High Wheelspace Temperature

D

Thread Starter

dontknowman

Hi to all

We"ve done an HGPI on a frame6 turbine with S.T mark5 control system. first stage nozzle and first stage buckets were replaced. After starting the unit we saw wheelspace thermocouples 2AO1,2 and 3FO1,2 are high. In 18MW and 45" centigrade ambient both are above alarm (alarm is 510" for 2AO1,2 and 493" for 3FO1,2). Other wheelspace thermocouples and condition is normal. By reviewing the before HGPI data we noticed that this thermocouples temperature had been high (in 25MW and 25" ambient above alarm).

Washing has been done two times but nothing has been changed. Mechanical department says cooling path of stage two nozzle is blocked and rotor must be removed.

Now my question is what are the other possible causes for this event and is there any danger for turbine if with this condition works?
 
dontknowman,

The subject of suddenly high wheelspace temperature readings after a maintenance outage has been covered many, many times on control.com. Use the search term:

"improper insertion"

as one of your search criteria (WITH the double quotes), and you should find many threads about what can cause high wheelspace temperatures.

Were new T/Cs installed, or the previous T/Cs re-used?

Usually, if it's only one side of a wheelspace that's reading much higher or much lower than the other side, it's the result of improper installation/insertion. If it's both sides, it could STILL be improper insertion of the thermocouples; it's very difficult to get them to seat properly in the spaces in the nozzle blocks.

If you're certain the wheelspace thermocouples were installed properly, then there's some problem with the seals between the rotor and the diagphragms (nozzle blocks) or with the cooling air flow through the machine. Both are really hard to diagnose, without pulling the rotor out of the machine.

And, since you didn't provide the before-outage T/C readings, it's difficult to tell how bad the problem might be. Wheelspace temperatures are a measure of how well the seals below the turbine nozzles/and buckets are preventing hot gases from getting down near the turbine shaft and causing heating problems. There is also usually some internal gas turbine cooling air passages which can direct compressor extraction air into the wheelspaces to help with cooling. If hot gases are leaking into the wheelspaces or there's insufficient cooling air flow, that's NOT GOOD for the turbine shaft or for the internal alignment of the turbine long-term.

Are you certain the proper T/C termination methods were followed when installing and connecting, or re-connecting, the T/Cs to the extension wire that runs to the Speedtronic turbine control panel? Cold junctions can introduce errors in the readings, sometimes large errors (as much as 50 deg C), sometimes just a couple of degrees C. So, wiring errors could be the cause. That should be investigated before taking the machine apart.

But, getting those T/Cs into the little "nubs" on the nozzle blocks from 1 meter or more away through small little tubes is really difficult. When the machine is open and the rotor is out, do take a look at where the T/C "thermowells" are located, and then at how the T/Cs are pushed through tubes into the openings to the "nubs". And, remember that the idea is that all T/Cs must either be touching the nub, or not touching the nub so that they are read uniformly. And, realize that's a very difficult thing to do with the rotor installed and the upper casing on. It takes a GREAT deal of convincing to get the mechanical department to allow the wheelspace T/Cs to be installed BEFORE the rotor is installed, but if the coils of the T/C sheaths can be properly protected from the feet and tools of the mechanics it usually leads to fewer problems during re-start. But you have to get the mechanics to at least TRY not to break the T/C sheaths, and that's almost impossible to do. Much less to get them to actively try to protect the T/Cs....

Another way the seals can leak hot gases into wheelspaces is if the rotor is not in the proper location during operation--meaning the thrust is not correct, the axial position is not correct. So, if the mechanical department have other suspicions--such as the axial position float not being correct--then they might indeed have the answer to the problem of high wheelspaces.

But, it's always a difficult to tell. And no one wants to take the rotor out again, only to find it's an instrumentation problem--especially if it's only an insertion problem, which is difficult to prove with the rotor in or out. It just takes care when inserting the T/Cs, and that's not always easy. There's a saying: "There's never enough time to do it right the first time. But there always seems to be enough time to do it over again." I don't like that saying, but unfortunately it's sometimes true.

Check to be sure the T/C terminations are all correct. And, work WITH the mechanical department to understand ALL of their concerns. And, considering trying to remove and reinsert one or more of the affected T/Cs (with the unit shut down and cool), to see if that has any positive effect on the readings before the decision is made to take the machine apart--unless the Mechanical Department knows something they're not telling (like axial position/float readings).

But, long-term operation with high wheelspace T/C readings--especially when both sides are reading high--is NOT recommended. And in case you're wondering, the cutoff point for an upper limit of "high" and for how long is "long-term" is always a site thing. Some sites have more tolerance for risk; others have very little tolerance for risk. So, that's a site call--always!

Hope this helps!
 
D
HI CSA

This thread belongs to almost 6 weeks ago. But I have done something and there is some conclusion which I should say for feedback:

Last month I checked all the thermocouple paths from local JB (on turbine wall under the greeting) with a thermocallibrator and all thing was normal. We read on HMI the same value which injected. Then I replace one of the thermocouples( 3FO1). I noticed that this thermocouple was not in its tube!!!

After starting the unit this temperature was normal. almost 60" decreased. So we suspected more to an instrument problem. Last weak I replaced the other three thermocouples and after starting the unit all the temperature was normal.

That was unbelievable but all thing was IMPROPER INSERTION.

So thanks for your knowledge sharing and your permanent and tireless guiding to those who have many question on this field and need help.

Regards
 
dontknowman,

Thanks very much for the feedback--it's very helpful!!!

And thank you for the kind words; I'm happy to be of help whenever possible.
 
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