Extended Gas Turbine Turning Gear Operation


Thread Starter

Imran Wazeer

During turning gear operation, the turbine buckets are not centrifugally loaded as they are during normal machine operation. Without the centrifugal load, the buckets traverse from side to side as the rotor revolves. This motion can cause the wheel dovetails to lose material with extended turning gear operation.

So what would be the maximum for which GT can be put on turning gear?
Imran Wazeer,

> So what would be the maximum for which GT can be put on turning gear?

It would be most helpful if you could provide details about how you operate, or plan to operate, the unit on turning gear (cooldown). 24 hours? 48 hours? Indefinitely?

There is a minimum amount of time the unit is supposed to be on turning gear--unfortunately, it's not the same for all GE-design Frame 9E heavy duty gas turbines. The time is defined by the packager (GE; Alstom; John Brown; BHEL; etc.) in the control system provided with the turbine and auxiliaries. It's commonly approximately 24 hours--but there is always a caveat with whatever the minimum amount of time the unit must remain on turning gear before an operator is allowed to take the unit off turning gear--and that is, that the highest wheelspace temperature should be less than approximately 150-200 deg F (approx. 65-93 deg C). So, even if the minimum time limit of the control system has expired the unit should still not be taken off cooldown (turning gear) until the internal metal temperatures have dropped low enough so that heat will not "flow out" along the turbine shaft and damage the bearing material. So, the unit should remain on turning gear (cooldown) with the Aux. L.O. Pump running to keep oil flowing to the bearings to keep them cool, in addition to preventing rotor sag.

It surprises most people to learn that it's not the turbine section of the rotor that is susceptible to warping. The turbine section is very short (only three stages). The compressor section of the rotor is MUCH longer--and MUCH heavier--that the turbine section. Turning gear (cooldown) is more about preventing the compressor section from warping than the turbine section. And, as was mentioned above, L.O. flow to the bearings while the shaft and internals are hot is necessary to prevent the hot shaft from damaging the soft bearing material. So, turning gear (cooldown) has multiple functions--and they must all be considered when deciding how soon to take the unit off turning gear (cooldown). And, of course, the bucket rock/dovetail wear must also be considered when deciding how long to leave the unit on turning gear after is can be safely taken off turning gear.

Most plants these days are on a mission to minimize electric consumption/usage in the plant and so they want to take the unit off cooldown (turning gear) as soon as is practically possible. When the unit is on cooldown the Aux. L.O. Pump motor is running, the L.O. Mist Eliminator Motor is running, and the turning gear motor (if so equipped) is running. The Aux. L.O. Pump Motor will usually be between 50-90 HP, and the Mist Eliminator Fan Motor will usually be between 1-3 HP, sometimes a little larger, and the turning gear motor can be anywhere between 3-7 HP, sometimes smaller. So, taking the unit off turning gear will usually reduce electric consumption considerably above leaving it running indefinitely.

So, there is a minimum time the unit must remain on turning gear (cooldown), and then there is the bean-counter permissive (that it must be taken off turning gear (cooldown) as soon as possible).

Depending on how long between the time the unit can be taken off cooldown and when it's expected to start again, most sites try to strike a balance so that the unit isn't on turning gear (cooldown) unnecessarily, but that the cycling of the auxiliaries is kept to a minimum--if for no other reason (and it's a VERY important reason--to the bean counters) than to reduce electric consumption in the plant.

I believe GE and/or the packagers have published information about their recommended practices for reducing bucket rock and dovetail wear. I don't know what the GE publication number is, but I'm pretty sure there is at least one official publication that talks about bucket rock/dovetail wear on turning gear (cooldown), and I also believe there is at least one TIL (Technical Information Letter) or PSB (Product Service Bulletin) about the phenomenon, also. So, check with your OEM/packager for their publications and recommendations, and then resolve that information with your plans for turning gear (cooldown) operation. Every plant has some unique requirements that other plants don't have, including supervisory- and management preferences--all in addition to the bean counter directives.

Hope this helps! The answer really depends more on how you operate, or plan to operate, the unit when it's on turning gear (cooldown). Along with the manufacturer's recommendations. And site conditions/requirements. It's not really a simple answer like one would expect it to be--unfortunately.

>> So what would be the maximum for which GT can be put on turning gear?

> It would be most helpful if you could provide details about how you operate, or plan to operate,
> the unit on turning gear (cooldown). 24 hours? 48 hours? Indefinitely?

This reply would be consistent with GE Frame 7Bs to Frame 7F+e:
For the 7Bs, the min TG time is ~ 24 hrs after a run to base load (3rd stage wheelspace temps less than 250-300 deg F to take off TG). There is no need to continue to operate the lube system at this point after TG shutdown.

For the 7F+e-the wheelspace temps need to be much lower{as they are more "delicate" machines) max 150 deg F.- and this unit needs to keep the lube system in operation, even after TG shutdown, until the oil temp approaches ambient(when I say this, I'm not referring to ambient weather temps, as the hydraulics will keep the oil temp well above outside temps-maybe 130-150 deg F)

Over the three-plus decades I've been working on GE-design heavy duty gas turbines (specifically Frame 9E and Frame 7E/EAs), the minimum times for cooldown operation have ranged from 13 hours to 48 hours--these numbers were from factory settings provided in the Speedtronic turbine control panels and documented in the Control Specification drawings.

I've never been able to determine how these numbers were chosen, or who reviewed their selection before shipping the turbine and control system to understand how there could be so much variance in the factory-supplied settings. It's always been rather hit or miss. I even recall some Frame 7s in Indonesia which had a cooldown timer of 10 hours, which was much too short (in my opinion, and in the opinion of the factory engineers I discussed the setting with).

I've always been told--and believe--that the proper deciding factor for when a unit should be taken off cooldown is when the highest wheelspace temperature is less than approximately 150-200 deg F--which is lower than the temperature at which the bearing babbit material begins to melt/deform (approximately 300 deg F, or approx. 150 deg C). This prevents heat from "flowing" out along the shaft and getting to the bearings--especially the #2- and #3 bearings of Frame 9Es--which is what the original poster of this thread asked about. F-class turbines have similar considerations, but are only two-bearing machines (the turbine has only two bearing) and have some different considerations than GE-design B/E-class heavy duty gas turbines.

So, it's not appropriate to say the minimum cooldown <b><i>time</b></i> should be [this] or [that] for any machine. Operators should be taught and the SOPs (Standard Operating Procedures) they use and refer to should always mention that for long-term zero-speed operation the unit should not be taken off turning gear until the highest wheelspace temperature is less than what will result in bearing damage. Even if the cooldown timer has expired and will let the operator select COOLDOWN OFF--a turbine should not be taken off cooldown if the turbine hasn't sufficiently cooled to prevent bearing damage--OR unless the site plans to keep the Aux. L.O. Pump running for some time while at zero speed. Some sites with a lot of turbines and a lot of experience with operating turbines have decided that number can safely be 250 deg F (approx. 120 deg C), while others have standardized on 200 deg F (approx. 93 deg C), and still others choose the very conservative 150 deg F (approx. 65 deg C).

And, I want to be very specific here about cooldown (turning gear) operation: It's purpose is to prevent the axial compressor rotor from bowing as the axial compressor section of the turbine is MUCH longer and heavier than the turbine section of a GE-design heavy duty gas turbine. HOWEVER, there is NO temperature sensor(s) monitoring the axial compressor rotor temperature, and there is no rotor eccentricity sensor monitoring rotor condition. The <b><i>presumption</b></i> is that if the wheelspace temperatures are low enough to prevent damage to the turbine bearings when L.O. flow is removed (when cooldown (turning gear) is selected OFF) that the axial compressor, and turbine rotors, are cool enough to prevent warping. And, that's a reasonably certain condition/presumption. The only reason we're talking about wheelspace temperatures is to prevent heat from the internal gas turbine areas from heating up the turbine shaft and that heat from then getting transferred to the bearings without sufficient cooling from L.O. flow. And, again, all of this is because there aren't temperature sensors or an eccentricity monitor on the axial compressor--which is the component which is most likely to warp or bow if the unit goes to zero speed while still hot/warm.

But, I think we digress from the original poster's question: How long can a unit be on cooldown (turning gear) without causing excessive damage to turbine bucket dovetails? And, again, the OEM has published information about that, and my recommendation was that the minimum amount of time should be as described while asking the original poster for how long they would expect to operate the turbine on turning gear. So, it was decided to provide some background information on the minimum time (since most sites these days want to minimize the cooldown time to minimize electrical consumption) and refer the original poster to the OEM guidelines for their turbine(s) so they can decide if they've been or plan to be excessive in their turning gear operation.

There's little benefit in operating the unit on turning gear indefinitely--which many sites used to do, just to keep the unit ready to start at a moment's notice. It just wastes energy (which does cost money), and if left on for excessive amounts of time can cause wear and damage to the bucket dovetails. A lot of sites believe they MUST operate the unit on turning gear for some minimum period of time before initiating a START, and while it's advisable to do so--it's not required. Unless the turbine has been sitting for several weeks at zero speed it's not absolutely necessary. (If it was, it would be programmed into the START sequence.) If the turbine has only been at zero speed for a day or three or four there's really no need to put the turbine on cooldown before starting. The vibration might be a little higher than normal during acceleration, but it will soon settle out. But, if the turbine has been sitting at zero speed for weeks, or months, it's absolutely advisable to put the unit on turning gear for a few hours before initiating a START; the axial compressor shaft will have deformed very slightly under its own weight having sat at one position for so long. Cooldown operation will help to reduce the sag so reduce the vibrations during starting--but they will still likely be a little higher than normal anyway, or higher than a unit which had only been at zero speed for a day or four before initiating a START.

And, ambient temperatures have some effect on this as well.

Of course, the F-class turbines are an exception to this--and just about every other rule. As they are only two-bearing machines they have a much higher tendency to bow under their own weight at zero speed. They don't deflect as much as one would expect, but they do bow as quickly or quicker than their heavier-duty brethren. And, so cooldown operation before starting an F-class turbine is almost always recommended as they don't take rocking-and-rolling as well as their heavier-duty brethren, either.

sggoat, we try to be as responsive as possible to the original poster's stated questions and conditions as we can be. Of course, sometimes, not much information is provided--and, as in this case, it would have been better if the original poster had specified the amount of time they expect to have have left their Frame 9E on turning gear in order to give a more helpful answer.
I thought I was pretty specific in stating the references to 7Bs and 7F machines. I wanted only to give reference to these Frames so the poster might get a better idea of what they had - and could expect-I was assuming (maybe this was presumptuous) that the 9s would be similar.

All 12 Bs I was in charge of went by the third stage forward TCs being less than 250 deg F before ceasing TG operation. This Usually occurred after 24 hrs on TG. There was no cooldown oil supplied after stopping. The units might sit a month or 2 before being commanded to start. The TGs were operated once per week for 1 hour-this being the answer to the notorious third stage bucket rock problem - limiting TG operation while at least circulating the oil.

The F machines have yet to experience this problem severely, as they are too young, but the bucket gaps are getting wider as we speak-most of them are in combined cycle service, so they don't get a lot of TG time, but the Peaking Units are already experiencing wider bucket gaps.

Kudos to GE for finally realizing the need to spin these guys at a faster rate (30 RPM) so the buckets don't get a chance to 'rock'

This additional information is very useful, and would have been even more so had it been included with your earlier post.

I have to say, though, if the turbine control systems of the 7Bs you were in charge of used 3rd stage forward wheelspace temperatures as the permissive to allow operators to select COOLDOWN OFF then the sequencing/application code in the turbine control systems was unusual because usually, at least for decades, the permissive was simply a timer which started either during coast-down after flame was lost, or when the unit reached zero speed and the unit automatically started cooldown operation. I'm not saying it's wrong or that it's bad; I'm only saying it's rather unique amongst the fleet of GE-design heavy duty gas turbines.

And, as for what is done with F-class turbines, well, they are different machines with different requirements and should be discussed in a different vein. Offering information about F-class turbines might be appropriate if the original poster didn't state the type/model of unit he was inquiring about--but in this case, the original poster did state the unit was a 9E.

I'm only offering this because the information provided (by BOTH the original poster AND respondents) should be as clear and concise and relevant as possible--even if, quite often, the information provided by original posters is quite lacking. If our experience is not exactly on the machines the original poster is asking about, or the original poster has not provided sufficient information, then it's appropriate to offer information and state whether or not it's applicable or similar enough to what the original poster is inquiring about to be of help. It would be nice if people--original posters asking for information/help as well as respondents--all adhered to a loose set of guidelines for posting, and that's all I'm suggesting here, a loose set of guidelines, something I've been trying to implement by example on this, and other, forums I post to. Basically, it's just to try to get original posters to post as much information as they can--with actionable data--recognizing that although they live and work with their machines day in and day out that we don't know really anything about their machine and how it's operated and what's been done in trying to troubleshoot and resolve a problem. And, when information is provided in response to a post that we provide relevant information to the extent possible, and state any differences or possible discrepancies to prevent any misunderstandings.

Since I've never stated the loose guidelines, I am now (not that a lot of people who do or will post will read this information--but it's a start). And, that's all I'm suggesting here--a loose set of guidelines for posting to keep things short and simple to the extent possible, and to prevent providing unnecessary, inapplicable or incomplete information--and if providing additional information or related experience to clarify as such.

It's great to have more people with experience willing to share and post replies. It would be great if we were all "on the same", so to speak.

By the way, what kind of control systems were used on the 7Bs you were in charge of?

The Control system was OEM Mark I. They didn't have a "cooldown off" selection since the Operator Interface was nothing more than a Control Panel in the Start house for the 12 machines. The Operator would check the wheel space temps, and if low enough, would turn the Master Selector Switch to "OFF" and give the unit a 'stop' command. This would stop the TG and shut down the lube system.

I need to apologize profusely here as I mistakenly referred to the "Third Stage Forward temperature" as the deciding factor in shutting off the turning gear. The actual temp should be FIRST stage forward.