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When increased vibration levels threatened to load-limit the operation of this power station turbine, engineers were able to exploit their training in rotating machinery testing techniques to keep the machine in-service with minimum impact to operations.
Australia’s Northern Territory is a vast outback region rich in natural beauty and mineral deposits. With a land area almost the size of Alaska, a small, dynamic population, and its proximity to Asia making it an important gateway to countries such as Indonesia and East Timor, the state is currently enjoying economic growth driven by investment in major infrastructure projects in Transport, LNG, Gas-to- Liquids, Minerals, and Power Generation.
The Power and Water Corporation (PWC) is the Northern Territory’s provider of electricity, water and sewerage services. Current installed capacity is 380 MW with the Territory’s largest gas turbine power station (254 MW) located at Channel Island, on Darwin Harbour’s Middle Arm.This power station consists of five GE Frame 6 gas turbines, two of which are used in a combined cycle configuration with one Mitsubishi steam turbine, and one GE LM6000PC aero derivative gas turbine.
This GE LM6000 turbine, “Unit 7”, is an important peaking unit on the PWC grid, supporting domestic and industrial demand particularly through the critical wet season periods when power demand in the tropics hits its ever-increasing peak.
In the second quarter of 2006, Unit 7 was taken offline for general maintenance and to have the inlet gear box (IGB) repaired. After the unit was brought back to service, high vibration was measured at the turbine rear frame accelerometer filtered to LP (low pressure) shaft speed (TRF LP channel) while operating within the 10-15 MW range. The unit exhibited frequent vibration alarm conditions of above 1.5 inches/sec pk in this load range. However, at high load the TRF LP vibration remained below 1 inch/sec pk.
Following this initial observation, PWC restricted the machine from prolonged operation within the low load range until the unit could be taken offline for further investigation.
Finding fault
In the last quarter in 2006, Unit 7 was scheduled for a unit control upgrade, replacement of the thrust balance valve, and inspection of the complete engine package. With the PWC engineering team having a strong background in rotating machinery testing techniques, such as vibration analysis, and the confidence to tackle complex rotor dynamics problems in the field, the shutdown provided a window of opportunity for the PWC Diagnostic Engineers to prepare for more advanced machinery diagnostics than provided by the turbine supervisory system.
Hence, during recommissioning, vibration testing was carried by PWC utilising two Bently Nevada ADRE 208 data acquisition instruments connected to a Bently Nevada 3300 protection system. Using these instruments for the data acquisition exercise provided the additional advantage of ensuring data format consistency with the GE Installation and Field Service standard, so if necessary the data could be sent back to GE Aircraft Engines’ diagnostic specialists in Cincinnati for a second opinion.
The term ADRE was introduced by Bently Nevada in the early 1980s, meaning “Automated Diagnostics for Rotating Equipment”, an innovative software program which automated the then labour-intensive task of replaying rotating machinery vibration data through analogue tracking filters to manually plot out each diagnostic data set.
The ADRE 208 units were first introduced in 1993, allowing diagnostic engineers to capture up to 16 channels of machinery data simultaneously. Data could be captured automatically during machine transients or for steady-state testing and presented in the correct formats for analysis, reporting and balancing.
Although the 208s have recently been superseded by the newer ADRE 408s, which support 24 channels of turbo-machinery data capture per unit and allow for up to four units to be interconnected, many practitioners in the machinery diagnostics field still rely on the 208 as a dependable workhorse for rotating machinery analysis. After analysis of the initial machine start-up data, it was apparent that the behaviour was consistent with the previously identified high amplitude vibration at the turbine rear frame sensor (TRF), closely related to the LP shaft speed.
It was established that the vibration response appeared repeatable from run to run, and largely unchanged from earlier in the year, so the PWC team assessed there was no progressive deterioration to machine condition and could confidently judge that there was no likely risk to machine integrity provided they could run the machine within the recommended limits for vibration amplitude.
Discussions with GE Engineers led to a conclusion of unbalance of the LP rotor, with possible causes being blade damage, erosion, or a mechanical or thermal related incident during the previous overhaul. In any case, the only permanent solution would be a workshop inspection and precision reassembly.
But, with the Northern Territory wet season fast approaching, unplanned downtime for Unit 7 was not feasible, so with further collaboration with GE, PWC decided to carry out an in-situ trim balance of the LP rotor to reduce the TRF LP vibration and remove the current operational limitations.
Trim balancing of the LP rotor is rarely performed insitu, as a change in residual imbalance often indicates an underlying problem demanding inspection and repair. The nature of vibration measurement and typical thermal related vibration response on aeroderivative turbines make field balancing challenging, but fortunately, the LM6000 is equipped with coupling hubs at either end of the turbine, enabling addition of balance weights when necessary.
Time for trimming
As the LM6000 was fitted only with multi-tooth wheels for speed detection on the HP and LP rotors, no phase-locked Keyphasor signal was available for calculating the 1X vibration vector needed for rotor balancing. Together with GE Field Service personnel it was decided to install a temporary phase reference signal at the main gearbox input shaft. This would enable collection of synchronous, transient data vibration data for best diagnosis capability.
Although a true keyphasor could not be fitted, it was possible to use the Bently Nevada TK16 Keyphasor Multiplier/Divider to modify the speed sensor pulse according to the gear ratio, thus generating a steady simulated phase reference signal that could be used for balancing.
The resulting signal provides only a relative phase reference rather than an absolute phase reference, so for each run of the machine, the phase will change. Nevertheless, this technique does provide good diagnostic information that might otherwise be unavailable. Due to the complex gear ratios of the accessory gearbox, three TK16s had to be brought in from the GE Singapore office for this purpose. The machine was started and ramped up to full load of 42 MW, remaining on-load for six days when the opportunity arose to add trim balance trial weights. The unit was cooled overnight in order that the initial trial weights could be installed.
The initial trial weight of 91 grams at the exhaust end balance plane (gearbox and generator are coupled to the compressor end) was based on the machine response to the weight used in a previous balance job on this machine, while also not exceeding the “10 percent rule” (the centrifugal force generated by the trial weight should not exceed 10 percent of the rotor weight). Of course, as with all good field balancing, final selection of weights comes down to the available selection of standard weights and unoccupied balance holes.
Without knowledge of the rotor dynamics of the LP shaft, it is difficult to predict how a mass addition at the exhaust end balance plane will affect the casing vibration (velocity) vectors at the CRF and TRF locations. With industrial gas turbines, X/Y proximity probes generally give good insight into the mode shape of the rotor as it changes speed, so a more scientific assessment may be made of the best trial weight location. In this case, however, the best available information from experienced practitioners suggested adding the trial mass at the coupling roughly opposite the TRF LP filtered 1X velocity vector.
Interestingly, two solution points, low load and full load for both TRF and CRF LP filtered measurements, had to meet the vibration limit of less than 1 inch/sec pk. This meant a total of four vibration vectors to be minimized by a single mass addition. Adding to the complexity, vibration vectors at low load and full load were almost opposite in phase.
So, a balance solution for low load behaviour could lead to unacceptable vibration response at high load, and vice versa. Therefore, a compromise had to be found which would allow the turbine to operate within limits through the entire load range, lifting the previously imposed vibration-related operating limitation.
The initial trial weight actually caused deterioration in the TRF LP filtered vibration response at full load conditions, although the CRF LP response remained well within limits at all times. However, by reviewing the calibration data and performing various manual calculations and checking predictions, a compromise solution was determined: the initial weights were removed and a final balance weight of 32.9 grams @ 75 degrees right from TDC installed.
The machine was shut down after the calibration run and a final correction mass added within hours, freeing the machine for service. Unit 7 was then restarted, ramped up to full load and heat soaked, before unloading to 5 MW to collect final balance data. Throughout the testing the measured 1X amplitude at the TRF LP channel remained below the required 1 inches/sec pk. At this point it was agreed that the unit could be returned to unrestricted operation.
Full availability asset
As a result of the in-situ trim balancing carried out by PWC staff, Northern Territory’s Power and Water Corporation has been able to operate Unit 7 across all load ranges without exceeding vibration limits.
The operation limitation imposed earlier has been lifted, returning full availability to the asset. In fact, Unit 7 has since recorded 99.96 percent availability and has provided the state with uninterrupted electricity during the wet-season, when demand peaks at 258 MW. In addition, PWC has been able to avoid an unplanned outage to swap out the engine for inspection and shop balancing off-site, saving over a week of production and many hundreds of workshop hours. The investment in technical training and retention of corporate skills has been illustrated to return good dividends for both the business and the customers.
Author details: David Rossi, Technical Specialist, PWC; Ron Atwell, Generation North – Operation Manager, PWC; Bas Meys, Mechanical Engineer, PWC; David Nguyen, Senior Mechanical Service Engineer, PWC; Chris Engdahl, Field Application Engineer, Optimization and Control, GE Energy.
