TL;DR
A 1x vibration increase that tracks with load or temperature is consistent with thermal rotor bow or slow mass unbalance. Compare vibration vectors at multiple load points and contact the OEM if the vector changes.
What you might see
- shaft vibration above normal at 1x running speed
- rotor vibration proximeter reading above alarm setpoint
- vibration increasing with load or temperature
- vibration probe alert on turbine control system
Likely causes
Gradual rotor mass unbalance from rotor body distortion or loose balance weight
Thermal unbalance from uneven heat distribution in the rotor body during load changes
Bearing clearance increase from wear, allowing the rotor to run off-center
Electrical unbalance from a partial ground on the rotor winding heating one side preferentially
Required tools
- Plant DCS vibration trend data or offline vibration analyzer
- Bearing temperature trend data
- Rotor ground detection system readout
Safety first
- Large spinning rotors store enormous kinetic energy. Keep all personnel clear of the shaft line during operation. Only use long-reach tools or permanently installed sensors to monitor vibration on a running machine.
- Do not continue operating a large generator with vibration above the OEM trip setpoint.
Procedure
- 1
Pull vibration trend data from the plant DCS or turbine monitoring system. Plot amplitude and phase angle at 1x speed over time to identify whether the change is gradual or sudden.[1]
- 2
Record vibration at multiple load and temperature points. A 1x vector that changes with megawatt load or hydrogen temperature suggests thermal origin.
- 3
Check the bearing metal temperature RTDs and bearing oil drain temperatures. Rising bearing temperature alongside vibration suggests bearing wear.
Warning: Large rotating machinery at speed is extremely dangerous. Never approach or attempt to measure directly at a bearing on a running machine unless you are using long-reach calibrated probe extensions from a safe zone. - 4
Check the rotor field current and look for insulation resistance changes (using the rotor ground detection system) to identify a partial rotor ground.
- 5
Compare current vibration vectors to the OEM's acceptance template for this machine at this operating condition. If within limits but trending, flag for the next planned outage.
- 6
If vibration exceeds the alarm limit and continues rising, consult the OEM and evaluate an expedited outage for rotor inspection and balance.
Sources
GE GE Gen6 / Gen8 Generator (Large) general technical documentation, GE
Large rotating machinery vibration analysis and rotor dynamics, general power generation references (general)
More guides for GE GE Gen6 / Gen8
How to diagnose high stator winding temperature on a GE Gen6 / Gen8 generator
Elevated stator winding temperature with normal load usually means reduced cooling water flow or a blocked cooling passage. Check cooling water flow rate, inlet temperature, and compare RTD readings across all stator slots.
How to detect and respond to a hydrogen gas leak on a GE Gen6 / Gen8 generator
Declining hydrogen purity or elevated makeup demand signals a leak. Check the shaft seal oil system first since shaft seals are the most common hydrogen leak path on large generators.
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