Thermowell

A thermowell consists of a tube closed at one end and mounted on the wall of the piping or vessel within which the fluid of interest flows.

Since more mass is present with a sensor-well assembly than with a probe directly immersed into the fluid, the sensor's response to changes in temperature is slowed by the addition of the well.

In modern high-strength piping and elevated fluid velocities, each installation must be carefully examined especially in cases where acoustic resonances in the process are involved.

For low pressures and temperatures, Teflon may be used to make a thermowell; various types of stainless steel are typical, with other metals used for highly corrosive process fluids.

In liquids and in high-pressure compressible fluids, a smaller but nonetheless significant component of motion in the flow-direction is also present and occurs at nearly twice the vortex shedding rate.

The in-line resonance condition may govern thermowell design at high fluid velocities although its amplitude is a function of the mass-damping parameter or Scruton number describing the thermowell-fluid interaction.

For Reynolds Numbers associated with the Drag Crisis (first reported by Gustav Eiffel) 100,000 < Rd < 1,000,000-3,000,000, the shedding forces are randomized with a corresponding reduction in magnitude.

The ASME Performance Test Code PTC 19.3TW-2016 ("19.3TW") defines criteria for the design and application of rigidly supported thermowells.

Coatings, sleeves, velocity collars, and special machined surfaces such as spirals or fins are expressly outside the scope of the 19.3TW standard.

[2] Catastrophic failure of a thermowell due to fatigue caused the 1995 sodium leak and fire at the Monju Nuclear Power Plant in Japan.