Wheelspace Thermocouple Exact Location

Hi Users,

We have a 20MW GE's gas turbine. It's a 2 stage turbine.
We have 8 wheelspace thermocouples installed on the machine.

If we take the turbine wheel and nozzle arrangement as
N1 1st Stage Nozzle
T1 (Turbine Stage 1 wheel)
N2 2nd Stage Nozzle
And T2 Turbine stage 2 wheel

Then, the wheelspaces would be between N1 T1, T1 N2, and N2 T2.

Corresponding to these the Wheelspace TCs would be in the arrangement as

N1 T1 1FO2 & 1FO3
T1 N2 1AO2 & 1AO3
N2 T2 2FO2 & 2FO3


There is no wheelspace on T2 Aft outer. Still, the wheelspace TC nomenclature suggests so... Could anyone please explain where exactly 2AO1 and 2AO2 are installed.


Let's try this:<pre>
1st 1st 2nd 2nd
Stage Stage Stage Stage Exhaus
Nozzle Blade Nozzle Blade Casing
___ ___ ___ ___ _____
|sss| _|rrr|_ |sss| _|rrr|_ |sssss|
_________|sss|- |rrr| -|sss|- |rrr| -|sssss|
sssssssssssss| |rrr| |sss| |rrr| |sssss|
_____________|1 |rrr| 2|___|3 |rrr| 4|_____|
The hot gases from the combustors flow into and through the turbine in the "picture" above from left to right, through the 1st stage turbine nozzles, through the 1st stage turbine, into the second stage turbine nozzles, and through the second stage turbine and into the exhaust.

So, the "spaces" marked "sss" are stationary--meaning they don't move when the unit is running. The "spaces" marked "rrr" rotate when the unit is running. (The "rrr" is the turbine shaft and turbine "discs" or turbine "wheels"--to which the turbine buckets (that's what GE calls blades) are affixed. There are also labyrinth seals below the nozzle segments and the exhaust casing which couldn't be shown in the "picture" above, which help to isolate the wheelspaces from each other. The labyrinth seals are on the shaft and the undersides of the nozzle segments and the exhaust casing.

The numbers, 1, 2, 3 and 4 represent where the various wheelspace T/Cs would be sensing the temperature of each wheelspace. (The wheelspace T/Cs are inserted through the outside of the combustion casing, through tubes and into small passages inside the nozzle segments, and are to come to rest inside small bulbous protrusions (called "nipples") on the outside of the nozzle segments, below the "angel wing" seals (represented by the "-_" symbols). The nozzle segments, and the exhaust casing, each have angel wing seals on them--which do NOT rotate. And, the bottoms of the turbine buckets have seals on them which DO rotate when the shaft rotates. Both the stationary and rotating angel wing seals are very thin, and have tapers to a very fine edge--not quite a knife edge, but very thin. And, they are very close to each other--the rotating and stationary angel wings in each wheelspace.

The area where the "1" is located is the 1FO wheelspace (1st Stage Forward, Outer). The area where the "2" is located is the 1AO wheelspace (1st Stage Aft, Outer). The area where the "3" is located is the 2FO wheelspace (2nd Stage Forward, Outer). And, finally, the area where the #4 is located is the 2AO wheelspace (2nd Stage Aft, Outer). Each wheelspace has two thermocouples in it--one on the left side of the unit, and one on the right side of the unit.

The very close proximity of the two angel wings of each wheelspace (the rotating angel wing and the stationary angel wing) actually for a seal when the shaft is spinning at rated speed. And it is the seal that helps to keep the hot gases from getting down into the wheelspace area where it doesn't belong and can cause damage to the stationary components as well as the turbine shaft and turbine disc(s) (wheel(s)).

There is also a slight positive pressure in the wheelspace (designed to be just slightly higher than the pressure above between the nozzle segment and the rotating turbine buckets (blades) that also helps to keep the hot gases from getting into the wheelspaces.

By keeping the hot gases flowing through the nozzles and buckets--and NOT leaking down into the wheelspaces--the maximum amount of energy can be extracted from the hot gases as they flow through the turbine nozzles and buckets. Any hot gases which leak down into the wheelspaces (below the angel wings, or "inside" the angel wings) is lost efficiency, AND can also cause problems with nozzle segments and turbine rotors and disc(s) (wheel(s)).

The 2AO wheelspace T/Cs are often inserted from the load tunnel area, through tubes close to the #2 turbine bearing.

The best way to visualize all of this is when the upper half of the turbine casing has been removed, and when the turbine rotor is still in the lower half of the casing. When the lower halves of the nozzle segments are still in place it's very easy to see the angel wings on the stationary and rotating components. Then when the rotor is removed, you can look down into the wheelspaces and see the bulbous "nipples" where the wheelspace T/Cs are slid into to sense temperatures in the wheelspaces.

<i><b>Ideally,</i></b> the wheelspace T/Cs are slid into the tubes and the bulbous "nipples" at exactly the same depth on each side of each wheelspace. But, in the real world--that almost NEVER happens, and so after the wheelspace T/Cs are re-inserted into the tubes and bulbous "nipples" instead of reading the same temperature on both sides of the same wheelspace they can sometimes read VERY different temperatures. And, that's almost always the result of improper insertion and unequal insertion depths.

It's virtually <b>IMPOSSIBLE</b> for the temperature on either side of a wheelspace to be much different from the temperature on the other side of the same wheelspace. Why, you ask? Because, in the case of a Frame 5 GE-design heavy duty gas turbine the shaft, and the turbine discs (wheels) are spinning at approximately 5100 RPM--and that is causing the air in the wheelspace to be mixed by virtue of the rotating turbine discs. Unless THE EXACT leak of hot gases is impinging on (pointed at AND blowing directly on) the bulbous "nipple" where the wheelspace T/C tip is located it's virtually impossible for the temperature of one side of a wheelspace to be 100- or even 200 degrees (C or F) different from the other side of the same wheelspace--again, because the turbine discs are spinning at 5100 RPM and mixing the air in the wheelspace preventing almost without any possibility such a hot (or cold) spot in a wheelspace.

Anyway, hope this helps! Wheelspace temperatures are monitored because its desirable to keep the hot gases passing through the turbine section passing through the turbine section, and not leaking down into the wheelspaces where they can cause damage. And, by keeping the hot gases out of the wheelspaces the turbine section can extract the most energy from the hot gases--making the unit as efficient as it can be.

And, with VERY FEW exceptions--it's virtually impossible to have true high differential temperatures in the same wheelspace. That's ALMOST ALWAYS the cause of improper insertion of the wheelspace thermocouples.
Dear CSA,

I am truly grateful to you for having taken time to give such an elaborate explanation. the representation of rotating and stationary parts is awesome and clears the entire picture... Thank you very much for the detailed explanation...

Sadiya Mariyam Khan

You are very welcome. I'm glad it was helpful. I wasn't sure if the "drawing" would be clear.

Thank you for the feedback!

This might sound as a bit of a silly question. But, why do Wheelspace TCs have been numbered as 1FO2 & 3, 1AO2 & 3, 2FO2 & 3 but then 2AO1 & 2?

What could be the reason of starting with 2 for 1FO, 1AO and 2FO and then at 2AO it's 1 & 2 again?

Thanks and Regards
Good question. I've often wondered that myself.

I have never found someone who knows the answer to that question, and in my experience wheelspace T/C designations only usually refer to left and right locations. I did notice one on a very old machine with -2 & -3 designations that one side (the left side if I recall correctly) was in the upper half of the casing, whereas most of the units I have worked on had both wheelspace T/C's in the lower half of the casing. But I've seen newer units with both the -2 & -3 designations in the lower half casing, so I haven't been able to decipher a pattern that applies to all machines.

I did hear of a unit that had three T/C's per wheelspace--one in the upper half casing, and two in the lower half casing, but I have not seen such a unit myself.

There is an occasional contributor to control.com who might know the answer, or the history of the designations. Hopefully he will see this thread and enlighten us!