Temperature sensors or indicating type temperature gauges are not directly inserted into the process pipe, unless these are used to measure the outside temperature of process pipe, instead, these are used with thermowells. By using thermowells, sensors and gauges will not interfere with the process line operation, and the users are able to perform the maintenance procedure of the process line more easily. These thermowells can be used in a high steam line or Vapor line. These are directly welded onto the socket or pipe so can be a semipermanent. Therefore, the user must carefully decide its material and specification before welding process is performed.
Selection of thermowell
In general, the thermowell material chosen for the installation is governed mainly by the corrosion condition the thermowell will face. Recommended material for various services are given in the corrosion table. Occasionally, the material consideration is one of strength rather than corrosion. For example, a stainless steel thermowell may be required for a high pressure water service where otherwise a brass thermowell woule be satisfactory from a corrosion standpoint.
The distance from the end of the well to the underside of the thread or other connection means (Designated as "U") is the insertion length.
■ Bore size
Almost any installation uses several type of temperature measuring instruments. The selection of a standard bore diameter can produce extreme flexibility within the plant.
■ Tapered or straight type
Tapered type thermowells provide greater stiffness for the same sensitivity. The higher strength to weight ratio gives these thermowells higher natural frequency than for equivalent length straight type thermowells, thus permitting operation at higher fluid velocity.
Wake frequency calculations in accordance with ASME PTC 19.3
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