Hi all,

Here we are facing a strange problem since few days. During premixed mode, 1st stage forward inner temperature sensors (TTWS1FI1 and TTWS1FI2) reading difference is high. I am attaching here photo. We think some problem may occur at thermocouple. After checking, it looks, TC is ok. Also CTDA is normal. So, why this difference? (Alarm limit for differential temp. 65.5 degree Celsius)

http://tinypic.com/usermedia.php?uo=XaTRJmVkC3fReLoeRIXUbYh4l5k2TGxc#.V_f29LlhnMI

Please suggest us, what the other things are responsible for this.

Thanks all

 

niazbibiyana,

Was there any maintenance carried out where the wheelspace TCs were removed?

Do you have more than one TC in each wheelspace? if yes, then you need to trend and monitor these TC values to ascertain whether both in the same wheelspace are having the same value and behaving in a similar manner.

Thank you.

 

niazbibiyana,

What is the differential at lower loads, before the unit transfers into Premix? Is it very near the high differential limit?

Has the differential been increasing over the last few days/weeks/months? Whenever a problem like this comes up, it's always best to understand what the historical behaviour has been--because if the differential has been historically high, and very close to the high differential limit, then it might not be so worrisome.

If however, the differential has been increasing quickly over the last few days/weeks, then it might be some kind of blockage or leak developing in the cooling and sealing air system.

Also, what has the axial position been doing over the last few days/weeks/months? The seals are affected by axial position and if the thrust bearing is wearing, this could be contributing to, if not the root cause, of the problem. Premix mode usually occurs at higher loads, when the axial movement in the thrust direction is greatest--and when temperatures are highest.

You need to look at historical trend data--and keep an eye on what's happening to the temperatures now, especially if you don't have any or much of any historical data. Trending, as analyzing historical data is called, is always most helpful in troubleshooting these kinds of problems.

A lot of people don't pay attention to values until they hit an alarm--this is a BIG mistake. Someone (the operators and their supervisors) should ALWAYS be trending values (all operating parameters) to detect changes BEFORE they are alarmed by the control system. Even gauge readings from parameters that aren't sensed by the control system using transmitters or other sensors should be trended--and checked at least once per day. I've seen cooling water pump discharge pressures dropping by 0.07 barg per day over a two week period--recorded by outside operators on the log sheets. And then when the pump discharge pressure goes low enough to alarm they get all upset and wonder what's happening and why. Well, the pump suction strainers were plugging (choking), causing the pump discharge pressure to drop--and it could have all been anticipated if someone (anyone) had bothered to notice the pressures had been steadily dropping (not even the outside operators who had been writing down the values noticed, or if they did, they didn't say anything to anyone).

Again, when a problem like this "comes up" one has to know what has been happening prior--and you didn't provide that information. So, it's really impossible for us to say what the problem is or what the problem might be. It could be the differential has historically been high and very close to the limit, and as the machine approaches another maintenance outage, or if it's being started and stopped (and/or tripped and stopped) a lot this could be contributing to leaks or blockages of the cooling and sealing air system.

Most times when these kinds of wheelspace temperature problems come up one will also find that the other wheelspace temperatures are also trending higher. Not always, but many times.

So, you need to have a look back at the historical trend data to see what's been happening, how long it's been happening, and then and only then can you start to formulate what the problem is or might be.

Hope this helps!

 

Great stuff CSA, but how many years do we need to keep telling ops. and maintenance staff to do this before they start doing it?

 

CSA

actually, from last 2 weeks, we observed this differential value. Previously, difference was 5-8 degree Celsius. But now it is about 40 degree celcius through all load condition. Maintenance team did not do any work on this, they only checked after this problem detection.

I am saying some parameters.

During base load and premix mode (DLN 2.0+ï¼::
Load- 230 MW, CTIM- 28.5, CPD- 14.1 bar, 1st stage forward inner temp sensor (1) - 420 degree celcius , st stage forward inner temp sensor (2) - 460 degree celcius , ctda- 402 degree celcius and ttxm- 624 degree.


During lower load and piloted premix mode (DLN 2.0+ï¼::
Load- 98 MW, CTIM- 33, CPD- 8.9 bar, 1st stage forward inner temp sensor (1) - 408 degree celcius , st stage forward inner temp sensor (2) - 446 degree celcius, ctda- 368 degree celcius and ttxm- 650 degree.

Hope, After shutdown maintenance team will check this issue.

thanx..

 

Until the OEM or packager provides proper documentation of how wheelspace temperatures are measured, and what affects wheelspace temperatures, and people actually read and comprehend the documentation--people will still be asking the same questions and people will still be answering the same way.

Likely for at least a couple more decades, anyway.

 

niazbibiyana,

Thanks very much for the feedback. You haven't said which temperature increased, or if the two increased but that one increased more, or if one decreased more than the other and both decreased slightly.... There are very many different scenarios (read below) and information like this is important to have and to be able to make good comments without writing a LOT of words and generalizing.

From the description of the developing differential it would seem there is a problem brewing.

The wheelspaces are where the turbine discs ("wheels") rotate, between the nozzle segments and the exhaust (for the last stage turbine wheel). The discs/wheels are what the turbine buckets are attached to. If hot combustion gases are allowed to leak down into the wheelspaces then the efficiency of the turbine section will decrease (slightly) because hot gases aren't traveling axially through the nozzles and buckets producing forces to rotate the turbine-generator shaft.

Also, if the wheelspaces don't get sufficient cooling or hot gases are leaking into them (which reduces the effect of the cooling air flow) then the discs/wheels can be overheated, which reduces the expected life of the discs/wheels.

Unfortunately, there is no way to measure cooling and sealing air flow-rates to the various wheelspaces. So, this is a difficult problem to troubleshoot while the unit is running, and even when it's not. It might be possible to use a borescope to inspect the area between the first stage nozzles and the rotor to see if there is any visual evidence of overheating (discoloration), but that's about it to the best of my knowledge.

Best to keep an eye on the temperatures and if they continue to increase, especially if they increase or start to increase quickly you should probably contact your turbine service provider for more assistance and guidance. Since the first stage forward inner wheelspace is "below" (closer to the shaft; "underneath" the first stage forward outer wheelspace) it's pretty difficult to imagine there isn't something amiss with the angel-wing seals between the inner- and outer wheelspaces.

Another thing to realize about wheelspaces is that the discs/wheels are spinning (at 3000 RPM in your case) at rated speed. This SHOULD cause a good deal of mixing of the air in the wheelspace so, GENERALLY it's very hard to get a high differential when running, unless the T/C is failing (which they can do intermittently, but not usually gradually), or there is some problem with the wiring in the sheath of the T/C (creating a bond or cold junction somewhere other than at the tip of the T/C), or there is a problem developing at some wiring connection between the T/C and the Mark VIe T/C input terminals. (And, although it's highly unlikely, it could be a failing T/C input channel on the Mark VIe. This could be verified fairly easily by just exchanging the two inner wheelspace T/Cs wires--AFTER inspecting all of the terminations along the length of the wire between the T/C and the input terminals).

It could be there is some kind of leak developing between first stage turbine nozzle segments in the close vicinity of the highest wheelspace temperature, or that somehow cool air (from the axial compressor discharge) is getting though the nozzle segments near the colder thermocouple. But, all of these possibilities involve opening the turbine section to inspect, though a talented borescope operator might be able to do some good sleuthing and snaking.

This is most very likely not a control system problem (though I've been wrong in the past, and I will be wrong at some points in the future); contrary to popular belief Speedtronic turbine control systems (of which the Mark VIe is a part of the family) do not control cooling and sealing air flows, nor, again, is there any cooling and sealing air flow measurement (that I've every encountered on F/FA-class machines; I'm pretty sure the OEM has found a way to measure these and many other flows in the new HA-class turbine!).

Please write back to let us know what you find!

 

CSA,

I have noticed that "1st stage forward inner temp sensor (1)" has been decreased from before. But "1st stage forward inner temp sensor (2)" has not been changed.

By the way our management decided to do boroscope after shutdown within few days.

But after increasing this differential value, we noticed that, our turbine efficiency did not reduce. So, may be no leaking of exhaust to wheelspaces.

I will be connected with you. Later I will inform you after finishing maintenance. Thanx for your genius explanation.