Crank Speed for Rapid Cooling


Thread Starter


Our gas turbine is GE MS9371 9FB.

Until today we have applied turning gear natural cooldown operation without force cooling. But especially in maintenance periods, I need rapid cooling methods/application via selecting crank speed with LCI to accelerate turning gear shutdown process. In natural cooling, following shutdown, when wheelspace temperature is less than 150 F, turning gear is being off after around 60 hours via, and this natural cooling causes waste of time.

GER3620 is giving general information for us but it is not adequate.

Do you have any application or operation methods for this topic?
CRANK cooling, or as it's sometimes called, forced cooling, is best accomplished by trending the wheelspace temperatures during CRANKing. You will observe at the beginning of the crank cooling the wheelspace temperatures will drop fairly rapidly, then over time they will level off until they don't drop very much at all. At that point, it's best to stop CRANKing, usually for a couple of hours. This allows the heat in the metal to work its way "out" to the surfaces. You may even see the wheelspace temperatures start to rise ever so slowly.

Then re-start CRANKing while observing the wheelspace temperatures. You should see the wheelspace temperatures drop fairly rapidly again, and then, again, begin to level off. Once they have leveled off, repeat the steps above, until the highest wheelspace temperature has dropped below approximately 135-140 deg F (this because there is still some residual heat in the unit).

DO NOT turn on the turbine compartment vent fans during this CRANK cooling, and DO NOT open the turbine compartment doors and leave them open. I believe the OEM even recommends against turning on the #2 bearing vent fan, but I have heard mixed feelings about whether or not this is potentially injurious.

One of the things to watch is the LCI cooling water temperatures, and the bridge temperature (if available). This is really the limiting factor.

I believe very recent versions of GER-3620 have some guidelines about rotor end-of-life calculations involving extended CRANK periods (for any reason).

Many sites just start CRANKING, and they CRANK, and they CRANK, and they CRANK, and they CRANK, and they .... But usually that's not as effective as cycling CRANK periods watching the wheelspace temperatures as the indication of when to stop and when to start. Cyclic CRANKing is usually much more effective. (The OEM used to sell a forced cooling option that just cycled CRANK periods based solely on time, and while this was more effective than continuous CRANKing, it wasn't as effective as using the wheelspace temperatures as indicators.)

SOME sites also force the IGVs to the full open position during CRANK cooling, but I personally do not like doing that on F-class machines (they are very unforgiving machines and the internal compressor clearances are very important to performance and efficiency). I might consider doing this during latter CRANK period, but NOT at the beginning as the temperature differentials are just too high. (One thing that happens when the IGVs are opened during CRANKing--whether it's while force cooling or when off-line water washing--is that the torque required to turn the rotor increases and can add to the load on the starting means (the LCI in your case). I've seen units with electric motor starters (9Es and 6Bs and 7E/EA units) actually drop so much speed that 14HM drops out and that can cause the CRANKing to be aborted by the Mark* in some cases. So, that's another consideration to be aware of depending on the machine and the type of starting means, though it's probably not so applicable to F-class machines with LCIs.)

Finally, at some point someone at the plant is going to look at the "bill" for CRANK cooling.... It's NOT cheap; LCIs use a LOT of electrical power, and it's generally not for free. You're talking about hours of CRANKing, and since LCIs can consume several MWs over those hours, it eventually begins to add up. Best to do some quick calculations about the cost (electrically) of CRANK cooling, just to have an answer when someone (finally) asks, "How much is this costing?" (It might even be a good idea to let management/ownership know in advance of starting this practice; it's a cost, and not a small one, though in the overall scheme of an outage, or several outages, it might be economical--but, still, best to consider this in your analysis and planning.)

Hope this helps!