We have 6 GTG frame 9 mark VI control. After major inspection (MI) of one of our gas turbine when we are going to give command for cooldown mode, torque converter (TC) output not rotating. Other checks/observation are:

1. Torque converter input shaft rotates.

2. From inspection window we have check the operation of VIGV inside torque converter, found working normal.

3. Fixed the PG at supply & return line found pressure in normal range that confirm that fluid going from pump to turbine.

4. By decoupling the AGB & TC coupling , TC output shaft rotate freely.

5. During inspection & final alignment turbine & generator shaft rotate ok.

6. Checking jacking oil pressure of generator shaft, found normal and generator shaft lift observed ok.

But when all things combines torque converter output shaft not rotating.

 

viralmehta,

What Process and/or Diagnostic Alarms are annunciated when you try to initiate Cooldown?

GE-design Frame 9E heavy duty gas turbines employ several different cooldown methods--one of them is called "slowroll" which uses the torque converter to <i>maintain</i> approximately 30-50 RPM when the unit is coasting down from a STOP or trip. For these units, if the shaft is at zero speed and COOLDOWN ON is initiated, the electric starting motor has to start and spin the turbine-generator shaft up to more than approximately 100 RPM, and then the electric starting motor is de-energized, and the torque converter switches from starting mode to cooldown mode (solenoid 20TU-1 is used for transmitting high torque, such as during starting and acceleration; and 20TU-2 is used for cooldown torque operation, which takes a portion of the Auxiliary L.O. Pump discharge and ports it through the torque converter to <i>maintain</i> RPM at 30-50 RPM during coastdown from a higher speed (be it approx. 100 RPM during COOLDOWN ON from zero speed, or when coasting down from higher RPMs after a STOP or trip).

Starting and slow-roll occur when the variable torque converter guide vanes are at MAX torque position. Do you know if the torque converter guide vanes are at MAX torque position during your COOLDOWN ON initiation? (Firing occurs at MIN torque.)

So, if your units use slowroll, when you click on COOLDOWN ON the electric starting motor has to start, and approx. 5 seconds after that 20TU-1 has to be energized and if the variable torque converter guide vanes aren't already at MAX torque setting they need to be moving to MAX torque setting) and the shaft should break away from zero speed. Once the shaft gets above approximately 100 RPM, the electric starting motor should shut down, 20TU-1 should de-energize and 20TU-2 should energize. And the shaft speed should start decreasing but the torque converter should be able to maintain approx. 30-50 RPM (if the L.O. Temp is above approx. 27 deg F; cold L.O. temp causes the oil film/wedge in the bearings to be very thick and the torque converter can't maintain RPM with the additional energy required to overcome the oil viscosity).

Was the torque converter removed and re-installed? Were the torque converter limit switches properly re-adjusted or re-connected properly (if they were moved/changed during the outage)?

If the bearings (turbine and/or generator) were changed during the outage sometimes the clearances are so small with new bearings (even if they're in the allowable range) that rotation can be difficult. I have had to use a bottle jack under one of the load coupling bolts at the turbine end of the generator to help get the shaft spinning on one occasion--and once it started spinning the unit was left on CRANK for several hours, and then fired and run up to FSNL for a couple of hours before shutting down. When the unit reached slowroll speed it had a little difficulty maintaining 30 RPM, but after successive starts and run, especially with load, everything worked out okay. (You put the bottle jack on a FIRM support such that the jack can move once the shaft starts turning and doesn't get hit by the other bolts passing by, with the bottle jack shaft under a coupling bolt, and the pre-load the bottle jack with some pressure, and then initiate a START with CRANK mode selected. The bottle jack will fall out once the coupling starts rotating (so have it secured with a rope or something--and have people out of the compartment for safety reasons, because you'll have to remove the coupling guard to do this--SAFETY , SAFETY, SAFETY!!!). I used a large beam of wood under the bottle jack placed against the foundation to support the bottle jack when placing it. DO NOT put the bottom of the bottle jack against the horizontal joint of the generator end-shield!

Please write back to let us know what you find and how you resolve the issue!

 

Dear viralmehta and dear All;

About bearings replacement, I had same experience as CSA one but on a smaller 10 MW gas turbine after gen's bearings replacement. We had to use a large beam to free rotor movement and after few startups, problem is gone but we still keeping the large beam as special tool

THINK SAFETY! We had to make a risk assessment, HAZOP and wok procedure to do this task.

Regards
Karim

 

Dear Viralmehta,

When u give the cool down command, 88CR start and 20TU energized. If shaft not rotate "Shaft Break away failure" alarm will come and 88CR trip. Try the cool down by Lube oil tank temperature nearly 50 degC and Torque adjuster angle manually raise up to 100 deg.