ArticleA Five Year Case History of a 150 MW Turbine Generator
by Kevin R. Guy
This is an excellent long term case history showing the complexities of large turbo-machinery. It would be very good for beginning and intermediate vibration analysts and engineers as well as a good reference for advanced vibration analysts and engineers that may not have a lot of experience around large turbo-machinery. It is a very practical paper detailing five years of analysis and inspection attempts to improve the operating condition of a turbine generator set. The paper is very descriptive of the information available at the different investigation points and uses tables, plots, and sketches very well in describing the conditions noted. It is an excellent case history documenting attempted analysis and corrections to improve the vibration levels of the machine at different points in time; vibration analysis, inspections, and attempted balancing were performed with differing results at the different times pending the limitations of the operational demands of the utility. There were several different issues that had to be corrected prior to finally achieving the desired results.
“Background Information: The owner of this unit is a small utility with a total generating capacity of 400 Megawatts. Due to the lack of generating capacity it is imperative this unit run to supply the local municipality. Since this is a small utility, cost cutting measures lead the plant to look to secondary resources to engineer and supervise turbine overhauls and repair work. This started in the early to mid 1990’s. The unit was scheduled for a complete overhaul and controls change out in October 1996. Once the unit was put back in service high vibration amplitudes were experienced on the Low Pressure Section of the unit. Vibration was over 8.0 mils (pk-pk) at 3200 rpm.
The unit is instrumented with shaft riders installed at 60 degrees above the horizontal joint. Velocity sensors are mounted on the shaft riders. These are original installation General Electric instrumentation. The utility had no vibration; phase or amplitude, data from before the outage to compare with the post outage data. It was at this point the five year case history began.
Fall 1996: The initial involvement was due to the unit not making it up to speed during the first run after the complete overhaul. The unit was tripping at 3200 rpm. The first critical on this unit is at 43.5 Hz (2610 rpm) so this unit was tripping well above the critical. Turbine operating speed on this unit is 3600 rpm. The unit has an exciter driven through a gearbox connected to the generator. The exciter runs at 1200 rpm (Figure #1).
A program was initialed to balance the Low Pressure Rotor at around 3200 rpm. Results from the balancing were not effective (Table #1) with balance response somewhat odd. The addition of weight (shots original to shot #3) had little effect, even with doubling the weight. Rotating the weights 180 degrees did allow the unit to run at speed; however, the vibration was still unacceptable.
It was decided to shut the unit down and inspect the turbine for mechanical defects. What to look for would be decided by the company who supervised the turbine overhaul. One concern was the balance sensitivity during the turbine balancing below running speed. The unit was very insensitive to the amount of weight installed.
The unit was placed back in service three days later with nothing found during the further inspection. Vibration increased with load and the phase angle moved against rotation. Vibration amplitudes reached 6.0 mils (pk-pk) plus at 149 MW (Figure #2).”
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