11.0 Vibration and Balance
Gas gen critical speeds: - 1st 4600 cpm - rigid body mode
2nd 5950 cpm - rigid body mode
3rd 20860 cpm - bending mode
Gas generator balance limits are specified on drawing CT5020 and are specified as 9 gm inches at the CT journal and 3 or 7 gm inches depending on phase angles at the inlet journal.
PT critical speeds: - 1st 4065 cpm - rigid body mode
2nd 13760 cpm – bending mode
The first gas generator critical speed at 4600 cpm is only one which has the potential have any real influence on the behaviour of the turbine. See paragraph 11.3.
Traditionally, the TB vibration limits applied solely to casing levels at prescribed points on the turbine and package. During factory engine testing the limit is 9 mm/s rms. In practice levels in excess of 7 mm/sec would be considered a bit on the high side and at least investigated to understand the reasons, even if nothing was done about it. Alarm and shut down are usually set to 10 and 13 mm/sec rms respectively.
Sometimes customers request acceleration set points but it is not really possible to give a single value as the relevance depends on the frequency of the vibration. In this case it is better to make a record of a benchmark spectrum and look for differences if the all pass level increases for any reason. It should then be possible to identify any offending frequency and take the appropriate corrective action.
Displacement vibration measurements are an option with the TB and the test acceptance limit at full load is now 50 μm pk to pk all pass; and 19 μm pk to pk for discrete non synchronous components. At all other loads there is a simple limit of 65 μm with no restriction on the frequency composition.
Installation warning and shut down levels are normally set to 65 and 90 μm pk to pk respectively.
The two types of vibration monitoring require different bearing types throughout the core turbine.
With displacement monitoring three lobed bearings should be used because they provide better control of rotor dynamics. It is very important that the displacement vibration protection equipment is fitted and operational when the 3 lobed bearings are installed. Failure to activate this equipment can lead to bearing failure in some situations; the two elements are mutually dependent upon each other.
For casing velocity vibration monitoring, offset half bearing shells are fitted, but it has to be accepted that the dynamic behaviour of the turbine may not be as good. There are several common vibration situations for the TB, either in form or frequency. These are:-
11.1 Rocking gearbox
Sometimes the gearbox and its supporting pedestal can be affected by a natural frequency vibration which occurs at somewhere between 1500 and 1800 cpm. Unfortunately, this coincides with the operating speed of an alternator driver and can lead to high levels of low speed synchronous vibration. It usually manifests itself as a side to side rocking of the gearbox on the pedestal. Only a few units become susceptible to this vibration but because it is driven by a natural resonance it is often difficult to deal with. It is essential that the drive train is in perfect balance to minimise the excitation and in some cases it has been necessary to provide added stiffness by welding extra support members into the base plate. Mechanical drive units are not affected by this problem.
11.2 Power turbine whip/whirl.
Although it is not a common occurrence the power turbine inlet bearing can experience a non synchronous vibration at about 3300 cpm. Again this is associated with a resonance and is triggered by dimensional discrepancies with the bearing / housing. It usually only shows itself with 3500 rpm mechanical drive gearboxes via displacement probes although in extreme cases it can be picked up with seismic measurement with offset half bearings.
Gas gen critical speeds: - 1st 4600 cpm - rigid body mode
2nd 5950 cpm - rigid body mode
3rd 20860 cpm - bending mode
Gas generator balance limits are specified on drawing CT5020 and are specified as 9 gm inches at the CT journal and 3 or 7 gm inches depending on phase angles at the inlet journal.
PT critical speeds: - 1st 4065 cpm - rigid body mode
2nd 13760 cpm – bending mode
The first gas generator critical speed at 4600 cpm is only one which has the potential have any real influence on the behaviour of the turbine. See paragraph 11.3.
Traditionally, the TB vibration limits applied solely to casing levels at prescribed points on the turbine and package. During factory engine testing the limit is 9 mm/s rms. In practice levels in excess of 7 mm/sec would be considered a bit on the high side and at least investigated to understand the reasons, even if nothing was done about it. Alarm and shut down are usually set to 10 and 13 mm/sec rms respectively.
Sometimes customers request acceleration set points but it is not really possible to give a single value as the relevance depends on the frequency of the vibration. In this case it is better to make a record of a benchmark spectrum and look for differences if the all pass level increases for any reason. It should then be possible to identify any offending frequency and take the appropriate corrective action.
Displacement vibration measurements are an option with the TB and the test acceptance limit at full load is now 50 μm pk to pk all pass; and 19 μm pk to pk for discrete non synchronous components. At all other loads there is a simple limit of 65 μm with no restriction on the frequency composition.
Installation warning and shut down levels are normally set to 65 and 90 μm pk to pk respectively.
The two types of vibration monitoring require different bearing types throughout the core turbine.
With displacement monitoring three lobed bearings should be used because they provide better control of rotor dynamics. It is very important that the displacement vibration protection equipment is fitted and operational when the 3 lobed bearings are installed. Failure to activate this equipment can lead to bearing failure in some situations; the two elements are mutually dependent upon each other.
For casing velocity vibration monitoring, offset half bearing shells are fitted, but it has to be accepted that the dynamic behaviour of the turbine may not be as good. There are several common vibration situations for the TB, either in form or frequency. These are:-
11.1 Rocking gearbox
Sometimes the gearbox and its supporting pedestal can be affected by a natural frequency vibration which occurs at somewhere between 1500 and 1800 cpm. Unfortunately, this coincides with the operating speed of an alternator driver and can lead to high levels of low speed synchronous vibration. It usually manifests itself as a side to side rocking of the gearbox on the pedestal. Only a few units become susceptible to this vibration but because it is driven by a natural resonance it is often difficult to deal with. It is essential that the drive train is in perfect balance to minimise the excitation and in some cases it has been necessary to provide added stiffness by welding extra support members into the base plate. Mechanical drive units are not affected by this problem.
11.2 Power turbine whip/whirl.
Although it is not a common occurrence the power turbine inlet bearing can experience a non synchronous vibration at about 3300 cpm. Again this is associated with a resonance and is triggered by dimensional discrepancies with the bearing / housing. It usually only shows itself with 3500 rpm mechanical drive gearboxes via displacement probes although in extreme cases it can be picked up with seismic measurement with offset half bearings.
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