Heating and Ventilation Logic for GE-9E GT

When two of our GE-9E gas turbines were commissioned we observed that time for both GT after a shutdown the ventilation fan for Turbine compartment (88BT) was running for compartment cooling as well as Turbine casing cooling. The fan was stop after certain time period and again start after 15 min and run for 1-hour and again stop. this was continued till the highest wheelspace temperature reduced to 300 degF. But at present we are observing the 88FV fan keep running for long time rather than the previous operations of 88BT. We notified to GE local T/A about the matter, but did not get any satisfactory answer. Can anyone has the correct and authentic answer of running 88BT for the cooling purpose?
 
rokan_123,

Only GE can answer your question with any authority (correct and complete purpose). And GE in Belfort, France, has authority for determining how the Frame 9E machines are to be operated and controlled and protected. (And they have a pension for making things complicated and difficult to understand--just relaying the truth. Don't shoot the messenger!)

I'm not sure what Belfort is labeling as 88FV; 88LV was the Load Compartment Vent fan for many years. And 88BA was the Accessory Compartment Vent fan for many years. But, I'm not familiar about 88FV. Can you be more specific?

The purposes of the turbine compartment vent fan are:

--Cool the turbine compartment when flame is present in the combustors
--Evacuate the compartment of hazardous gases (both when the turbine is running (flame is present) and afterwards, when vapours may be present from piping/tubing leaks--including L.O. leaks)
--Prevent the turbine compartment temperature from getting too high during Cooldown (after shutdown or trip)

The whole business about running the turbine compartment vent fan until the highest wheelspace temperature is suspect and, in my personal opinion, ill-advised. When the unit is on cooldown cooling the axial compressor- and turbine casings using the turbine compartment vent fan can cause them to "shrink" faster than the rotor. This can--and HAS--led to compressor rubs and turbine bucket tip and shroud block damage because of uneven cooling of the casing and rotors.

The standard, tried-and-true, decades-old logic was to start the turbine compartment vent fan as soon as flame was detected in the combustors, and then during a shutdown or trip to leave the compartment vent fan running until the compartment temperature was less than approximately 200 deg F (I can't remember if it was 26BT-1 or 26BT-2). Then it was determined that leaving the compartment vent fan running after flame was extinguished was leading to compressor- and/or turbine rubs, and so that practice was stopped. Then, some machines were shipped with logic that cycled the turbine compartment vent fan periodically (on and off) until the compartment temperature dropped below approximately 200 deg F. (NOTE: The fan cycled on and off until the temperature dropped below the switch setpoint; it didn't run until the switch dropped out, and then re-start when the switch picked up; it ran for a period of a few minutes (usually minutes; and then was shut down for a period of 10 or 15 minutes; and this cycle was repeated until the temp switch dropped out. If the compartment temperature increased above the switch setpoint again, the fan did not start cycling again.)

With the advent of DLN-I combustion systems and the greatly increased number of gas fuel piping flanges and the possibility for more leaks, it was then decided to leave the turbine compartment vent fan running for about 15 minutes after reaching cooldown, and then cycle it periodically until the compartment temperature dropped to less than approximately 200 deg F as described above.

Now, it seems that someone (Belfort?) has decided to keep the turbine compartment vent fan running continuously until the highest wheelspace temperature reaches some setpoint--which is DEFINITELY going to cause uneven cooling rates of the casings and the rotor.

Now, any other compartment vent fan will likely not have the same logic. In the Load Compartment, for example, there's no fuel so the need to evacuate the compartment of fuel vapors is unnecessary. And, while there may be fuel flowing through the Accessory Compartment, it's not typical to keep the Accessory Compartment Vent Fan running after the unit is shut down (though in some hot climes that may be recommended; there's a LOT of heat coming off the L.O. Tank during cooldown).

But, in the end GE Belfort has the last word--unless you own the machine and it's under warranty or LTSA (Long Term Services Agreement) and then you're free to make the unit and auxiliaries operate as you see fit. The OEM doesn't really have a lot of machine operating experience--they're manufacturers. Sometimes, the owners and operators understand the nuances of operating GE machines better than the OEM. And, there may be something about the technical regulations and standards in your part of the world that dictates other operating considerations.

While on cooldown, the turbine compartment needs to be kept ventilated--primarily for removing combustible vapors, but also to keep the compartment temperature from getting excessively high to prevent damage to wire insulation (which are usually rated at from 90-110 deg C or higher). Using the turbine compartment vent fan to cool the turbine and axial compressor--NOT a good idea, even in an emergency. It has happened MANY times that compressor- and turbine rubs have caused serious problems with efficiency and operation, caused by cooling the axial compressor- and turbine casings too fast.

And, since the turbine compartment vent fan can't effectively cool turbine wheelspaces to any great degree, basing the time to shut off the vent fan on turbine wheelspace temperature is, well, ..., just asking for trouble. Especially if the turbine is in a temperature climate (not tropical or desert). Ambient air being drawn through an axial compressor and turbine by the "chimney effect" of the hot exhaust is the best way to cool BOTH the rotor and casings--from the inside out--without causing rubs and damage.

Again, all compartment vent fans are not going to have the same purpose--and will not necessarily operate in a similar manner. You have done the right thing to ask about the purpose of the turbine compartment vent fan (88BT), but without knowing exactly what 88FV is, we can't say for sure.

I know there were some Frame 9Es which were shipped with very small turbine compartment vent fans--and those were just used to ensure that combustible gases were being evacuated, and only provided a small amount of compartment cooling. There were even motor-operated dampers on some of the air inlets of those units to try to help keep the turbine compartment "bottled up" and to prevent excessive cooling of the axial compressor- and turbine casings and reduce the possiblity of rubs. Truth be told, if there was no ventilation in the turbine compartment when it was on cooldown it would be best for the turbine and compressor, but there is the issue of fuel leaks and combustible vapors and preventing wire insulation damage from excessively high turbine compartment temperatures during cooldown.

Hope this helps!
 
> Thanks for writing back, but can you please tell us what 88FV refers to?

In our plant the FVs are the "Liquid Fuel Compartment Fans".
 
>> Thanks for writing back, but can you please tell us what 88FV refers to?

> In our plant the FVs are the "Liquid Fuel Compartment Fans".

The previous statement is for our 6FA+. In our 6FA, the FVs are the LF-AA-WI enclosure fans (in the 6FA+ the liquid fuel pumps have an independent enclosure).

Sequencing in the 6FA+ is slightly different to that in the 6FA.

In the 6FA+ the permissive to start the FVs (L4FV1) is the liquid fuel forwarding pumps master control signal (L4FD):

L4=1 & water wash not selected => L4FD=1 => L4FV=1 => if there's no fire detection or motor fault, the fan will start. The moment L4FV drops,there's a motor fault or fire detection,the fan stops.

In the 6FA the permissive to start the FVs (L4FVRUN) is the atomizing air master control signal (L4AKRUN). In this case there's a stop signal time logic.
 
Dear CSA & REI

Sorry for the delays for the simple reply..the 88FV means Liquid Fuel Compartment Vent Fan,a closed enclosure inside Aux compartment where Main Fuel pump PF1 & Flow divider 88FM with fuel pressure control valve & High pressure filter arrangement installed.

Mr.CSA describes with good explanation of running of Turbine compartment vent fan(88BT) after a shutdown/trip.Consideration of Wheelspace temperature on the running of Turbine compartment vent fan is that when wheelspace temp is high/higher side then there is a possibility of forming vapour of Lube oil & liquid fuel oil if there any leakage due to heat radiation from Turbine hot paths as explained by GE T/A....the answer is logical...isn't it?

Regards,
Rokan
 
rokan_123,

> Consideration of Wheelspace temperature on the running of Turbine compartment vent
> fan is that when wheelspace temp is high/higher side then there is a possibility of
> forming vapor of Lube oil & liquid fuel oil if there any leakage due to heat
> radiation from Turbine hot paths as explained by GE T/A....the answer is
> logical...isn't it?

Wheelspace temperatures are always going to be higher than compartment temperatures, just by virtue of the fact that they are internal to the gas turbine, and adjacent to the path of hot combustion gases through the turbine.

If the consideration is that leaking fuel or L.O. can become vaporized and cause an explosion hazard, well, the same is true when the turbine is running and the compartment vent fans are running. Even the best computer-based dynamic flow simulations won't ensure that pockets of combustible vapours from hydrocarbon leaks won't collect in pockets in the turbine (or liquid fuel) compartment--whether the turbine is running or not.

One purpose of the compartment vent fans is to try to remove or prevent combustible vapours and gases from collecting and exploding--but to keep the compartment vent fans running after a trip or shutdown until the wheelspace temperature cools to a particular level is absolutely going to ensure that the axial compressor- and turbine casing are going to cool faster than the rotor and this will cause rubs at the same time they're circulating air to prevent the collection of combustible vapours from any source.

Do the compartments not have combustible gas detectors? Or are the combustible gas detectors located in the vent fan ducts? (Which I've ALWAYS thought is a terrible idea, because to work properly the sensors need to be kept clean--and blowing unfiltered air from a compartment which is likely to have some liquid fuel and/or L.O. leaks is going to contaminate the sensors and reduce their ability to effectively sense combustible gases. Even the oils from human hands can adversely affect some combustible gas sensors (even the Bently-Nevada combustible gas sensors!).

Using the compartment vent fans to protect against explosive concentrations of combustible gases is the correct and proper thing to do--but basing the time to shut down the fans on internal gas turbine wheelspace temperatures, and leaving the fans running after flame has been extinguished in the combustors for long periods of time is going to cause rubs. There's no doubt about it. Now, that won't happen in the liquid fuel valve compartment, but it WILL happen in the turbine compartment. (And, what does wheelspace temperature have to with liquid fuel valve compartment temperature?)

And the turbine compartment vent fans aren't going to protect the inside of the turbine from hydrocarbon vapours due to leaks in the turbine which could be ignited from the high temperatures in the turbine. So, why use wheelspace temperature as a determining factor for when to shut down the turbine compartment vent fans?

Why not install multiple temperature sensors (T/Cs, or RTDs) in the compartments in strategic locations and use the average or the highest temperature to cycle the compartment vent fans? This way excessive cooling of the axial compressor- and turbine casings can be limited, and ventilation of the compartments can occur on a periodic basis. And, for units with combustible gas sensors in the vent fan ducts this will also allow periodic "sampling" of air for combustible gases.

But to even suggest that wheelspace temperature is relative to compartment temperature is just, ..., well, ..., it's not very, ..., logical. The cooling rates of the casings and the rotor have to be considered--and using wheelspace temperature doesn't do that. And running the compartment vent fans continuously after a trip or shutdown until the internal wheelspace temperature cools to a particular temperature will certainly ensure the compartment temperature is likely below the point at which combustible hydrocarbon vapours could form from fluid leaks--but why not just monitor compartment temperature in that case.

And why run the compartment vent fans continuously after a trip or shutdown when the risk of rubs is high?

Is that logical?

I think not.
 
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