Thermowells are tubular fittings designed to protect temperature sensors used in industrial monitoring processes. Processes like these most often exist in the cosmetics, food processing, pharmaceutical petrochemical, and refining industries, along with all those others that deal in processing. Typically, they’re installed in piping systems. Temperature sensors protected by thermowells include thermocouples, resistance temperature detectors, and temperature probes.
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How Thermowells Work
Thermowells provide a barrier between sensors and the outer atmosphere after the sensors are placed inside them through their open end. When a sensor is placed inside a thermowell, it is almost always spring-loaded, an action that makes sure that the outside metal tip of the sensor touches the inside metal tip of the thermowell. This metal-to-metal contact ensures that the heat transferred to the thermowell wall by the process fluids is then passed onto the sensor, so that it can work. The atmospheres in which they may be immersed include those that feature the presence of high flow velocities, high pressures, or corrosive substances.
Often, thermocouples cannot function without thermowells, because the substance or environment in which they are being placed is so incompatible to contact with them. In addition, not only do thermowells protect sensors from the hazards of being physically immersed into inhospitable environments and/or substances, they also protect sensors from secondary process forces, such as corrosion, vibration, abrasion, and mechanical force.
Materials Used in Manufacturing Thermowells
Thermowells are typically constructed from strong metal bar stock materials like stainless steel (any grade) and carbon steel, lesser metals like monel and brass, and, on occasions of low temperatures and/or pressures, Teflon.
Thermowell Design and Customization
Material construction is decided per a material’s corrosion resistance, chemical reactivity, and reaction to temperature changes, and how these behaviors match the needs of an application. Thermowells are made using a few different frequently implemented designs, such as sanitary, retractable, threaded, weld-in or screw-in, and flanged. These design names refer to the connection type used to mount the thermowell into the process stream. "Sanitary," for instance, refers to a sanitary cap process connection. In addition, they are typically available in a variety of standard shapes and sizes and can usually be custom ordered.
Customizable configuration factors include bore diameter, stem length, and lagging extensions. When selecting or designing these, thermowell manufacturers consider the thickness of the substance the thermocouple will be monitoring and the pipe fittings and insulation. Bore diameter, which is the size of the thermowell opening, and stem length, which is the distance from the thermowell opening to the wall of the pipe or tube, are decided in part by substance thickness. Meanwhile, fittings and insulation impact the nature of a lagging extension, which is an extension of the thermowell’s hex length that allows it to extend through insulation or walls.
Benefits of Using Thermowells
Thermowells are an excellent investment not only because they protect sensors, but also because they provide system operators with much easier access to sensors and thermocouples. This makes all the difference when the system needs checking, a sensor is in need of removal or repair, or when a probe needs to be moved to a different point inside the processing system without any disruption to the process itself.
Points to Consider When Purchasing Thermowells
Thermowell users need to be on the lookout for a few potential problems associated with them. One of these is flow-induced resonance. Flow-induced resonance is a condition that occurs when vortex shedding, or an oscillating flow, takes the place of regular flow. This condition, which is mostly a risk for cross-flow applications, frequently leads to the fatigue failure of both the thermowell and the sensor inside.
Another condition for which to be watchful is bending fatigue. Bending fatigue occurs at the base of a thermowell. If a thermowell is bombarded with extreme force, it may experience catastrophic bending and failure all at once. Thermowells working in low-pressure environments can fall prey to bending fatigue as well, but in their case, the failure is gradual, many times beginning a string of sensor failures. To learn more about thermowell selection, installation, and care, interested parties should speak at length with an experienced, trustworthy manufacturer.