Frame 9E 88BT Logic

H

Thread Starter

Haris

Hi,

We have a frame 9E Gas Turbine with Mark VI control.

What is the Logic of Turbine Compartment Vent fans during shutdown and Trip? As far as manual is concerned it mentions that both fans should trip at flame out and restart after one hour and remain running until the wheelspace reaches 300 F. But I have checked the m6b and past trends which state otherwise and the fan continues to run after flame out, until the wheelspace condition is met. This is probably causing rapid cooling of the turbine.

Can anyone Please advise regarding this scenario. I can share more details if required.
 
S
Hello Harris,

for a 9FA gas turbine, BT fans start with either Gt start or with flame on and stop when flame is off and temperature switch 26BT-1 is de-energized. turbine compartment lower than 40 oC or something like that. The BT fans still operate during shutdown or trip.
 
Harris,

The philosophy--and resulting logic--for turbine-, accessory- and load compartment vent fan operation is dynamic, meaning that is has evolved, and continues to evolve, over time--in this case, decades.

It would be necessary to know a lot about how the compartment enclosures are configured and what the philosophy was at the time the unit was installed/commissioned. For, example, some units used to start the vent fans when a unit START was initiated, and keep them running until the unit was taken off cooldown (ratchet; slowroll; turning gear).

It was changed to start the vent fans when flame was detected in the turbine, and then leave them running until the unit was taken off cooldown.

Then it was changed to stop the vent fans when compartment temperatures dropped below the setpoint of a temperature switch that was mounted in the compartment(s).

Then it was changed to stop the vent fans when flame was lost in the compartments, and start them when on cooldown if the compartment temperature exceeded a setpoint, and shut them down again after it dropped by a certain setpoint.

I've never seen logic that looks at wheelspace temperatures to determine when to start or stop vent fans--that would certainly seem to be excessive. Vent fan operation will cause the turbine and compressor shells to cool faster than the rotor and can result in rubs of rotating blades/buckets with the casing(s).

Lately, the philosophy seems to be to stop the vent fans as soon as flame is lost in the turbine during a trip or shutdown, and then cycle them as necessary to prevent the compartment temperature(s) from getting "excessive"--all while allowing the compressor- and turbine casings to cool without the vent fans continually drawing air into and through the compartment and cooling the casings faster than the rotor.

But, to properly answer the question about how it's done at your site--it would be necessary to examine the sequencing/application code in the Speedtronic panel at your site. That's the ONLY description of operation that matters--because that's what's controlling the unit and its auxiliaries. Not what's written in some paragraphs in some section of the manual, or even the Control Specification. What's programmed in the Speedtronic is what's actually controlling the unit and its auxiliaries.

And, depending on the vintage of the machine(s) at your site, the sequencing/application code can be different from other GE-design Frame 9E heavy duty gas turbines. From a turbine standpoint all Frame 9Es are very similar, but, from an auxiliaries standpoint they can be very different.

Hope this helps!
 
Dear CSA,

Thanks for your detailed reply. i went through our m6b and have found the following start-stop logic. The problem of rapid cooling is probably resulting in failure of shaft breakaway from zero speed. And we are looking that whether or not we need a logic modification.

Start Logic

1. (L4BTRUNX) If turbine compartment temperature > 50ºC AND compressor inlet temperature < 15ºC AND rpm > 300 then
a. After 5 minutes it will check the above condition again and the process will continue.

2. (L4BTRE) If compressor inlet temperature is less than 15ºC AND Compartment temperature is greater than 50ºC AND wheel space temperature is greater than 149ºC AND no fault on motor AND no fire protection trip then

a. The fan starts and keeps running as long as the condition remains true.
OR
(L4BTW) If Fan permissive (L4BT) to Start exists AND wheel space temperature is greater than 149ºC AND compressor inlet temperature is greater than 15ºC AND no fault on motor AND no fire protection trip, then

b. The fan starts and continues to run until conditions remains True.
OR
(L14HM) If speed is greater than 300 RPM AND no fault on motor AND no fire protection trip, then

c. The fan starts.

Stop Logic
1. L26WSBTSTP
Heat Vent Turbine Compartment Fans Stop due to High wheel Space Temp
 
Malik787,

Without being able to see the relay ladder diagram or function block diagram representation of what you are attempting to describe I can't comment.

To my personal way of thinking, cycling turbine compartment vent fans based on wheelspace temperatures is, ..., well, ..., er, ..., er, ..., uh, ..., pretty absurd. But, the GE Belfort (in Belfort, France--which should say a lot about the situation) have their own ideas and no one is going to deter them from implementing them. Even when tried and true logic has been around for decades, they are going to needlessly complicate things just because they can.

Sorry; unless you can post the "rungs" from the Speedtronic in a format we can all see and view--and do so without any typographical errors--it's just too hard to try to understand a written description such as you've tried to post. Especially when Belfort has written the logic you're trying to describe!
 
Hello Friends,

Just to share my experience on this topic, I had been to a site in Oman, The GT had new design of air ducts looks like Octopus ducts blowing straight on to upper half of Turbine shell in the centre. The unit was on reliability run for a month period. The company who installed was the first to develop such huge cooling ducts with a huge blowing motor 88BT1, 2 thinking they need more air to cool the unit due to ambient in gulf region high.

On first shutdown of unit and when first ratchet cut in we heard rubbing sound every 3 minutes for 5-6 hrs. The reason was turbine cooling shell got cooled earlier than rotor. Luckily there was no damage, Later on the duct was modified and directional louvers were installed to control and divert the flow away from turbine and compressor shell

So As Dear CSA have mentioned this has to be site specific.

Mambo
 
Malik787,

With just a cursory glance at the application code, I would say that shutting down the turbine compartment vent fan(s) by looking at wheelspace temperatures is most likely going to result in problems for the machine (compressor rubs; bucket tip rubs). The wheelspaces are inside the machine, and turbine compartment ventilation fans circulate air around the outside of the machine. If anything, if one wants to cool the internals of the machine the faster the way to do that is by cranking the machine until the wheelspace temperatures stabilize, shut it down for about 30-60 minutes, then crank it again until the wheelspace temperatures stabilize again, then shut it down for 30-60 minutes, and so on until the desired internal temperatures (as measured by the wheelspace thermocouples) are reached.

The reason for cycling the cranking periods is that during cooling the wheelspace temperatures will level off (stabilize) and continued cranking will not all the differential to increase to allow more heat to travel "out" from the internals of the machine. Shutting down the unit for 30-60 minutes will allow heat to radiate out increasing the differential between the metal and the air temp--so that when the unit is started cranking again that more heat will be removed faster. Until the differential levels off and stabilizes, at which time it's necessary to stop the unit and wait another 30-60 minutes for the internal temperatures to radiate out again.

This won't be seen by monitoring the wheelspace temperatures--that is, the radiating out of the internal temperatures won't be seen by monitoring the wheelspace temperatures. But, it does happen--it's just not easily monitored.

Cyclic crank cooling is the only way to drop machine internal temperatures quickly--even faster than continuous crank cooling.

But, cycling turbine compartment vent fans on wheelspace temperatures is, really, just asking for problems. Begging for it, in my opinion. The turbine shell and compressor case will cool much faster than the internals when the unit is on cooldown (ratchet; turning gear; slow roll) and this will result in contact between rotating and stationary parts, which will result in damaged blade/bucket tips and increased internal clearances where the rubbing/damage has occurred which will also decrease efficiency.

Unless there's something we don't know about the configuration of these machines (and there may be!), <b>based on the information provided</b> cycling turbine compartment vent fans based on wheelspace temperatures is going to most likely result in rubs and damaged blade/bucket tips.

You didn't provide any of the Control Constant values, and, again, I just had a very quick look at the logic and re-read what you originally wrote. And, if there's something about these units which makes them different from other similar GE-design Frame 9E heavy duty gas turbines, then it's difficult to say exactly why this was done--except that it's typical of GE Belfort to needlessly and unnecessarily complicate turbine auxiliary operation. (This may be BHEL "logic", too; they have been known to try to emulate their French brethren from time to time.)
 
Dear CSA,

Thanks for your detailed reply. i went through our m6b and have found the following start-stop logic. The problem of rapid cooling is probably resulting in failure of shaft breakaway from zero speed. And we are looking that whether or not we need a logic modification.

Start Logic

1. (L4BTRUNX) If turbine compartment temperature > 50ºC AND compressor inlet temperature < 15ºC AND rpm > 300 then
a. After 5 minutes it will check the above condition again and the process will continue.

2. (L4BTRE) If compressor inlet temperature is less than 15ºC AND Compartment temperature is greater than 50ºC AND wheel space temperature is greater than 149ºC AND no fault on motor AND no fire protection trip then

a. The fan starts and keeps running as long as the condition remains true.
OR
(L4BTW) If Fan permissive (L4BT) to Start exists AND wheel space temperature is greater than 149ºC AND compressor inlet temperature is greater than 15ºC AND no fault on motor AND no fire protection trip, then

b. The fan starts and continues to run until conditions remains True.
OR
(L14HM) If speed is greater than 300 RPM AND no fault on motor AND no fire protection trip, then

c. The fan starts.

Stop Logic
1. L26WSBTSTP
Heat Vent Turbine Compartment Fans Stop due to High wheel Space Temp
Dear Malik 787,

I can confirm you by ( having an Frame 9E Mark6e app code) that some "ADD/MODIFICATIONS " have been brought to 88BT Controls/sequencing logic.

Please see herbelow comments added to the Mark 6e Rung :
ADD NEW CODE FOR BT FAN CONTROL AS THE 26BT TEMP SW IS BAD**
1. BT ALWAYS ON IF FLAME
2. AFTER FLAMEOUT BT WILL BE OFF 15 MIN ON 30 MIN UNTIL L69TWW=1
IE MAX WHEELSPACE BELOW 300 F


Looks like your solution :
Stop Logic
1. L26WSBTSTP
Heat Vent Turbine Compartment Fans Stop due to High wheel Space Temp

Has been changed and now L69TWW is now taked into account for the stop/start logic sequence on Flame out event.
As written by Engineering L26WSBTSTP WAS "BAD" solution.

Please tell us what you think about that .

Regards,
Controls Guy25.
 
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