Thermowells are cylindrical fittings used to protect temperature sensors installed to monitor industrial processes. A thermowell consists of a tube closed at one end and mounted on the wall of the piping or vessel within which the fluid of interest flows. 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 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 fluid, the sensor's response to changes in temperature 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 fluid temperature, and since the walls of the thermowell conduct heat out of the process, sensor accuracy and responsiveness is reduced by the addition of a thermowell. Traditionally, the thermowell length has been based in the degree of insertion relative to pipe wall diameter. This tradition is misplaced as it can expose the thermowell to the risk of flow-induced vibration and fatigue failure. When measurement error calculations are carried out for the installation, for insulated piping or near-ambient fluid temperatures, excluding thermal radiation effects, conduction error is less than one percent as long as the tip is exposed to flow, even in flanged mounted installations. Arguments for longer designs are based on traditional notions but rarely justified. Long thermowells may be used in low velocity services or in cases where historical experience justified their use. In modern high-strength piping and elevated fluid velocities, each installation must be carefully examined especially in cases where acoustic resonances in the process are involved. The response time of the installed sensor is largely governed by the fluid velocity and considerably greater than the response time of the sensor itself. This is the result of the thermal mass of the thermowell tip, and the heat transfer coefficient between the thermowell and the fluid. A representative thermowell is machined from drilled bar stock to ensure a proper sensor fit (ex: an 0.260-inch bore matching an 0.250-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 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. The use of welded sections for long designs is discouraged due to corrosion and fatigue risks.
Materials and construction The thermowell protects the instrument from the pressure, flow-induced forces, and chemical effects of the process fluid. Typically a thermowell is made from metal bar stock. The end of the thermowell may be of reduced diameter (as is the case with a tapered or stepped-shank thermowell) to improve the speed of response. For low pressures and temperatures, Teflon may be used to make a thermowell; various types of stainless steel are typical, with other metals used for highly corrosive process fluids. Where temperatures are high and the pressure differential is small, a protection tube may be used with a bare thermocouple element. These are often made of alumina or other ceramic material to prevent chemical attack of the platinum or other thermocouple elements. The ceramic protection tube may be inserted into a heavy outer protection tube manufactured from silicon carbide or other material where increased protection is required.
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