We have GE Machine Frame 6, 30 MW.

I want to know, how long we should run Ratchet?

Can we safely stop Ratchet below wheel-space temperature 100 degree centigrade? or must be below 50 degree centigrade? what is the criteria to stop ratchet?

 

This topic has been covered several times on control.com. Use the search term 'cooldown', because a ratchet mechanism is one of the methods used for the cooldown of a GE-design heavy duty gas turbine.

Before the wheelspace T/C temperatures were used as the permissive for allowing a unit to be taken off cooldown, there was just a timer, sometimes set to 24 hours or more, and usually the machine was cool enough to be taken off cooldown before the 24 hour timer expired. Now that the wheelspace temp feedback is used as the permissive, the typical time for cooldown has been reduced, significantly in some cases (depending on ambient and other site conditions).

Remember also that the wheelspace temperatures are not measuring the temperature of the turbine or compressor rotor (shaft) and that is the primary purpose of cooldown: preventing a hot shaft from taking a "set" (warping) if shut down and left in a stationary position for very long.

But, it's not the only purpose of cooldown. If the shaft temperature in the bearings is high, then it will soften, deform, or even melt the bearing material if L.O. flow is not maintained to the bearings for cooling.

There is no method for directly measuring the shaft temperature, so the wheelspace T/Cs are used as the next best estimate of internal machine temperatures. Before bearing metal temperature T/Cs were used, there was no way to monitor the temperature of the bearings.

So, cooldown is not only about protecting the shaft, it's also about protecting the bearings. Other than power consumption for the pumps and motors, it generally isn't harmful to leave the unit on cooldown longer than is necessary.

But, please search past threads on control.com, and then tell us when you think it's safe to take the unit off cooldown, and under what conditions it can be taken off cooldown if absolutely necessary.