hi guys
am new to the control.com

in ge gas turbine frame 9 e if the turbine tripped suddenly, as per ge manual it is to be kept on turning gear within 15mts. other wise keep 48hrs at rest then put on turning gear. otherwise rotate with ratchet. but ratchet was not provided.

so how to do.

pl.reply.

 

Welcome; we're always looking to add to the community here at control.com!

This is a dilemma. Many Frame 9Es were provided with a ratchet which can be attached to the generator end of the load coupling. It ismeant for rotating the unit during maintenance procedures, and can be installed and removed, though it's not an easy process. And installing it in an emergency situation when the Load Compartment is hot is even more difficult.

Another issue with the ratchet mechanism is that it requires manual operation, meaning that someone must physically be operating the hydraulic controls until the unit can be cranked or it's sufficiently cool to stop ratcheting.

Some Customers have left the mechanical components in place, forgoing the installation of the coupling guard. The hydraulic portion of the mechanism can be installed fairly quickly. But, in my personal opinion this is risky and unsafe.

If one can't get the unit on Cooldown within about 15-20 minutes of reaching zero speed after being tripped or shut down, the key is to keep the Auxiliary L.O. Pump running to maintain L.O. flow to the bearings for cooling until the highest wheelspace temperature has dropped below approximately 95 deg C (approximately 200 deg F).

 

And, to complete the "cooldown" procedure if the unit can't be put on cooldown within 15-20 minutes of reaching zero speed or it can't be maintained on cooldown until the highest wheelspace temperature is approximately 95 deg C (approx. 200 deg F) or cooler, the key is to just keep the Aux. L.O. Pump running to maintain L.O. flow to the bearings to keep them cool. If you don't keep the bearings cool while the turbine and compressor rotor are still hot, the rotor will "wick" some of the heat away from the turbine and compressor and heat up the bearing material. The bearing material used in GE-design heavy duty gas turbines begins to soften and melt at temperatures above approximately 150 deg C (approx. 300 deg F).

The real key, if the unit can't be put on cooldown or maintained on cooldown is to *wait*. I've seen and heard of more than one compressor rotor damaged because people use hydraulic jacks or chain falls to try to turn the rotor before it has sufficiently cooled. A hot compressor rotor (the heaviest part of the shaft) will bend under its own weight and if hot enough the rotor blades will actually come in contact with the bottom of the compressor casing. If the rotor is allowed to cool sufficiently, it will actually straighten almost back to normal, and what little "bow" is left in the shaft can be "run out" by cranking or firing the unit for some period of time before trying to accelerate it to rated speed. But, most managers and supervisors panic and try to turn the shaft using artificial means and actually do more damage and lose more production while performing repairs than if they'd just wait at least 24 hours, or 48 hours as the manual says. Patience is *the* word in this case, otherwise it becomes "forced outage" (two words, but you get my meaning) to repair the rotor.

For units that have a ratchet mechanism, if the rotor can't be turned by the ratchet, the same thing holds true. Once it hasn't turned for about 15-20 minutes, just force the ratchet mechanism off and leave the Aux. L.O. pump running, and wait. Then do the same things: crank, FIRE, accelerate with AUTO mode.

How long to wait really depends on how hot the rotor is when it reached zero speed and couldn't be kept on cooldown. The basic guideline is to wait until the highest wheelspace temperature has dropped to about 65 deg C (approx. 150 deg F) which is usually a very safe and conservative temperature. The 48 hour thing is a conservative estimate of how long it would take to reach this temperature and that depends on a lot of things, such as whether or not there is a tall stack or a short one; whether or not there is an HRSG on the turbine exhaust; how hot the rotor was when it couldn't be put on or maintained on cooldown; what the ambient temperature is; etc.. *DO NOT* open the turbine compartment doors or turn on the turbine compartment vent fan(s)! This will cause the compressor casing to cool faster than the compressor rotor and cause the rotor blades to come into contact with the casing even faster! Time and patience are the only things that will prevent the possibility of great damage.

Once the wheelspace and compressor rotor have sufficiently cooled, and the unit can be put on cooldown again, it's best to just CRANK it for some period of time (20-30 minuites, possibly longer). Then select FIRE mode and allow flame to be established and some heat to be applied to the rotor. The unit will accelerate very slowly as it warms up. After 20 minutes or so, then just select AUTO mode and the unit will accelerate to rated speed (FSNL, Full Speed - No Load). There may some higher-than-normal vibration, but it should smooth out very quickly.

Again, time and patience. Don't try to force the rotor to turn, especially if it's really hot (immediately after a trip or shutdown from high load). Use the time to try to repair or understand why cooldown couldn't be achieved or maintained.

In an emergency situation, the Aux. L.O. Pump can be shut down for some time, as long as somehow (either by cycling the Aux. L.O. Pump on for a minute or so every 7-10 minutes) or by cycling the Emer. L.O. Pump (which usually happens automatically approx. every three minutes or so) in order to try to cool the bearings.

So, the important things are to keep L.O. flow to the bearings while you wait patiently for the shaft to cool sufficiently, then crank/FIRE the unit at low speed to help remove the slight remaining bow, and then accelerate to rated speed.