Generally thermocouple probe is enough to sense the temperature of the environment.Working condition in plants etc however is not so favorable for probes and they can be easily damaged For example places like high pressure steam,Flowing fluid etc can easily corrode away the probe Also it is required that the process is not disturbed when we insert a probe through a hole .
So insertion must be airtight.For this reason a thermowell is used.They provide protection from corrosion and are also useful for easy replacement of probe without opening up the process.For this reason they are installed airtight into the system.
They are of 3 types based on installation method.
1.) welded 2.)Flanged 3.) threaded You van easily identify them by picture .
While selecting a thermowell you should look for these factors
1.)Length Thermocouple are tip sensing unlike rtd which are stem sensing. therefore length is important to consider because while using thermocouple probe the tip of the probe should be touching the well bottom otherwise it will increase the time delay in measurement which in fact is unavoidable due to insertion of thermowell but it should be minimized to reduce error in measurement.
There are two types of length stated manufacturer.
Insertion length. or “U”length This is the length of well in the process of measurement. A properly installed element: in liquid, the element should be immersed up to point where it is most sensitive plus one inch.In air or gas, the element should be immersed up to point where it is most sensitive length plus three inches.
Extension length This length is called “T” length and is the extending length ewhich is to extend thermowell and probe through wall or insulation of material. Between two length is the connection mechanism ie flanged threaded or welded.
Bore size In case of thermocouple bore size is used greater then the size of probe so that more that one probe can be inserted.General optimum bore size is
They are of 3 types based on installation method They can be bore drilled
1.)Welded This connection can be used when the process is not corrosive and routine removal is not required because welding frequently is not economical and is time consuming.This technique is suitable for high temperature & pressure application. As welding make thermowell quite airtight.
2.)Flanged Appropriate for large pipe diameter.Preferable if there is a need for more frequent well replacement such as high corrosion rates SO flanged connection is economical and easy to reinstall Such a technique is more and for high-pressure application.
2.) Threaded suitable for smaller diameter well threaded well are easy to install when diameter of well is small., which are not likely to be changed frequently
For high corrosive environment flanged is used where frequent changing is for non corrosive environment and smaller diameter.
You van easily identify them by picture
Stepped Tapered or straight.
Stepped is usedto increase time response.Reduced diameter causes easy thermal flow.Tapered is in between strength (vibration resistance) and (time response)straight high strength or vibration resistant
Tapered length are more resistant to vibration of fluid. Vibration is caused from a phenomenon called wake frequency.Actually fluid flowing around well forms a kind of turbulence and if the frequency of wake is equal to (something called the) natural frequency of well the well may vibrate with maximum amplitude and break of.So the natural frequency quoted as velocity rating of well supplied by the supplier should be higher that the fluid velocity .
So in places like high speed fluid like outlet of high power motor a tapered connection will be better to use.
Material Choose the material according to the corrosiveness of the environment.Generally it will not be a bad idea to choose a material of higher corrosive resistant than your current material if you are trying to replace a pre installed well.
There are corrosion charts available from thermowell manufacturers that list the preferred material for different conditions.