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Thermowells are tubular fittings used to protect temperature sensors installed in industrial processes. A thermowell consists of a tube closed at one end and mounted in the process stream. A temperature sensor such as a thermometer, thermocouple or resistance temperature detector is inserted in the open end of the tube, which is usually in the open air outside the process piping or vessel and any thermal insulation. Thermodynamically, the process fluid transfers heat to the thermowell wall, which in turn transfers heat to the sensor. Since more mass is present with a sensor-well assembly than with a probe directly immersed into the process, the sensor's response to process temperature changes is slowed by the addition of the well. If the sensor fails, it can be easily replaced without draining the vessel or piping. Since the mass of the thermowell must be heated to the process temperature, and since the walls of the thermowell conduct heat out of the process, sensor accuracy and responsiveness is negatively impacted by the addition of a thermowell.[1] To be representative of the average temperature of fluid, the thermowell must extend a few per cent of the inside diameter of the process pipe or vessel (Normally 1/3 rd of the pipe inside diameter).[2] A rule of thumb that is sufficient for most industrial applications (approximately 1% accuracy) is to ensure that the thermowell projects 5 times its own diameter into the process plus the length of its sensing element. So, for a grounded thermocouple spring-loaded into a thermowell with a 1-inch-diameter shank and a tip thickness of .25 inch, a typical immersion length would equal 5.5 inches (1 inch OD x 5 + .25 inch well tip thickness +.25 inch sensor wall thickness = 5.5 inches).[3] A thermowell is typically machined from solid bar stock and gun-drilled to ensure a long straight bore that closely approximates the diameter of the installed sensor (ex: a .260-inch bore matching a .250-inch sensor). A thermowell is typically mounted into the process stream by way of a threaded, welded, sanitary cap or flanged process connection. The temperature sensor such as a thermometer, thermocouple or resistance temperature detector is inserted in the open end of the thermowell and typically spring-loaded to ensure that the outside tip of the temperature sensor is in metal to metal contact with the inside tip of the thermowell.
Normally a thermowell is installed for easy maintenance. If a temperature needs to be measured in a pipe or on a vessel that has a high pressure, temperature or corrosive substance, a thermowell is normally installed that will enable you to remove the sensor without the danger of the substance causing injury. A thermal paste is also normally inserted into the thermowell to ensure good heat transfer between the thermowell wall and the sensor. If no thermal paste is available, you can use oil or copper grease.
Thermowells are used for protection of temperature measuring elements like thermocouple, RTD from direct contact of process fluid. Temperature elements are inside the thermowell and thus protected from process fluid.
Generally they are made of SS316 or SS304.
Thermowells are used to guard temperature sensors such as thermocouples, thermistors and bimetal thermometers against damage from excessive pressure, material velocity and corrosion. They also increase the longevity of the sensor, allow sensor replacement without draining the system and reduce the probability of contamination. Thermowells designed for high pressure applications are typically machined from bar stock to ensure integrity. Smaller thermowells for use in low pressure environments may be constructed from tubing with one end welded closed.
Selecting the right material is crucial to the longevity of a thermowell. The type of chemical, temperature and flow rate the thermowell will be exposed to should be considered when specifying the material. The corrosive effects of chemicals are increased at higher concentrations and temperatures. In addition, particles suspended in fluid can cause erosion. The proceeding list includes some of the most frequently used materials for the construction of thermowells:
Thermowells are used in high pressure areas like steam, acid, ammonia, etc to protect temperature sensors (like RTD, Thermocouples) and for human safety. Temperature sensor can easily be removed without shutting down the whole process using thermowell.
Thermowells are manufactured from solid bars stock, 316 Stainless Steel, Inconel 600, etc.
