Recently during Steam turbine shutdown, we got in trouble when the AOP tripped. One of the lugs at the terminal had overheated due to lose connection (most probably). The EOP could not sustain barring as the discharge pressure was 1.9kg/cm2. There is an interlock of pressure to be more than 2.5kg/cm2.

Then the pressure interlock was looped and barring was started with EOP till AOP was normalized.

I look after gas turbine operation. My question is what best option we have in case ratchet motor fails (for any reason) during shutdown?
Or at any time during cooldown?
Can we have a standby ratchet motor?
Is providing as airtight closed compartment as possible an option so that the heat does not escape?

 

Seeker1988,

This topic has been covered many times before on control.com. You can try using the search terms:

+cooldown
+"hydraulic ratchet"
+"ratchet trouble"

and other combinations.

Very briefly, with GE-design heavy duty gas turbines one has about 20-30 minutes to get the ratchet/shaft turning after reaching zero speed if the turbine is "hot" before the axial compressor shaft starts bowing under its own weight. (The turbine section of the GT shaft is very short, only two or three stages, and the axial compressor section is MUCH longer, 16 or seventeen stages, and heavier. It's the axial compressor section that will bow if not turned when warm.)

If the shaft can't be rotated (ratchet or cranking) within about 20-30 minutes of reaching zero speed, it's very important to keep the flow of lube oil to the bearings going (to cool the bearings so that heat from the shaft doesn't damage the bearings).

And, then just wait. Be patient and wait. Yes, keep the turbine compartment doors closed, and don't run the turbine compartment vent fans. That will cause the axial compressor casing the cool, and shrink, faster than the axial compressor rotor which will increase the likelihood of rubs and damage.

DON'T try to turn the shaft--even with the ratchet--for approximately 24 hours. It will bow (bend) as it cools, but then as it cools further it will very nearly return to straight, not perfectly straight after about 24 hours.

By then, hopefully the ratchet/cooldown mechanism/equipment will have been repaired and the unit can be put on ratchet/cooldown for a few hours to help remove any remaining bowing. If the unit can be started in FIRE mode and run for about 15-20 minutes in FIRE mode the heat and RPMs will also help to reduce the bowing.

But, GT shafts (the axial compressor shaft, specifically) are not as subject to bowing and internal damage as steam turbine shafts. The internal clearances of steam turbines, and steam turbine packing, are MUCH tighter than GT (axial compressor) internal clearances and because the shafts are longer they don't return as easily to nearly straight as they cool as GT (axial compressor) shafts.

Patience is the key--and continued efforts to try to turn the shaft after about 30 minutes at zero speed when it's warm will almost always result in axial compressor blade rubs and/or damage. It's EXTREMELY difficult for many operators and Operations Managers and Plant Managers and the Mechanical Department to just "sit and wait"--but it's the proper and prudent thing to do for GE-design heavy duty gas turbines. It's NOT written in any manual anywhere, but it is industry-wide, standard and accepted practice.

Or, it will become standard practice after the axial compressor is damaged by continued efforts to turn/start the turbine when it hasn't been on ratchet/cooldown for more than about 30 minutes....

Hope this helps!

P.S. Are you suggesting to have a second, standby ratchet pump and mechanism all wired-up and tubed-up and ready to just "switch over" in the event of a failure of the original ratchet pump/mechanism? I've implemented one of these, but it was expensive and was never used before the turbine was shut down and relocated to another part of the world (it was in place for almost 20 years). It was entirely a manual operation to switch multiple valves, and manually switch solenoids, manually switch discrete inputs, and manually switch motor starters, etc. And, in the end, there's still singular jaw clutch solenoids and one jaw clutch and they're still single points of failure. The best idea is just to understand what to do, have spares ready to install, and perform regular maintenance and upkeep.

 

Thank you for the reply.

 

What if the ratchet fails when the wheelspace temperature is around 200 deg. C?

Once it happened that the ratchet timer T2HR didn't reset to zero (for L20CS and L33HRF to change state) and kept increasing. ratcheting didn't happen for an hour or so then.

 

Seeker1998,

The bearing material commonly used in GE-design heavy duty gas turbines melts at approximately 200 deg F. Heat from wheelspaces will heat the shaft and that heat will be transmitted to the bearings. That's why it's critical to keep L.O. flowing to the bearings when the ratchet/cooldown mechanism isn't working. Especially when the wheelspace temperatures are above the melting point of the bearing material.

Usually the operator isn't able to select Cooldon OFF until a sufficient amount of time has passed so that the highest wheelspace temperature is approximately 50 deg F below the melting point of the bearing material. (I believe some GE-design control systems now allow an operator to select Cooldown OFF based on actual wheelspace temperature--instead of time.

Again, this has all been covered before many times on control.com and this--and many other topics related to GE-design heavy duty gas turbine operation and control--are accessible using the 'Search' feature.