ASME PTC 19.3 TW 2010,Thermowells Performance test code

You can read an opinion on velocity collars on Emerson's "Process Experts" web site,

http://www.emersonprocessxperts.com/

Scroll down to the 3rd article, Velocity Collars are Not the Answer to Thermowell Resonance.

I suspect that prevention of breakage and the resulting loss of integrity was the thrust for the new standard, but that's speculation on my part.

The revised PTC 19.3 TW-2010 was revised due to a thermowell failure at a nuke palnt in 1985 that shut the plant down for 15 years. The thermowell passed the 1974 version of the code.

The new code takes into account the lift and drag forces on a thermowell and it must meet certain criteria to use 0.8 as the critical factor. The in-line resonance band is a concern when the thermowell and flow properties calculate that the well will be in this area because of the fluid velocity. The equations provide the criteria for determining this resonance affect.

As for velocity collars - There have been several finite analysis calculations run and they have determined that, unless it is a tight fit, velocity collars still allow the thermowell to vibrate sufficiently enough to cause a failure. For best results, it is better to calculate the insertion length of the thermowell from the unsupported length. As a company, many of our customers require the wake frequency calculation to be performed on all thermowells, not just those in high energy systems.