Thermowells are recommended whenever a temperature element is to be inserted into a process where corrosion, pressure, abrasion, or shear forces may threaten the life of the element. In addition thermowells allow for a defective instrument to be removed without shutting down or draining the process. Thermowells are among the simplest yet least well publicized accessories used in industrial temperature measurement applications. There are many variations of two basic kinds; low pressure and high pressure.
They are used to provide an isolation between a temperature sensor and the environment, either liquid, gas or slurry. A thermowell allows the temperature sensor to be removed and replaced without compromising either the ambient region or the process.Low Pressure
Fabricated Pockets are precision components manufactured to the highest standard from pipe, tube and bar, to serve as protective devices for primary sensing elements of all types.
High Pressure
Solid Drilled Thermowells are precision components manufactured to the highest standard from bar or forgings, to serve as protective devices for primary sensing elements of all types. A wide range of possible products are available. Conditions of pressure, temperature and corrosion resistance govern the size, shape and selection of materials to ensure optimum dependability system response and accuracy.
Wake Frequency Problems
When fluid flows past a thermowell inserted into a pipe or duct, vortices 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.
Corrosion Factors
Material chosen for Thermowells is governed by the corrosion conditions the well will face.
Thermowell Material is an important factor when considering the thermowell design.