Thermowells: A Reliable Solution for Effective Performance of Temperature Element

Introduction to Thermowells

Thermowells, as their name suggests, are the ‘wells’ designed for thermal measurement.

For the measurement of process temperature in various industrial applications, two type of contact temperature sensors are used namely RTDs (Resistance Temperature Detectors) and TC (Thermocouples). Selection of these sensors depends on the particular application and the range of temperature which is to be measured. These are installed in the pipeline / column / vessels. Thermowells provide the protection to RTDs / TCs against the unfavourable process conditions like corrosion, high pressure and velocity of the medium (air or liquid), chemical reaction, mechanical failure etc caused when these are installed in the process. In general, inset element of RTD and TC consist of PT-100 and two dissimilar metals (Chromel and Alumel in ‘K’ Type) respectively which are insulated from filled Magnesium Oxide (MgO) and covered by a sheath of SS-316 / Inconel-600 / Any other Sheath Material as per applications.

Thermowell provide the outside protection to this sheath. Furthermore, with thermowell installation we can interchange the sheath or probe of temperature element without stop the industrial process. Hence Thermowells are protection assembly for sensors made from single Barstock or seamless pipe or combination of seamless pipe and Barstock rod welded together. All three combinations are closed from one end which is inside the process and other end is connected to the instrument.

Classification of Thermowells

The classification of thermowell is based on the type of process connection used, shape of their shank and constructional design.

As per the 1st classification criteria (process connection), there are following 3 types of thermowells:

• Threaded type thermowell: Threaded thermowell consist of either a a combination of barstock connection and seamless tube or a rigid barstock rod. Such type of thermowell has threads at its one end which is connected to process. These threads may be of parallel or tapered type and male or female based on the process requirement. These thermowells can be attached directly to the process, or sometimes a weld-in stub or fitting may also be employed.

Diag-I: Layout of a threaded type thermowell

• Flanged type thermowell: Flanged Thermowell also made from either solid barstock rod or with a combination of barstock connection and seamless pipe. The difference exists in terms of process connection. In this category of thermowells, a flange is utilized in place of threads. The counter flange is located in the process which is connected with thermowell flange with the help of nut-bolt and gasket. These thermowells are preferred in higher pressure applications and the rating of flange is also by process pressure. A flange may be full penetrant weld or thread with thermowell.

Diag-II Layout of the flanged type linear thermowell

• Welded type thermowell: This type of thermowells are used mainly in those applications in which the process is non-corrosive with lesser periodic maintenance or regular removal of thermowells not required. This type is more reliable against the high pressure applications as it is welded directly to the process

Diag-III Layout of the welded type thermowell 

As per the 2nd one (shape of their shank), there are following 3 types of thermowells:

• Taper type thermowell: Taper type shank thermowells are the most popular design applied in process industries. In this type, the outside diameter of thermowell reduces with the immersion length. So, there is a difference in root diameter (near the process connection) and tip diameter (bottom end inside the process) of thermowell.

Diag-IV: Layout of a flanged narrow thermowell

• Straight type thermowell: Straight type shank thermowells are straight in the shape from process connection to tip side. So, the root radius and tip thickness of linear thermowells are same. These type of thermowell has better mechanical strength but the response time is comparatively slower than a taper shank.

Diag-V: Layout of a straight type thermowell

• Stepped type thermowell: Stepped type shank also have different root and tip diameter but this reduction is in the form of step. In case of taper thermowell, diameter reduced continuously with length but in stepped type, it steps down only at one point. In general, smaller diameter is the last 50mm or it may be changed as per the requirement. The smaller tip responds more quickly to temperature changes, but when it comes to mechanical strength, these thermowells are less robust than the other two.

Diag-VI  Layout of stepped type thermowell

As per the last one (constructional design), there are following 3 types of thermowells:

• Built-up or Fabricated type thermowell: These type of thermowells are made from two parts, which are welded together by refereeing suitable welding procedure at the tip side. Part-1 is a seamless tubing and part-2 is a rigid barstock rod of same material. Thus, stem of the thermowell will be tubular and tip will be a barstock either narrow or linear. This type is preferred for low pressure applications and where it is allowable to install thermowells having immersion length more than 500mm. Connection of thermowell with vessel/column can be done by providing suitable threads/flange / weldolet. Sometimes, thermowells are made from single tube without any welded barstock tip.

Diag-VII Layout of the built-up type thermowell

• Solid Barstock type thermowell: These type of thermowells are made from only part-2, which is solid drilled barstock rod without any seamless protection tube. Such thermowell doesn’t have any weld joint inside the process. This kind of thermowell is the most prevalent in process industries. Similar to built-up, suitable threads / flange / weldolet is used as a process connection. The rating of a flange is decided by the design pressure. In general, where thermowell immersion length is lesser than 500mm, barstock thermowells are preferred.

Diag-VIII Layout of a rigid cylinder thermowell

• Vanstone type thermowell: These type of thermowells are made from single barstock and there is no any weld joint in between flange and thermowell. Thus, the thermowell, stem, tip, and subcomponent are all crafted from a single barstock rod. These thermowells are used in the highest pressure applications where welding joint is not allowed. Such thermowells are connected in the process with a companion flange. Further, in order to ensure the sealing between thermowell collar and companion flange, a gasket is placed in between both of these. Nut-bolts are applied to tighten the Vanstone thermowell with companion flange in the process.

Diag-IX Layout of a vanstone type thermowell

Conclusion :

Thermowells are essential requirement of any process industry in order to provide necessary protection to any RTD or Thermocouple against the process fluid. These thermowells are classified based on their process connection, constructional design and the length. Tempsens offers a variety of thermowells which includes barstock, built-up, fabricated, flanged, vanstone, threaded, welded etc. as per the application and client’s requirement.