Hello everybody,

I want to know which material is better for Thermocouple used for coal fired boiler. Inconel600 sheath got burnt in the furnace in 1 month. Can I go for SS446? Please advise.

 

Hi,

If your temperature inside the furnace where the thermocouple installed is very high (above 1000 deg. C) it is suggested to go for Non Contacting temperature measurements such as Pyrometers.

With best regards,

S.Elavazhagan

 

Sir Thanks for the reply. our temp. is 850 Degc Max.

 

You can try SS446 or inconel 600 grade materials to be used as thermowell. This can surely withstand those temperatures.

 

875 degC is not too high for furnace application.

Few points worth noting are:

1. Thermowell's MOC, profile selection is important to ensure that necessary mechanical/thermal integrity is retained. Thermowell in this application require periodic inspection and may be, rotation.

2. Sheath dia for the T/C should be 8mm or better.

3. T/C should have Titanium Stab with Incoloy 800/H, which many manufacturers provide.

4. If selected T/C is K-type, junction degradation (and hence inaccuracy) occurs with time in hydrogen ingress. T/C needs periodic check in temp bath and replacement in 2/3 years time. R-type is present preference in similar application but is costly.

If you take care of point 2 & 3, you should get good life of T/C. Hope above clarifies.

Regards.

 

Dear Sir;
The Thermocouple sheath dia. is 16 mm & T/C is K type. Which material SS446 or Inconel 600 is better for this application? What should be the Sheath material for T/C? same as thermowell? Also whether Mineral Insulation to Sensor will help to reduce the problem?

 

Will you please provide specification of existing thermowell & thermocouple?

How often do your thermowells fail? In how many cases damage of TC may be attributed to thermowell failure?

Do you normally maintain temperature up/down rate ( X degC/hr) for your furnace? Is it automatic or manual?
How frequently furnace trips? Do you experience failure of TC mostly after furnace trip?

Regards.

 

These are ungrounded K type having length 700 mm & 16 mm Dia. Thermowell failure is once in a 6/7 months, but the T/C failure is ofter in 2/3 months. so we can say that in only 1 out of 4 cases the T/C failure is attributed to thermowell failure.

The furnace is coal fired FBC type havind Silica in side. It's temp. Drafts are maintained manually. Tripping is very rare. but the temp. profile changes very frequently. (Up's & downs).

Also Clincer formation is on of the major problem.

 

Few observations are,
1. Rate of thermowell failure is high. Design & location of installation to be looked into.

Is this problem persisting since the furnace is in operation or it is a recent development. If it is a recent development, we need to look into,
a. How the furnace operating condition has changed with time.
b. Whether the replacement items (TC/TW) are in accordance with original specification.
c. Whether the laid down inspection procedures are being followed.

2. T/C length is high. Can you go for a shorter length but still getting a representative temperature? Look into the internal flow profile and confirm. What is the furnace wall thickness?

3. Wide temperature swing at faster rate in furnace will reduce T/C life, where you may find open circuit across terminals.

4. How to avoid/overcome clincer formation.

Wherever failure of T/C is not attributed to TW failure, do you find physical damage of sheath of T/C?

Please provide the complete specification of thermowell and thermocouple. It appears you need a root cause analysis where you may have to address other issues too.

You may contact me offline at abhijit(dot)goswami(at)rediffmail(dot)com.

Regards.