from +0.010 in to +0.040 in, and front-standard vibration rises from 0.8 to 1.6 mils peak-to-peak. Lube oil supply is 45 psig at 120°F. The heater level control has failed, and the heater high-level alarm is on; the extraction line drain trap is cold. The operator proposes to reduce load to 20 MW, keep the unit on line, and monitor the bearing every 15 minutes because the trip setpoint is 250°F.
1. Interpret the combination of thrust-bearing temperature, differential expansion, and vibration. What is the most likely mechanism, and why is the heater level alarm important?
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The three indications together point to thrust-bearing distress rather than a simple oil-temperature problem. A 55°F rise in 20 minutes, a 0.030 in shift in differential expansion, and a doubling of vibration indicate rapid changes in axial thrust and rotor position. The heater high-level alarm and the cold extraction-line drain trap make water induction the most likely mechanism: the failed level control has raised the heater level above the extraction nozzle, and the extraction line is not draining. Water entering the turbine through the extraction nozzle raises axial thrust, cools and contracts the rotor, and can produce blade or gland rub. Bearing temperature alone could be oil or alignment, but the simultaneous expansion and vibration changes make thrust distress the dominant concern. The heater level alarm is the smoking gun and should be treated as an immediate trip condition, not a monitoring item.
2. State your immediate actions and defend why reduced load is not an acceptable alternative to a trip.
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Trip the turbine immediately, close the extraction nonreturn valve, open all extraction-line and heater drains, isolate and drain the heater, and maintain lube oil flow. Do not wait for the 250°F trip setpoint; that setpoint is a backstop, not an operating target. Reducing load to 20 MW may lower steam flow, but it does not stop water induction if the heater remains flooded and the extraction valve is not tight. Continued rotation with water in the machine can wipe the thrust bearing, cause rotor-to-stator rub, and turn a repairable bearing failure into a major rotor and casing failure. The correct judgement is to remove the energy source first and investigate afterward.
3. After shutdown, what inspections and instrumentation changes would you require before restart?
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Inspect the thrust bearing, thrust collar, diaphragm, rotor runout, seals, and extraction nonreturn valve. Test the heater level control, the high-level alarm, and the extraction drain trap, and confirm that the heater can be isolated and drained. Before restart, add redundant heater level transmitters with an independent high-high trip, extraction-line temperature and drain-flow indication, a thrust-bearing temperature trip that is independent of the monitoring system, differential-expansion alarms, and a water-induction detection system. Verify that the extraction nonreturn valve closes before the turbine is rolled and that the heater cannot back-fill the extraction line during a trip.