We want to change one wheel space thermocouple in GE frame 6 machine gas turbine at the third stage. can anybody suggest what are the precautions and procedure we have to take for that? and suppose there is punch regarding the length of the thermocouple to be inserted on the turbine side, from where to where have to measure this length? I assume from tip to up to ferrule joint, is it right?

 

Those wheelspace T/C's are suspect to breaking when you attempt to pull them from the machine. If it hangs up while trying to remove it, working the T/C in and out may help. A wheelspace T/C breaking in the machine can only be accessed for removal during a major outage when the casing gets opened. I work on a 7FA and have seen a couple of these break while removing. The replacement T/C should be a GE OEM T/C. Not sure why you are questioning the length.

 

Your assumption regarding the length is correct. But, if you use as replacement a TC with the same part number (strongly recommended), the length is already the correct one, the ferrule being pre-fixed in the right spot.

Now, regarding the installation, if the TC has a guiding tube that can be removed (the last piece that is inside the turbine casing), I would not recommend removing it. If it somehow comes out with the old TC, be extra careful when you put it back before inserting the replacement TC (the inner guiding tubes are fixed on the nozzle body with a metal band that do not cover completely the hole). I learned this the hard way when I dropped one guiding tube between the casing and the second stage nozzles ring. It took hours and tons of frustration to get it out without dismantling the casing.

When you insert the replacement TC, you can put some grease on it to make it slide easier. Moving it back and forth can help passing the more difficult points, but don't go very hard on it. At the end, if it doesn't go easily in, I normally use pliers to push carefully on the ferrule, but with a brand new TC you should not need it since it is still smooth and goes in easily.

I don't know if you worked until now with this kind of thermocouple, but you will learn very fast that they tend to bend quite easily. So, if it gets stuck, applying too much pressure will bend it. Since the difference between the diameter of the inner guiding tube and the diameter of the TC is very small, any small bending will make your job more difficult.

I hope this helps. Good luck!

 

Thanks for your reply.we replaced it successfully,removing the old one was little difficult but we could insert the new one easily and it is working fine.

 

Shyam,

Thanks for the feedback!