EPC School

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

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




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

1. Machine Thread (“MT” Type) recommended for wall thicknesses more than 3/4 inch;
2. 1-inch NPT Threaded (“T” Type) used for line sizes 6 inches and smaller, with wall thicknesses less than or equal to 3/4-inch;
3. 1-1/4 inch NPT Threaded (“T” Type) used for line sizes 8 inches and larger, with wall thicknesses less than or equal to 3/4-inch; and
4. 3-inch 150# Flanged (“F” Type) used for lined piping, atmospheric tanks and vessels.

Thermowell points to be taken care-