These are used in industrial monitoring processes which may include a corrosive substance or high pressure and flow velocity, and the thermowells provide a protective barrier between the thermocouple and the liquid, gas or environment which it is being used to monitor.
Used in applications for petrochemical, refining, food processing, pharmaceutical, cosmetics and other processing industries, temperature sensors and thermocouples are often subject to pressure or incompatible substance contact, therefore a thermowell is used to protect them. Aerodynamic or hydrostatic forces typically act upon the thermowells which can be designed to be low pressure or high pressure depending on the nature of the application. Thermowells further protect thermocouples from damage as a result of other process forces such as vibration, corrosion or abrasion, or from mechanical damage.
One of the main benefits to using thermowells, apart from the protection of the sensor or temperature probe, is that thermowells provide easier access to the temperature sensors and thermocouples within the measuring system. They facilitate in the checking, repair or removal of the sensor, or in the relocation of the probe to a different point within the process system without disturbing or compromising the process.
Common designs of thermowells include: threaded, flanged, weld-in or screw-in, retractable and sanitary. There are a number of standard size and shape thermowells which are suitable for use in many different applications; however, custom thermowells can be designed to fit into a process with specific requirements and restrictions. Some important factors to keep in mind when determining the most suitable thermowell for an application include: stem length, bore diameter, lagging extensions and the thickness of the substance the thermocouple is being used to monitor.
The bore diameter is the size of the opening that the temperature probe will be put into and the stem length is the distance from the bore to the wall of the tube or pipe. Lagging extensions take into consideration the pipe fittings and insulation. Material choice for the construction of thermowells should also be carefully considered with regard to chemical reactivity, corrosion resistance and a material’s reaction to temperature increase or decrease. Commonly used materials for thermowells include durable metals such as carbon steel and stainless steel, as well as other materials such as monel and brass. For flanged thermowells, appropriate flange size and rating will also need to be determined.