Thermowell types and their selection.

Thermowells are cylindrical fittings used to protect temperature sensors installed in the industrial process. A thermowell consists of a tube closed off on one end and threaded at the other end. 

it is an enclosure used to protect a Thermometer, RTD or thermocouple from exposure to hazardous materials or extreme environments.

According to the American Society for Testing and Materials (ASTM) has defined the term thermowell as – a closed-end re-entrant tube designed for the insertion of temperature sensing element, and provided with means for a pressure tight attachment to a vessel.

 Material: Manufactured from forged bar stocks / Pipes, thermowells are available in Carbon Steel, Stainless Steel, Monel, Hastelloy, Titanium,
molybdenum and chromium steel, Nickel etc.,

Selecting the right Thermowell material :

Selection will depend on the type of chemical, flow rate, and temperature to which the thermowell will be exposed. Also, the failure of a thermowell is due to vibration, caused by turbulent flow. In order to avoid vibration effects, the thermowell must of sufficient stiffness.

The use of Carbon steel is limited to applications with low temperature and pressure, whereas molybdenum and chromium steel is ideal for pressurized applications. Teflon thermowell for protection from corrosive fluids.

Themowell Terminology :

  • Bore Diameter (B): This is the inside diameter of the Thermowell.  
  • Tip Thickness (E): Thickness of the bottom portion of Thermowell.
  • Base diameter Dimension (Q): This is the outside dimension of the Thermowell shank and is a densest part because the outside surface area is exposed to the hazardous materials of the process.
  • Bore Depth (S): Total length of the bore.
  • Lagging Extension Length (T): The lagging extension commonly referred to as the “T” length is located on the cold side of the process connection and is usually an extension of the hex length
  • Insertion length (U): Thermowell immersion lengths are often called the “U” length which is the measurement of the Thermowell from the bottom of the process connection to the top portion of the Thermowell.  
  • Tip Diameter (V): Thermowell end Diameter.

Process Connections: Thermowells are distinguished according to their connection. Following are some popular types used in industries.

Flanged Thermowell : Flanged thermowell features a thermowell collar, which is located on the mating flange. This type of thermowell is used when frequent replacement or removal is required. Flanged thermowell is used in high temperature applications.

Threaded Thermowell : As the name implies, these thermowells are  screwed directly into the process piping or a vessel, which allows for easy installation and removal when necessary. While this is the most common method of mounting, it has the lowest pressure rating, and threaded connections are prone to leakage.

Socket weld Thermowell: These thermowells may be directly welded into the socket of weldolet.

Weld in thermowell: These thermowells are directly welded into the piping or process vessel. As the thermowell is welded into the pipeline, it only used for permanent connections. Used in application with the high-velocity flow, high temperature and/or high pressure.

  • Shank Construction: This is the immersion portion (shank) of the construction. Three categories of shank construction: straight, step and tapered.
  • The straight shank is the same size all along the immersion length. For general purpose, where additional strength is not required, the straight shank or stem is used.
  • The tapered version decreases gradually along the immersion length. Tapered thermowells are designed for use in high velocities, where extra strength is required. Special attention shall be given to the vibration effects caused by the fluid passing the thermowell.
  • Step shank Thermowells have the fastest time response because they possess the least material at the tip (smallest diameter).

Thermowell Immersion Length: Typically the minimum ” U ” dimension of a thermowell in the liquid process is a length equal to 5 times the diameter of the thermowell. For process involving gas or air, the minimum “U” dimension of a thermowell is equal to 10 times the outer diameter of the thermowell.