7FA Gas Turbine Crank Cooling

T

Thread Starter

Tom Murphy

Hello,

I have a 7FA question. Is it recommended not to crank cool a 7 FA Gas Turbine too many times. We do a lot here for maintenance. Is there a limit

There has been some controversy here lately 1.5 years into operating these 2 units at this site about Crank Cooling too much and that we should let the engine naturally cool on Turning gear for several hours then crank. With a hot machine we go from running to turning gear to crank. I cannot find anything in the GEK's where this is stated. Can you advise please?

Thank You
Tom Murphy
 
B

bicycle Mechanic

Without knowing how your unit is cranked, but if its an LCI, I would be cautious as to the extended length of run time. SCR's have a life span...which is unpredictable.

I like the idea of fast cooling the unit this way, if there was an emergency repair, and need to be back on line.

I look forward to hearing more on this.
 
Tom,
There is a general recommendation in GER-3620:

Prior to an inspection, it may be necessary to force cool the unit to speed the cool-down process and shorten outage time. Force cooling involves turning the unit at crank speed for an extended period of time to continue flowing ambient air through the machine.
This is permitted, although a natural cool-down cycle on turning gear or ratchet is preferred for normal shutdowns when no outage is pending.

Opening the compartment doors during any cool-down operation, however, is prohibited unless an emergency situation requires immediate compartment inspection—which requires that the doors be opened. Cool-down times should not be accelerated by opening the compartment doors or lagging panels, since uneven cooling of the outer casings may result in excessive case distortion and blade rubs that could potentially lead to tip distress if the rubs are significant.
 
Tom, I do not think there is a black and white answer to your question. This topic has been discussed here and on many other sites before. I will put it simply, since that's all I know.

Is crank(force) cooling good for a unit-NO.
Do a lot of sites do it-YES.

Why would someone do something to their unit that they know is not good for it-MONEY

GE has produced several documents that discuss estimating turbine parts life. To attain the longest parts life for a gas turbine you would start a machine and run it for a long period of time(read months), then shut down, perform maintenance and the restart and run for months again.

How do you minimize the life of parts for a gas turbine.

1) Start and stop the unit each day, or do multiple starts and stops in the same day.
2) Run liquid fuel.
3) Run the unit at "peak" load.
4) Trip the unit from full load.
5) Perform lots of fast load starts.

Thermal cycles of hot gas parts have the most effect on reducing parts life. Accelerating the heating rate of a component(read fast load start), or increasing the cooling rate of a part(read force cooling), will cause increased thermal stress and lead to reduced parts life, and more maintenance.

But lots of sites do force cool for the same reasons you guys do I suspect. It saves time. You can water wash sooner, start taking the machine apart faster, whatever. But doing this has a cost, and hopefully someone understands this.

In your case I would suggest that spending as much time on turning gear as possible before force cooling is best. Whatever mode allows for the slowest rate of temperature decrease will be better for the turbine.

Good luck Tom, keep the questions coming!
 
Dear Gents,

we are also operating six GE 7FA DLN 2.6 Gas turbines in saudi arabia, we do force cool the turbine through proper procedure

1. After M/C shutdown once it reaches to Turning gear speed (6 RPM) - six hours of natural cool i.e on turbine gear speed

2. after six hours we crank the machine at cranking speed ( 850 RPM ) for approx 5 hours ( or once highest wheel space temperature drops below 65 degc ) on colder days the crank period would be less due to low ambient temperature. it may take 6 hours! to bring the wheel space temperature during peak summer

Hope this helps!!

regards
KKK
 
What is the importance of wheelspace temperature bringing up/down in this case?

> 2. after six hours we crank the machine at cranking speed ( 850 RPM ) for approx 5 hours ( or once highest wheel space
> temperature drops below 65 degc ) on colder days the crank period would be less due to low ambient temperature. it
> may take 6 hours! to bring the wheel space temperature during peak summer
 
Randy,

On a GE-design heavy duty gas turbine it's important that the internal temperatures of the gas turbine be below a certain value before the unit is taken off "turning gear". It can take as long as 24 hours, sometimes longer, for the unit to cool sufficiently while on "turning gear" to take it off "turning gear" to prevent the axial compressor rotor from sagging, or, bowing, under it's own weight while it's still warm and the metal temperatures are in the elastic range.

If a unit is taken off "turning gear" prematurely for some reason (before the internal temperatures have cooled sufficiently) the axial compressor rotor can develop a "bow" or "sag" which can cause high vibrations during the next start attempt. Reasons can include off-line compressor water washing, and repairs to turbine or auxiliary components which require the lube oil and/or the hydraulic systems to be shut down quickly.

So, "forced cooling" is used when it's necessary to take the unit off "turning gear" sooner than would otherwise be possible by simply letting it cool naturally while on "turning gear." Forced cooling involves using the starting motor/cranking motor to rotate the unit faster (at a higher RPM, sometimes called "cranking speed") than the "turning gear" mechanism, using the air flow through the axial compressor to cool the turbine- and axial compressor internals quicker.

The problem with doing this "a lot" is that the faster cooling rate contributes to the thermal shocks of start-up and shutdown and emergency tripping that decrease the hot gas path parts life. Turbines would LOVE it if you started it once, ran them up to Base Load, and left them there for months at a time. Base Load (for a GE design heavy duty gas turbine) means the internal temperatures are kept constant regardless of ambient or machine conditions, and that means there is less stress on the hot gas path parts. They are heated once, during starting and loading, then loaded to Base Load where the internal temperatures are constant. That's the best operating conditions.

Start-ups and shutdowns cause thermal cycling of the hot gas path parts. And so does "forced cooling" of the unit internals.

Wheelspace temperatures in a GE-design heavy duty gas turbine are the closest approximation of the temperature of the shaft, which is what needs to cool sufficiently to be able to take the unit off turning gear without causing the axial compressor rotor to bow or sag. We don't know what kind of turbine you are working with, so we can't say for sure how internal temperatures are measured.

Hope this helps! Write back if you need more information--but be sure to tell us something about the type of turbine and control system you are working on. Remember, we aren't there with you and we don't know what you know and see every day.

Lastly, there is a wealth of information available here for GE-design heavy duty gas turbines and control system, but we don't have a lot of questions or responses for other turbines and control systems at this time. But, we'd like to!
 
T
At this site we have 2 7FA's on Barges. Operation is #2 Diesel Fuel. Many stops and starts due to dispatch needs. Water Injection for Base Load of course. CI every six months or so and HGP about 1 time a year or so. Some exhaust diffuser cracks that are repaired as needed In operation now for about 1.5 years. When not at Base load always at 70 MW. Overall operation is pretty smooth on diesel but I prefer gas of course. Thanks a lot everyone for answering my questions. This site is really helpful.
 
> Thanks a lot everyone for answering my questions.

your question is not clear, however, as is said previously in this thread, in GER-3620 you can find that the calculation of the Inspections intervals is based either on starts number or on operating hours depending upon your starting frequency, all what you cited as a particularities of your site such as (excessive starts number, water injection, using gasoil as fuel …) contribute to decrease inspection intervals, if you can clarify your question, we could be helpful.
 
A

Akram M. ALRAZEHI

Does anyone know what is the maximum allowable duration after shutdown of gas turbine before starting turning gear that before this time there is no danger of shaft sagging?

thanks in advance
 
The accepted "rule of thumb" (guideline) for most GE-design heavy duty gas turbines is that if the unit is hot and can't be put on turning gear within 20 minutes of reaching zero speed then just keep the L.O. Pump running to keep oil flowing to the bearings, and don't try to put the unit on gear. (Force turning gear off, if necessary!)

F-class turbines are two-bearing machines--with long compressors and a long distance between bearings. AND, the compressors and turbine discs are HEAVY. They really need to go on turning gear as soon as they reach zero speed--and certainly if they can't be put on gear in LESS THAN 20 minutes after reaching zero speed (especially if the unit was tripped while running at or near Base Load) then no efforts should be made to try to get it on turning gear after 20 minutes have elapsed after reaching zero speed.

Why? Because if jacks and/or levers are used to start the shaft turning it's very likely that axial compressor blades (rotating and stationary) will at the very least be bent or broken or abraded.

Just be patient, wait at least 24 hours (with the L.O. Pump running to keep the flow of oil to the bearings for cooling). Impatience has cost many sites a LOT of money and a LOT of lost production. I know of several sites that persisted for hours after failure to get the unit turning--and caused millions of USD in damages and lost production.

Again, F-class shafts are long and heavy, and 20 minutes is probably really above the maximum limit--especially if the unit was tripped from load and the shaft is very hot. If the unit was shut down normally, with a fired shutdown, then it had a chance to cool slightly--but, still; 20 minutes is about the maximum limit for such long, heavy shafts. If it were my employer's turbine and it tripped from load and couldn't be put on turning gear within about 10 minutes, I'd say wait the 24 hours.

But, then I've (rightfully) been accused of being very conservative. And it's not even my money at stake!

Hope this helps!
 
Thank you so much CSA for the beneficial information you gave,
its good to protect blades not to get hurt, but still have a doubt of the benefit of waiting 24Hrs regarding increasing the possibility of shaft sagging
 
RotoMan,

The 24 hours is to allow the shaft to deflect (sag) as it will under its own weight when it's hot and the metal temperatures are in the elastic range.

As the shaft cools, it will actually begin to straighten--though it won't return to exactly straight. Some time on turning gear (a couple hours minimum) and some cranking (an hour or more) will help to return the shaft to virtually straight.

Also, selecting FIRE mode and initiating a START will also help to warm the shaft and help to straighten it before just selecting AUTO which will accelerate the shaft to FSNL. The vibration levels might be a little higher than normal, but they will not be the next time the unit is started.

This has been experienced MANY times at HUNDREDS of sites around the world, especially sites that go black frequently (and there are some of those). Patience is the key--always--in this matter. Fortunately, gas turbine shafts don't require the efforts and time and cost that steam turbine shafts do when they become warped (many have to be removed and straightened using some extreme measures--and at a great cost, though it's usually cheaper than having to replace a shaft).

If you want to wait MORE than 24 hours, that's great, too. Most people are extremely nervous the first time around and want to get it back on turning gear as quickly as possible. The best indicator of shaft temperature are the wheelspace temperatures, and when they're done around 90 deg C or less then the shaft has sufficiently cooled that the risk of warpage and high vibrations on start-up are greatly reduced.

Sometimes, I recommend opening the IGVs while the unit is at rest to help facilitate the flow of air (due to convection currents in the exhaust) to help cool the shaft. BUT UNDER NO CIRCUMSTANCES should the turbine compartment doors be opened to try to help cool the shaft--that only cools the turbine and compressor casings faster and increases the risk of internal blades bending and breaking--even when the shaft isn't turning. Don't turn on the turbine compartment vent fans, either. Just let the natural convection currents pull air in through the inlet and through the turbine to help cool the shaft.

And, if you can convince Supervisors and Managers to wait longer than 24 hours, that's always better than less than 24 hours. Remember, the axial compressors are very heavy on F-class turbine, and the machines only have two bearings so longer is better, but 24 hours seems to be about right for most applications.

(Acutally, as you approach 20 or 24 hours it's probably not a bad idea to open the turbine compartment doors and/or to run the turbine compartment vent fans--just not at first. Let things cool naturally (there is really a HUGE amount of air that flows through the axial compessor turbine just due to the "chimney action" of the hot exhaust and that's what will cool the turbine rotor the fastest).)

Hope this helps!
 
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