I want to know that where and why we must use taperd shape thermowells. And for cylindrical ones as well. Is there any other shapes available? If any, what is the conditions to use them? Thanks for your help.
Might I recommend Tel-Tru Manufacturing's Guide to Thermowells for a suitable explanation, an extract from which is posted below:
Tapered or Straight Shank?
The velocity Rating Factor
Tapered shank wells provide greater stiffness for the same sensitivity.
The higher strength to weight ratio gives these wells higher natural frequency than for the equivalent length straight shank wells, thus permitting operation at higher fluid velocity.
Well failures, in most cases, are not due to the effect of pressure and temperature. The calculations necessary to provide adequate strength, under given conditions, are familiar enough to permit proper choice of wall thickness and material.
Less familiar, and more dangerous, are the vibrational effects to which wells are subjected.
Fluid, flowing by the well, forms a turbulent wake (called the Von Karman Trail) which has a definite frequency based on the diameter of the well and the velocity of the fluid.
It is important that the well have sufficient stiffness so that the wake frequency will never equal the natural frequency of the well itself.
If the natural frequency of the well were to coincide with the wake frequency, the well would vibrate to destruction and break off in the piping.
A recommended maximum velocity rating can be found on the Tel-Tru web site (http:www.teltru.com) for every standard well length and material catalogued.
It should be pointed out that the values given are extremely conservative, and intended primarily as a guide. Wells are also safe if the resonant freqency is well below the wake frequency or if the fluid velocity is constantly fluctuating through the critical velocity point. Nevertheless, if the installation is not hampered by the use of a sufficiently stiff well, we recommend the values given not be exceeded.
Thermowells are tapered so that the base has the mechanical strength to stand up to the forces of process conditions, namely pipeline flow. Flow stresses can be significant and result in failure. There is a calculation to check for mechanical resonance as well. The tip is
smaller so that there is less thermal mass and therefore quicker response to process temperature changes. The thermowell is a heatsink, and inserted in the process will have a temperature gradient from near process at the tip to some value in the direction of ambient at the connection.
Form follows function. The same considerations apply to stepped thermowells.
there are sizing codes (temperature.com has one) that deal with thermowell error and vortex shedding for common shapes.
what is not clear is if your original question is generic i.e. preference to one style over another or if it is application specific.
The old ASME PTC 19.3-2004 Temperature Measurement standard's thermowell calculation only applied to tapered shank thermowells. The current version ASME PTC 19.3TW-2016 applies to thermowells that are tapered, straight and stepped.
For temperature measurement purposes the skinnier and longer the better.
For mechanical integrity, check out www.jms-se.com/swiftycalc to see the impact of different shank style choices.