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Questions and Insights about Aeroderivative Gas Turbine
Some questions about the mechanical side of the aeroderivative GT.

Hi everyone,

After working on both heavy duty and aeroderivative gas turbines and being aware of the major differences of the control theory of both, I have some questions about the mechanical side of the aeroderivative GT.

I know that (correct me if i am wrong) the Aero GT shaft is much lighter than the HD, and the power output is a mix of low torque and high speed comparing to HD, but i want to understand the bearing types of Aero GT. the Gas Generator GG can achieve 10000rpm and for that we used a different type of oil (synthetic oil) for lubrication. we have roller bearings for journal bearing, and the thrust load is carried by the ball bearings, how is that?, how can we achieve those high speeds with those bearings? is the oil wedge still there?

My second question is according to the control theory and the auxiliary system, we do not have an electrical lube pump. we do have only a lube pump in the gear box combined with the scavenge pumps, and in the same time the synthetic oil tank is mounted on a high level. is this has a relationship with not having an electrical pump (means when the GT is stopped the bearing or sumps are always drown in oil?), during the cooldown the hot oil does not need a circulation? a fixed residence time in the bearing?

Thanks in advance

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Hi everyone,

Question to add to the above.

3rd...?
Can there ever be a scenario where hot gases escape in the forward direction. (Running...steady state.)

Any turbine....

Thanks in advance

Hi Eddie,

>Can there ever be a scenario where hot gases escape in the
>forward direction. (Running...steady state.)

>Any turbine....

Your question is very vague.

Plumbers have a saying: Poop only flows downhill.

In other words, things (good or bad) only flow from an area of higher pressure to an area of lower pressure, and only if there is a path. Pumps and compressors and the like can be used to create higher pressure, but there has to be a path, too.

This applies to all turbines. Ain't nothing going to go anywhere unless the area it is trying to go is at a lower pressure (regardless of elevation) and there is a path for it to travel. And, even if there is a path for something to travel and the pressure is higher where it starts than where the path ends, if there's no place for the something to go there will be no flow (if the end is full, for example).

This is pretty universal: Things flow from areas of higher pressure, or to areas of lower pressure (same thing, just from a different perspective).

And, most turbines are designed NOT to allow high pressure to flow "backwards" because that pressure (and flow) is used to develop torque (or thrust) to do useful work (generate electricity; pump water or oil; move an airplane through the air; etc.). A LOT of energy is expended to develop the pressure and flow of a turbine, and allowing it to "escape" to areas where that energy wouldn't produce useful work is wasteful. So, the design should be such that under normal operating, running, steady-state circumstances, all of the flow is in the direction to produce useful work by the turbine, and not in the "reverse" direction--if I understand your question correctly.

Hope this helps.

Is this a control question (though I maintain the original post was not a controls question, either....)?

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CSA

It is a control question. My sensor picked up hot temp spikes
In the bellmouth. I have no reason to suspect instrumentation but have strong reason to believe high pressure gas leak from combustor.

Just thinking out aloud and wonder if experts like yourself have come across any thing similar.

Regards

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Eddie,

My apologies. You didn't provide any background information whatsoever.

How long were the durations of the spikes?

How long were the durations between the spikes?

What kind of turbine (manufacturer; size/rating; type (heavy duty; aeroderivative)?

What kind of sensor?

What was the location of the sensor in the bellmouth? At the outer edges of the bellmouth? At the inner cone of the bellmouth?

Does the engine have any kind of inlet air heating or anti-icing system that uses axial compressor discharge air or something similar, or some kind of electric heating?

Have you reviewed the P&IDs to see if there is any path for combustor gases to make their way to the bellmouth?

You said this was during normal, steady-state operation. Was there something else going on at the time? Was a storm passing over the plant, with a low barometric pressure? Was there any sudden change in axial compressor speed, or IGV position, or bleed valve position? (You said running, steady-state, so that would seem to preclude any transient conditions.)

In general, this seems unlikely--but you say you have strong reason to believe the high temperature is coming from the combustor. Can you elaborate more?

In my experience, I have never seen hot combustor gases making their way to the bellmouth--but, most of my experience is with GE-design heavy duty gas turbines.

What kind of control system--you indicated Mark V/VI in your manufacturer categories. Were they any Diagnostic Alarms at the time of the spikes on cards which have the inlet bellmouth temperature sensors? If so, what were they?

What kind of sensors? In general, GE Mark* turbine control system thermocouple input channels read negative values when the circuit go open, and the predominant failure mode for thermocouples is to fail in the open circuit condition, not to mention what happens when wires break or terminations come loose. GE Mark* RTDs have been known to spike high when not properly terminated (and a lot of RTDs are not properly terminated on control systems and turbines and auxiliaries--especially if the RTDs were removed and replaced after a maintenance outage). There may or may not be Diagnostic Alarms with mis-wired RTD inputs. And, loose terminations in RTD circuits can also cause readings to intermittently spike--without Diagnostic Alarms.

Hope this helps! If you need further information, please provide answers to the above questions--even if they seem irrelevant.

Isulamu,

I am not an aero derivative GT expert. And these questions are really not controls-related questions, and I am tired of Droop and Isochronous Speed Control questions. So, I did a little research on bearings. An extremely useful World Wide Website is Wikipedia.com. Look up "thrust bearing" or "roller thrust bearing" or "spherical thrust bearing."

I also learned that bearings basically fall into three categories: hydrodynamic, hydrostatic and roller types. The first two use oil wedges for high loads, and roller bearings support loads differently. I have to believe, though I didn't research it for very long, that roller bearings also employ some sort of oil wedge type action but probably not to the extent the other two types of bearings do. Roller bearings are usually more expensive than hydrodynamic or hydrostatic bearings and are hot as easy to install, repair or replace. But, from the little research I did, roller-type bearings can be made for all types of applications, very often for thrust bearing applications, as well.

Yes, aero GT shafts are much lighter than heavy duty GT shafts, and generally spin at much higher speeds to develop similar torque. But, I believe that thrust in an aero GT is or can be handled in different ways, using axial compressor discharge pressure on "balance pistons."

Sorry; I don't have any more information to offer. They are good questions, just maybe a little off-topic for this forum. Again, try using your preferred World Wide Web search engine, and Wikipedia is always a good source of basic information on many topics (just not Droop Speed Control).

Hope this helps in some small way!

CSA

I apologise for my delayed reply, and I thank you for your efforts. I will be trying to spend some time finding more details and will share anything i find in the thread.

For my second question which has a somehow a connection with control is the cooldown of the machine and the bearings without having an electrical lube pump. from the other hand and according to the control philosophy, if the machine is shutdown after being at power state (flame detected), and before performing the next restart (during the next 4 hours) the shaft rotation is tested (rotates freely or not) before giving the ready to crank!

what may cause the shaft to not rotate freely? i think this has a relationship with the cooldown of the machine!

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Isulamu,

As mentioned, my knowledge of and experience with aeroderivative gas turbines is very limited.

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>what may cause the shaft to not rotate freely? i think this
>has a relationship with the cooldown of the machine!

EMRG_Shutdown..Non controlled shutdowns...

>EMRG_Shutdown..Non controlled shutdowns...

YES

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Hello Isulamu,

with which turbine are you working? I am working with solar mars 100 it is Aeroderivative Gas Turbine with two shaft.

Your gas turbine does not have post lube pump?
when you talk about cooldown how much is the rpm value of the NGP?

>with which turbine are you working? I am working with solar
>mars 100 it is Aeroderivative Gas Turbine with two shaft.
GE PGT25

>Your gas turbine does not have post lube pump?

It has no electrical lube oil pump for the GG shaft

>when you talk about cooldown how much is the rpm value of
>the NGP?

Zero speed L14HR=1