TL;DR
A 1x vibration that is high on roll-off but decreases with time is almost always a thermal rotor bow from cooling unevenly on the turning gear. Do not attempt to rush the start. Continue slow roll until eccentricity returns to normal limits before accelerating.
What you might see
- high 1x vibration when rolling off turning gear
- slow-roll bow detected on eccentricity probe
- vibration reduces after extended warm-up period
- turbine trip on high shaft vibration during start
Likely causes
Thermal rotor bow from uneven cooling during shutdown due to differential heat loss around the rotor circumference
Insufficient turning gear soak time after a hot shutdown
Steam admission at idle before the rotor has straightened thermally
Rotor rub during a previous trip creating a residual thermal imbalance
Required tools
- Eccentricity probe readout or DCS channel
- Turning gear status and oil pressure
- DCS vibration data
Safety first
- Never accelerate a steam turbine with eccentricity above the OEM limit. A rub at speed causes serious damage.
- Ensure all turbine casing drain valves are open before admitting steam to prevent a steam hammer or water induction event.
Procedure
- 1
Confirm the eccentricity probe reading at turning gear speed. If eccentricity exceeds the OEM limit (typically 2-3 mils peak-to-peak), do not accelerate the turbine.[1]
- 2
Continue operating on the turning gear. Log eccentricity every 15 minutes.
Warning: Attempting to roll through high vibration on a steam turbine with a bowed rotor risks a rub event that can cause severe blade and seal damage. - 3
Confirm that the turning gear oil system is maintaining adequate pressure. Low oil pressure increases the probability of rub.
- 4
Check that drains on the turbine casing steam admission paths are open to ensure no condensate has collected in the casing that would cause a thermal shock and worsen the bow.
- 5
If eccentricity does not reduce within 2-4 hours of continuous turning gear operation, contact the OEM. Persistent bow can indicate a rub or a previous event that requires inspection.
- 6
Once eccentricity is within the OEM acceptance band, proceed with the normal start curve. Monitor 1x vibration closely during roll-up through critical speed.
Sources
GE Vernova GE D / H Class Steam Turbine general technical documentation, GE Vernova
Steam turbine rotor bow, turning gear operations, and vibration start-up criteria, general power generation references (general)
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