5.3 First Stage Rotor Blades.
Apart from the 3000 variant it is not recommended that TBs operate continuously at low loads, typically less than about 600 - 700 kw. The reason for this is that it may lead to fatigue failure of first stage compressor rotor blades just above the root platform. The problem is caused by poor incidence leading to flutter of the aerofoil. Over the years a dozen or so have occurred, although it doesn’t happen in all cases of low load operation.
Ambient conditions and speed are the main influences although the exact trigger mechanism has never been fully identified. See section 15.0 - Blade Campbell diagrams.
One way round the problem is to fix the guide vanes as with the TB3000 but that will limit the available power to about 2100 kW. Alternatively the vanes could be set to +15 degrees for the start and change over to -5 degrees at the normal 1500 hp condition. This option requires changes to the control and fuel schedules. In principle this should be a sensible approach when the full power range of the turbine is required.
Both of the above options have been recommended to users. The fixed vane system has been used in the past but it is not known whether the 15 to -5 range has actually been implemented in the field.
The third stage rotor blade was redesigned with a thinner section circa 1980 to eliminate a failure inducing resonance. The current blade has a higher natural frequency and is not susceptible to the problem. It is believed that all these have been changed out but there may be units which are not under the control of Siemens and still contain the old design.
5.4 Third Stage Rotor Blade.
During the early 1980s problems were encountered with compressor surging after blade machining was brought in house and the aerofoils actually conformed to the drawing. The investigation resulted in small stagger angle changes to most stages to cure the problem. It is almost certain that all field units now have the current blade design.
6.0 CT Bearing Housing
The lower half of the housing is actually part of the centre casing with the top section being a casting. As the casing gets older it is not unusual for distortionof the bearing bore to occur and this can have a significant influence on the performance of the bearing and the rotor vibration characteristics. The real problem centres on the fact that the bottom half is all part of the substantial lower centre casing member whereas the top cap is a small relatively flimsy component. There is a resultant mismatch in the thermal and stress characteristics between the two which contributes to the problem.
There are two versions of the bearing cap; the original design and a later variant which is thicker with increased stability. It is possible to retrofit the more substantial cap but it does involve modifying the seal assembly to accommodate the larger part.
7.0 Bearings
Both offset half and three lobed bearings are utilised on the TB with the latter only being used if displacement vibration equipment is installed. See the section on vibration.
7.1 Bearing Clearances (inches):-
Offset half Three lobed
Compressor inlet 0.002/0.004 0.002/0.004
Compressor turbine 0.0047/0.0067 0.0075/0.0095
Power turbine inlet 0.0047/0.0067 0.0047/0.0067
Power turbine exit 0.0025/0.0045 0.0025/0.0045
Note the difference at the ct bearing location where the three lobed clearance is particularly large and therefore greater than would normally be expected.
This is necessary because the shaft expands rapidly during start and rapid load changes.
7.2 Bearing loads and oil flow requirements
The flow figures in the table below are not theoretical values but are based on
some field measurements taken circa 1980 plus some extrapolations and
estimates made by the writer. The units are imperial gallons per minute. The
compressor inlet and PT exit locations include the thrust bearing flows.
Location Comp inlet CT bearing PT disc end PT exit end
Engine driven
Offset Half 9.0 3.5 4.0 4.5
Engine driven
3Lobe 10.5 6.5 7.5 5.5
AC 3 Lobed 5.0 2.5 3.5 2.5
DC 3 Lobed 0 1.5 1.5 0
Static bearing
loads (lbs) 620 860 500 -50**
** This static load figure is negative because the journal naturally sits at the top of the bearing because of the disc overhung moment.
7.3 Bearing options
Several oversize bearing shells have been defined for special fault
circumstances. These are not necessarily formally issued or available but the following list dated 12/03/01 may act as a starting point if any activity is required. This would normally within the responsibility of the repair group. Some SGT200 bearing options are included in this list.
Apart from the 3000 variant it is not recommended that TBs operate continuously at low loads, typically less than about 600 - 700 kw. The reason for this is that it may lead to fatigue failure of first stage compressor rotor blades just above the root platform. The problem is caused by poor incidence leading to flutter of the aerofoil. Over the years a dozen or so have occurred, although it doesn’t happen in all cases of low load operation.
Ambient conditions and speed are the main influences although the exact trigger mechanism has never been fully identified. See section 15.0 - Blade Campbell diagrams.
One way round the problem is to fix the guide vanes as with the TB3000 but that will limit the available power to about 2100 kW. Alternatively the vanes could be set to +15 degrees for the start and change over to -5 degrees at the normal 1500 hp condition. This option requires changes to the control and fuel schedules. In principle this should be a sensible approach when the full power range of the turbine is required.
Both of the above options have been recommended to users. The fixed vane system has been used in the past but it is not known whether the 15 to -5 range has actually been implemented in the field.
The third stage rotor blade was redesigned with a thinner section circa 1980 to eliminate a failure inducing resonance. The current blade has a higher natural frequency and is not susceptible to the problem. It is believed that all these have been changed out but there may be units which are not under the control of Siemens and still contain the old design.
5.4 Third Stage Rotor Blade.
During the early 1980s problems were encountered with compressor surging after blade machining was brought in house and the aerofoils actually conformed to the drawing. The investigation resulted in small stagger angle changes to most stages to cure the problem. It is almost certain that all field units now have the current blade design.
6.0 CT Bearing Housing
The lower half of the housing is actually part of the centre casing with the top section being a casting. As the casing gets older it is not unusual for distortionof the bearing bore to occur and this can have a significant influence on the performance of the bearing and the rotor vibration characteristics. The real problem centres on the fact that the bottom half is all part of the substantial lower centre casing member whereas the top cap is a small relatively flimsy component. There is a resultant mismatch in the thermal and stress characteristics between the two which contributes to the problem.
There are two versions of the bearing cap; the original design and a later variant which is thicker with increased stability. It is possible to retrofit the more substantial cap but it does involve modifying the seal assembly to accommodate the larger part.
7.0 Bearings
Both offset half and three lobed bearings are utilised on the TB with the latter only being used if displacement vibration equipment is installed. See the section on vibration.
7.1 Bearing Clearances (inches):-
Offset half Three lobed
Compressor inlet 0.002/0.004 0.002/0.004
Compressor turbine 0.0047/0.0067 0.0075/0.0095
Power turbine inlet 0.0047/0.0067 0.0047/0.0067
Power turbine exit 0.0025/0.0045 0.0025/0.0045
Note the difference at the ct bearing location where the three lobed clearance is particularly large and therefore greater than would normally be expected.
This is necessary because the shaft expands rapidly during start and rapid load changes.
7.2 Bearing loads and oil flow requirements
The flow figures in the table below are not theoretical values but are based on
some field measurements taken circa 1980 plus some extrapolations and
estimates made by the writer. The units are imperial gallons per minute. The
compressor inlet and PT exit locations include the thrust bearing flows.
Location Comp inlet CT bearing PT disc end PT exit end
Engine driven
Offset Half 9.0 3.5 4.0 4.5
Engine driven
3Lobe 10.5 6.5 7.5 5.5
AC 3 Lobed 5.0 2.5 3.5 2.5
DC 3 Lobed 0 1.5 1.5 0
Static bearing
loads (lbs) 620 860 500 -50**
** This static load figure is negative because the journal naturally sits at the top of the bearing because of the disc overhung moment.
7.3 Bearing options
Several oversize bearing shells have been defined for special fault
circumstances. These are not necessarily formally issued or available but the following list dated 12/03/01 may act as a starting point if any activity is required. This would normally within the responsibility of the repair group. Some SGT200 bearing options are included in this list.
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