Restoring Gas Turbine to Service After Failure of Turning Gear


Thread Starter

Sarah Kwakumey

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In the process of planned shutdown of the GE frame 9E gas turbine, it failed to transfer from light crude oil to distillate fuel oil at 13mw. Investigation revealed that there was a spurious fire alarm. This may have affected the inability to transfer from LCO to DFO prior to shutdown. Also the bearing lift oil hose was found to have punctured resulting in lift pressure of 18psi. Shaft was left in standstill position when after staying at 6rpm for 80 minutes, the turning gear disengaged.

How does one go about the process of placing the gas turbine on turning gear again as part of restoring it to service?

First, the lift oil line has to be repaired. L.O. should be kept on to keep the bearings cool at all times when turning gear (cooldown) is not in operation until at least 24 hours after the unit reached zero speed and cooldown could not be maintained.

With sufficient time to cool, the axial compressor shaft (the longest and heaviest part of the turbine/generator shaft) will gradually return to very nearly straight. Simply putting the unit on cooldown (starting the turning gear) and running it for at least four hours will remove most of the bow.

Next, one can select FIRE mode, and start the unit. FIRE mode will establish flame and then reduce the fuel to the warm-up value, and hold it there as long as FIRE mode is active. The speed will increase slightly but level off at about 30-40%. One can click on STOP at any time if necessary.

After the unit has been in FIRE mode for about 15 minutes or so and the rotor has had a chance to build up a little heat and straighten nearly completely due to the combination of heat and rotational speed, one can simply click on AUTO and the unit will accelerate to FSNL in preparation for synchronization. The vibration may be a little high during acceleration but likely not high enough to trip the machine. One can hold at FSNL for a few minutes, if desired, or just synch and load the machine, not too fast, but not too slowly, either. And, any remaining vibrations should gradually fade away.

The topic of how to deal with a GE-design heavy duty gas turbine that was not on cooldown after a trip from load has been covered <i>many</i> times on Many times. There is a 'Search' field at the far right of the Menu bar at the top of every web page; the syntax isn't like using one of your preferred World Wide Web search engines, but it is powerful and fast. And, subjects related to GE-design heavy duty gas turbines, and their control systems, have been covered here for more than a decade--and there are lots of threads and information on MANY topics. Some not even really controls-related (we have our weak moments--like this topic, which is not really controls-related).

Not being able to maintain cooldown on a hot machine is not damaging to the unit--unless one tries to use drastic measures to turn the shaft while it's hot. The axial compressor shaft will bow--the metal is in the elastic temperature range--and if the shaft is turned prematurely (before it's had a good chance to cool, which is usually about 24 hours) there is a VERY good chance of severe damage to the compressor blades. Patience is the key--regardless of turbine Frame size or condition. It's also cheaper than the alternative (trying to turn the shaft when it's bowed and hot and hasn't had a chance to return to very nearly straight before being put on cooldown (turning gear; ratchet) for at least four hours).

Hope this helps! Please write back to let us know how you fare!