Exhaust Thermocouples are getting random failure in 6FA Machine

21 nos of thermocouples are available to measure exhaust temperature in 6FA machine.
its getting failure randomly its may be due to erosion and physical damage . Let me know any method or product to prevent the same.
Yearly 2 or 3 nos getting failure (not particular its random TC).
 

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Take a look in the "Search" feature of Control.com(top right corner) there are many articles on"Exhaust Thermocouples". What you have is not an unusual failure rate
 
By failure, do you mean that the measured value is offset more than 10 degrees, or do you just get an open thermocouple?
Can you specify which kind of thermocouple you (i.e. type K, J, S etc.) incl. the temperature range you want to measure?
What material is the outside of the thermocouple? Stainless steel?
 
These are Type K thermocouples made for a specific purpose, to measure the temp in the Exhaust duct of a Gas Turbine, normally 450-600 Deg.C. Don't have an exact spec. but I would guess Stainless
 
Ok, 450-600C is not very high in temperature and type K is more or less standard. Our factory makes many of those.
Is a flame or a certain gas in contact with the thermocouple?

Is the failure you have every 2-3 years just an open thermocouple? Or is it shorted somewhere inside the thermocouple so it doesn't measure at the tip but somewhere halfway?
 
21 nos of thermocouples are available to measure exhaust temperature in 6FA machine.
its getting failure randomly its may be due to erosion and physical damage . Let me know any method or product to prevent the same.
Yearly 2 or 3 nos getting failure (not particular its random TC).
Can you precise some points like:
-Is that "random reading /displaying or measuring for all Thermocouples??
-What part of load do this event occur??
-Mark** type?Do you get any alarms annunciated ??
-What have been done /change on the unit before this happen??
-What did you do for troubleshoot this ...

Can you clarify on "failure randomly" is that dispalying on Turbine controls system HMI is incorrect/fluctuating /swinging...

The more precise the informations that you give us , the more precise/ we can be to support/troubleshoot this issue.

Controls guy25.
 
With exhaust temperature measurements,in my experience what I found while doing root cause analysis for a diesel engine power plant was that the failure was mostly after a major shutdown was taken and during the removal or installation of thermocouples. Generally the nut and the ferrule, due to it's very high process temperature and due to the presence of sox & nox caused pitting and the ferrules and threads got siezed. So while removing, the old ones breaks at the seizure point. Also while installing if the thermowells and the therads were not cleaned, so some extra force and pressure was required to make it fit forcibly inside the thermowell, and as the t/c pipe material was quite delicate considering its width(6mm dia at the site)...so overtime it lead to breakage. I am not sure about how the situation is at your plant, but following few basic points might help
1. Apply high temperature anti sieze compound at the threads before installation;
2. During removal as pitting occurs, apply a chemical solution for removal of the pitting, and then remove the thermocouples;
3. Use of a heat gun can also be done, to heat up the material and then slowly make the nuts free.
4. Thermowells must be cleaned properly before fixing of thermocouple.
5. The exhaust gas must be analyzed to check for the presence of SOx and NOx. Depending on that the metallurgy of the thermocouple pipe insulation and thread and nuts must be chosen. If high SOx is present go for SS 300 with high chromium. If more NOx is present, go for material with high nickel. NOx is a big factor in material breakage as it increases hardness of a material and overtime that results in micro-cracks and breakage.
 
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