GE Frame 5 Ratchet Issue


We have nuovo pignon 5002c turbine. During major shutdown in facility where I work, after 6 hours an operator alarm us about problem in the turbine ratchet. We discover that ratchet didn't run for entire 6 hours. And after intervention in it work for just 1 hour (the sequance of cool down is 8 hours) .

The question is there is anybody deal with this situation or heard about bending of HP rotor? Because we are afraid run the turbine (even in crank), and we are very scared about losing the turbine.

If you're talking about the HP shaft, the heaviest--and longest--part of the shaft is the axial compressor. It does get hot, and the metal gets into its elastic range due to the heat. The axial compressor through-bolts (that hold the discs together) will stretch if left in one position for a long time when hot--but not to fear.

As the metal continues to cool the compressor through-bolts will actually return to very nearly their normal straight condition. There will be a little bow or sag and it will cause some vibration issues when you try to start the machine.

All you need to do is get the ratchet working again and leave it running for approximately 4 hours or so, and then I would recommend putting the unit in CRANK mode and giving it a START. The unit will accelerate up to purging speed. You can monitor vibrations as this is happening. The higher speed rotations will help to drive out more of the bow or sag.

After 15 or 20 minutes of CRANKING you should be able to select FIRE mode, and the unit should establish flame and hold at a low speed. This will also help to straighten the through-bolts further. Monitor the vibrations and if acceptable, select AUTO mode and the unit till accelerate to rated speed. The vibration might go up, maybe even cause an alarm as the unit goes through critical speeds, but it should drop back to very near normal.

That's all it takes. The axial compressor through-bolts will eventually return to straight if handled properly and there should be no damage.

The worst damage can occur when people get overly nervous and try to turn the rotor manually (with hydraulic jacks and rams and straps and such). If the shaft is bowed or sagged and the rotating axial compressor blades are in contact with the compressor casing rotating the shaft by any means can cause the blades to crack or break.

The next worst condition is to shut off the Aux. L.O. pump--which will remove the flow of L.O. to the bearings which is necessary to cool the bearings to prevent the heat from the shaft from causing the bearing metal to soften.

So, BE PATIENT when things like this happen. You said normal ratchet (cooldown) operation is eight hours, and you get more than 6 hours before the alarm was reported. Everything will probably be alright. I would recommend waiting four (4) hours--with the Aux. L.O. Pump running--before trying to take any action to rotate the rotor. I even suggest forcing the ratchet off, but leaving the Aux. L.O. Pump running. Just wait.

Then start slowly, with ratcheting first, then CRANKing, then FIREing, then FSNL. And all should be good!

By the way, this topic has been covered many times before on The 'Search' feature of is fast--BUT, the syntax of search commands is not like most popular World Wide Web search engines--so use the 'Help' function of the Search feature. Here's a suggestion:

+"ratchet trouble"

That should return many results. But, the recommendation is always the same: BE PATIENT. WAIT. Keep the Aux. L.O. Pump running (to maintain a flow of oil to the bearings for cooling). Then start with ratcheting, CRANKing, FIREing, and FSNL. And all should be good.