To restart GT at high speed right after trip


Thread Starter


Possible to start a machine (9FA single shaft) at high speed?

We have this problem that every time machine trip during startup, we have to wait for the turning gear to cut in (zero RPM), which we have to wait for easily 45 minutes.

What is the reason for this startup logic (machine can only start after turning gear cut in)? Any way to bypass it so that I can restart my machine faster, without jeopardizing the machine?

The answer to your first question is: No.

The answer to your second question lies in the type of turning gear and starting means used on the machine at your site. The logic is written to protect the machine and its auxiliaries.

The answer to your third question is to ask GE to see if there's anything which can be done for the unit at your site. If you're already asked and they've replied there's nothing to be done, or the cost would be very expensive then it's not likely anyone else would have a solution that would meet your needs.

The answer to the question you did not ask is to understand and resolve the condition(s) resulting in the trips during starting.

The question you did not ask was: How do I prevent trips during unit start-up? Trips of F-class machines should be avoided as much as possible. Resolve the issue(s) resulting in trips and you won't spend nearly 45 minutes waiting for the unit to coast down to a READY TO START condition.
Thanks for the reply. Indeed we tried very hard to prevent the trip during startup, and my machine startup reliability is as much as 98%. But sometimes s**t happens and this expediting startup time thingy is more like a damage control.

Before I follow up with GE, I want to get a second opinion from non-GE here. As far as I know, there are other machines out there can restart machine at speed as high as 200RPM. That makes me come out with this idea.

Process Value

GT High speed starting

hi expresso,

did you get this idea from highspeed starting of steam turbines?? what you have said is possible for steam turbines, and i have seen it done. but however this is not possible for GT even theoretically if you have a diesel engine or motor as a starting device. how are you going to connect to the jaw clutch if you do not have the turbine at zero speed. If you have a generator as the starting device, then may be it is possible but still would it not be sage to know why the trip happened and analyse/rectify it before starting the machine?
Dear Espresso, as with some of the replies.

Trips to F class machine=very bad!

S*** happens during start-ups, very normal.

Waiting for the machine to roll down to "ready to start" = not priceless. Any downtime = money!

We faced the same issue with an older GE Frame 5 machine with a jaw clutch which could not engage until the machine was at zero speed. We spent the money to install a different clutch and modify logic to allow the machine to restart "on the fly".

I should think logic could be modified to allow for an FA machine to start at a speed above TG speed, but this would require a lot of changes to logic for the control system. These changes won't come for free. This would be a situation best left to the OEM in my opinion. Many times they "seem" unaware to customers desires. In this case I would suggest you question them about this issue, I would not think you are the first to ever ask about this situation. If the price is right I am sure the OEM would be glad to take your money!!
Process Value,

This unit is not like the GE-design Frame 5 heavy duty gas turbines you have so much experience with in the last two years. The original poster stated the unit at his site was a Frame 9FA. F-class machines employ "static starters", where the generator is temporarily converted to a motor for starting the unit. There is no diesel engine, nor jaw clutch.

Some F-class machines have a SSS (Shifting, Self-Synchronizing) clutch to couple the cooldown motor and gear box to the turbine-generator shaft. Some have large turning gear mechanisms that must mesh with gears on the turbine-generator shaft for cooldown operation.

Process Value

>Process Value,

>This unit is not like the GE-design
>Frame 5 heavy duty gas turbines you have
>so much experience with in the last two

Thanks to CSA once again. Unfortunately what you have told is true; i do not have much experience in machines above frame 6FA. any idea on how much MW is consumed when the generator is used as a motor during startup. just curious to know.

Process Value

GT rolldown time reduction

Given that it is not possible to start the GT at high speeds. can roll down time not be reduced ? employing a friction braking at low speeds, or using axillary winding regenerative braking can do the trick right?? by using any of the above methods or by employing a combination of the two in my opinion the roll down time can be reduced. has anyone explored this idea? seems like a workable solution at first glance.

and MIKEVI what happened to the on the fly starting scheme? did it work well? can you provide more details of what you had done.
I think a lot would depend on the "capability" of the static starter in regards to how it can be operated or programmed to "synch" with the shaft speed/frequency in order to "catch" the unit as it was spinning down.

I don't think a static starter can start a machine from zero speed; the ones I've worked with need the turning gear motor to have the shaft spinning at a slow speed in order to "synch" the output of the starter with the generator rotor and then start accelerating it.

But, I don't know if there's an upper limit to the maximum speed the static starter can be programmed to synch to.

Interesting questions for GE or the packager of the unit. Hope the original poster will write back with any information obtained.

I believe we're talking on the order of several MW for the static starter of a 9FA machine. I've never actually been around the exciter to monitor the power drawn; perhaps someone else can provide more info on that.

As for friction braking or regenerative braking, money is the only obstacle, right?

Process Value

> Thanks to CSA once again. Unfortunately what you have told is true; i do not
> have much experience in machines above frame 6FA. any idea on how much MW is
> consumed when the generator is used as a motor during startup. just curious to know.

correction to the above post, i meant frame 6B not frame 6FA. refineries tend to use frame 6 and frame 5 machines only, and thus my limited exposure to other class machine operation.
GE has provided "flying restart" capability for some projects. I don't know what changes are required; I was not directly involved in it. I just know it was done. I believe it did require coastdown until 14HM reset.

As for friction braking, in addition to money, you need space to install the brake. You would likely have to add an extension to the generator shaft to do it. Either that or move the generator farther from the turbine and provide a longer coupling between the turbine and generator. Not something I would want to do! The only gas turbine I know of with a friction brake was a MS5002 marine application and that was on the LP shaft.
Dear ProcessValue,

I will try to not stray far from the topic of the original post. In the case of our Frame 5 machines we were able to reduce the unavailable time from almost 19 minutes to less than 8 minutes by replacing the original jaw type clutch with an SSS clutch, and some slight logic changes. In our case the unit is a peaker with no HRSG. The ability to restart the unit once it falls below 14HM had value for us in the event of a failure to lite or if our power control center made a poor decision of when to shut the unit down.

As CSA and Otised have discussed there are bound to be many more considerations for a static start, HRSG equipped machine, but if the poster was really serious I am sure GE would look at this for them and as mentioned I would find it hard to believe someone hasn't asked GE about this already.