TB5000 Useful Information Part 6

11.3 Gas generator ½X vibration

This can be a particularly troublesome and once again is linked to the critical speed at circa 4600 cpm. The problem shows itself as the gas generator approaches the IGV change over speed of about 9200 rpm and, although the speed falls a little and then rises to over 10000 rpm, the vibration usually locks to the speed as the load increases. Almost without fail it is an exact ½X component and if displacement probes are fitted it will show a forward rotation. It usually means there is a problem with the fit of the bearings giving the classical signs of looseness. It is mainly driven by the ct bearing but in some instances the inlet can also influence matters.
As well as non conformance of the housing diameters things to look out for are:-
• Lack of chamfer on the housing split line.
• Bearing location tang fouling on the housing.
• Incorrect nip; it should be about .004 ins. on the ct. It is a little less for the inlet bearing but it is very difficult to measure and get sensible figures.

• Correct bedding of the bearings within the housings.
• Housing discontinuity across the split line.
• Ovality of the bearing housings especially for the ct end.

Having identified how to address the 1/2X component it has to be said that in the vast majority of turbines the hardware does not experience any distress. It is known that many units without displacement probes do experience the problem but without the instrumentation the problem goes unnoticed. To the writers knowledge this issue has never led to components failures. This comment can also apply the power turbine vibration through whip / whirl.

11.4 CT2 Rotor Blade Fatigue Failures

There have been about 8 incidences of this problem in recent times but it is not seen as a generic problem because they have nearly all failed on turbines operated by one user, namely Conoco Phillips in Alaska. In all these cases CP themselves have taken the standard CT2 blades and applied various metallurgical treatments in an attempt to increase the service life. In principle the fatigue life of the blade should not have been affected to any significant degree.
However their experience has not backed this up and failures seem to occur at regular intervals. For some reason CP have not been deterred by the problems and to date are continuing to modify the standard blades supplied by Siemens.
 
There is an outside possibility that the cold ambient conditions are influencing the behaviour of the blades but at the moment this company does not believe that to be the case.

11.5 PT2 Rotor Blade Fatigue Failure

This problem is associated with mechanical drive units operating in very cold or arctic conditions. For mechanical drive units there is a 4th order interference with the fundamental blade frequency which can cause the alternating stresses to exceed the endurance limit if high powers generated in cold weather are utilised. A modified blade with a raised natural frequency is available in this situation. This increased frequency was achieved by thinning the aerofoil section. See section 15.0. TDN 89/063 provides additional background information.

12.0 Breathing

To state the obvious problems with the breather system usually means that there is more air than usual trying to pass through the various passageways there is a blockage in the system. Both of these result in high pressures;
usually within the oil tank.

Potential causes:-

• Failure of the compressor exit seal (bucket seal). This is a fairly
common problem and allows excessive amounts of high pressure air to enter the tank.
• Problems with the flame trap/coalescer – blockages etc.
• Overfilling of the oil tank which may restrict air flow.
• Blocked air holes in the drowned lube oil drain tubes which return the oil to the tank. This problem is often linked to over filling of the tank.
• Failure to fit the pt oil drain bypass pipe when 3 lobed bearings are installed.
Packages with depressed pressure in the enclosure may leak from the gearbox exit seal. In this situation a mod to pressurise to seal with air from the bleed band is available.

12.1 Nominal Breather Flows

Because of the relatively low pressures and temperatures for the TB the turbine does not have an external buffer air supply to the ct labyrinth seals. 

All flows are controlled by the internal pressures and clearances. The CT, PT and Tank breathers all exhaust through the low pressure discharge pipe via the coalescer and trap.   

 For the TB5400 the total breather flow is approximately 0.17 lb/sec For the TB5000 the total flow is marginally less at about 0.16 lb/sec. Of this, the CT accounts for about 0.07 lb/sec, the PT about 0.05 lb/sec and the remainder comes from the tank flow. These figures are very approximate

13.0 Rotor Overspeed

The question of whether to overspeed the rotating assemblies was reassessed in 2001 by Andrew Shepherd who concluded that:-
• All overhauled TB5000 rotors that are to be despatched without engine testing require overspeed.
• All new CT discs that are to be fitted to overhauled TB5000 rotors require overspeed. The compressor rotor does not require overspeed in this case.
• TB5000 compressor rotors that have any of the compressor shafts or discs replaced during overhaul do not require overspeed.
• TB5000 CT discs that have already operated in the field, but have had the CT blades replaced, do not require overspeed.

14.0 Performance

The four ratings are TB3000, TB4000, TB5000 and TB5200/5400. It is only with the most recent rating that the performance difference between liquid and gas fuel was formally recognised by quoting 5200 hp and 5400 hp zero loss respectively at the temperature limit. For the current range of turbines from SIT Lincoln the output power is a fixed quantity and the temperature to achieve this is set and limited during the works acceptance test. On the other hand, with the TB, the operating temperature is fixed and therefore the generated power can vary as a result of component efficiency variations. A performance degradation of 4% is allowed during the works acceptance test of overhauled turbines, although this is often not achieved and a concession may be needed to pass off the unit.

14.1 Performance curves
 
The performance group is the guardian of the performance graphs and it is impractical include all the available charts in this document in this document.
The short list below, particularly the 913 and 995 series should enable most simple gas related performance issues to be answered without problem. It is probably better to consult with the performance group if more specific information is required.
 
Chart Number                     Title
IC 913                                  series TB4900 Checking graphs
IC 995                                 Series TB5200/5400 Checking graphs
IC 857/7                               Compressor characteristic
IC 948                                    CT characteristic
IC 948/1                                    PT characteristic
IC 980/1t                               Test bay rating chart
IC 994/1                               Maximum operating temperature
IC 975                                  Nominal acceptance limits
Note that all generic performance graphs for the TB are prefixed by ‘IC’.
 
The major control parameter for the TB is the operating temperature Top, which is the difference between the exhaust stack temperature minus the compressor inlet temperature. In principle a constant Top value equates to a constant turbine inlet temperature although this is not strictly true at all ambient conditions. (see TDR 07/052)
 
Graph IC 975 shows the nominal temperature set points for the TB and includes a maximum continuous running temperature. However if proper provisions are made within the control system it is permissible to raise this maximum continuous value up to the warning set point to achieve extra power. There is a penalty in that hot component life is consumed at 04 times the normal rate.
 
14.2 Power shortfall

When new, most TB turbines easily generated the required power within the temperature limits but old or overhauled engines often struggle to achieve the rating. If there is a significant loss the most likely causes are:-

• Leakage past the ct bearing housing seal which abuts with the
upstream inner flanges of the quadrants. There is quite a high
differential pressure across this barrier and a gap of only a few
thousandths of an inch can have a significant affect. The actual air
leakage is less significant than the disruption it causes to the turbine
flow when it re-enters the gas stream.

• The piston ring seals on the downstream out flanges of the quadrant are another favoured leak path with similar consequences
• In general, the turbine section blades will be in good condition for a test, as it will have been critical to the integrity of the turbine to replace worn or damaged items. However the compressor blades are another matter.

These can become worn, corroded and damaged in so many ways that the compressor efficiency is compromised. Because of the diffusion activity within the stator the blades here can have a big influence on matters. In the past the writer has advised that the stator blades be replaced. After which the turbine performance was recovered.
 
14.3 Cold start performance
 
Due to changes in the clearances as the turbine heats up following a cold start there is a loss of almost 250 kw when the engine running at the nominal maximum operating temperature. Most of this fall off takes place during the first 5 minutes or so but degradation continues for perhaps 25 minutes until equilibrium is achieved.
14.4 Inlet and exhaust losses
 
These losses can significantly affect the generated power but the limits are really set by mechanical or flow problems within the turbine and in the extreme the pressure capabilities of the ducting.
The maximum inlet depression should be less than 15 inches water although levels up to 20 inches could just about be accommodated.
higher than this may have an adverse affect on cooling flows within the turbine.