Thermowell Design Guidelines

When fluid flows past a thermowell inserted into a pipeĀ or duct, vorticies form at both sides of the well. ThermowellsTheseĀ vorticies detach, first from one side, and then from theĀ other. This phenomenon is known as the Von Karmann effect. The frequency of shedding of theseĀ vortices is a function of the diameter of the thermowell, the fluid velocity and, to a lesser extent, the ReynoldsĀ number.The vortex shedding subjects the thermowell to a periodic transverse force. As the vortex sheddingĀ frequency approaches the natural frequency of the thermowell, the thermowell will oscillate, and is liable toĀ snap off. (The natural frequency of the thermowell is aĀ function of its shape and material of manufacture.)

The Von Karmann effect must be taken into accountĀ when designing thermowells of sufficient strength to withstand service conditions, and generallyĀ thermowells are chosen such that the sheddingĀ frequency is always less than or equal to 80% of theĀ natural frequency.

Thermowell Design Guide

Thermowell points to be taken care-

Thermowell dimension and well material shall be specified on data sheet and surfaceĀ finish shall be manufacturer’s standard. Tolerances are as follows:
  1. Bore, ā€œdā€ shall be 0.26 inch maximum to pass plug gauge of 0.254 inch OD.
  2. Bore /OD eccentricity shall be +/- 10 percent of minimum wall thickness.
  3. Bore depth shall be +/- 1/32 inch for depths to 30 inches.
  4. Overall length, ā€œLā€ shall be +/- 1/32 inch for lengths to 30 inches.
  5. Insertion Length, ā€œUā€ shall be +/- 1/16 inch for lengths to 12 inches and +/- 1/8 inch forĀ lengths over 12 inches.
  6. Head diameter shall be +/- 0.01 inch
  7. Shank diameter shall be +/- 0.01 inch
  8. Tip diameter shall be +/- 1/32 inch
  9. Tip thickness +/- 1/32 inch
  10. Thermowells shall have the tag number and material on the well head. For testĀ thermowells, provide 1/2-inch NPT brass plug with brass chain attached to head of wellĀ by non-welding technique. Bore drilling may be performed either by twist drill or gunĀ drill.

Thermowells purchased for installation on piping and vessels areĀ categorized into four types:

  1. Machine Thread (ā€œMTā€ Type) recommended for wall thicknesses more than 3/4Ā inch;
  2. 1-inch NPT Threaded (ā€œTā€ Type) used for line sizes 6 inches and smaller, withĀ wall thicknesses less than or equal to 3/4-inch;
  3. 1-1/4 inch NPT Threaded (ā€œTā€ Type) used for line sizes 8 inches and larger, withĀ wall thicknesses less than or equal to 3/4-inch; and
  4. 3-inch 150# Flanged (ā€œFā€ Type) used for lined piping, atmospheric tanks andĀ vessels.

Thermowell type1 Thermowell type2 Thermowell type3 Thermowell type4 Thermowell type5

Van Stone Thermowells

It is advisable to use full penetration welds for flanged thermowells to achieve the highest fatigue strength rating. Forged thermowells accomplish the same goal without welds, but at a much higher cost. Van Stone style thermowells accomplish this without the cost.

Van Stone termowells are machined from solid bar and are connected using a separate and reusable backing flange. Van Stone termowells mounted between mating flanges for process insertion.

Van Stone Thermowells

This design offers two advantages:

  1. Any weld stress is eliminated.
  2. The slip on flange not being exposed to the process conditions can be of a less expensive material.

Applications:

  • Chemical, process, apparatus and plantengineering; petrochemical; on/offshore
  • High chemical loads
  • High process loads

Also Read: Thermowell Insertion & ImmersionLength

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