Why thermo wells are used? What material is used in thermo wells?

Thermowells generally used to protect temperature sensors installed in industrial processes.And If the sensor fails, it can be easily replaced without draining the vessel or piping,the material selection for thermowell are used according to the process temperature and process fluid. 

A thermowell is a kind of fitting that is usually used for thermometer, it is important to use this fitting for maintenance purposes and it is usually made of brass.

To Monitoring heat from the pipe

Thermowells are tubular fittings used to protect temperature sensors installed in industrial processes. A thermowell consists of a tube closed at one end and mounted in the process stream.

Thermowells are additional external fittings for temperature used to avoid direct contact with industrial process and for easy replacement. Usually thermowells are made of Stainless Steel, Ceraic and Silica. 

Temperature Sensor protection & it is made up of SS-316

Its used to protect temperatuee sensors. SS 316

A detail answer is already added by Muhammad usman

Thermowells are used for the protection of temperature sensors, to prevent mechanical damaging of RTD/Thermocouple because these sensors are fragile and might be damaged if themowell is not used.

Used in high pressure areas like steam, acid, ammonia gas for the protection of both sensors and human.

Thermowells are either machined from solid bars stock or fabricated using materials like 316SS, Inconel 600 etc.

Thank you for the question Mr Tariq,

This article exposes nice infos about the topic.