Thermowells

Thermowells are used in industrial temperature measurement to provide isolation between a temperature sensor (often a thermocouple) and the environment whose temperature is to be measured.

They are intrusive fittings and are subject to static and dynamic fluid forces. These forces govern their design. Vortex shedding is the dominant concern as it is capable of forcing the thermowell into flow-induced resonance and consequent fatigue failure. The latter is particularly significant at high fluid velocities.

The ASME Performance Test Code (PTC 19.3) is the most widely used basis for thermowell design. It is currently being updated to cover a broader range of thermowell designs and fluid conditions.

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. A thermowell allows the temperature sensor to be removed and replaced without compromising either the ambient region or the process.

The most expensive, complex thermowells that we have ever seen were made from drilled molybdenum rods with an internal sheath of high purity aluminaLow pressure, moderate to high temperature environments are routinely provided with a thermowell variant called a protection tube that can be made of metal or high temperature glass or ceramic, again according to the conditions. Most high temperature industrial furnaces use ceramic or metal protection tubes, according the conditions.

Mineral-Insulated-Metal-Sheathed (MIMS) thermocouples have replaced many protection tube systems at temperatures up to about 700°C, although some newer sheath materials claim capabilities up to 1100°C or so. In base metal assemblies, these newer MIMS devices offer some improvement in response at moderate cost and are replaced themselves when they fail.
While the majority of uses of thermowells involve the more popular temperature sensors, such as thermocouples and RTDs, there is no fundamental technical reason why radiation thermometers can not be used to measure the temperature of the inner portion of a thermowell or protection tube and infer the process temperature on the other side. Not only is this a very common practice at high temperatures in process furnaces, but it is used at lower temperatures using low temperature, low cost radiation thermometer and ones with fiber optics as well.