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Temperature sensors like thermocouples, thermistors, and bimetal thermometers can be sensitive to damage due to pressure, material velocity, corrosion, and the like; Thermowells are used to guard them and hence increase the longevity of the sensor. They allow you to replace the sensors easily without draining the system. Thermowells for high-pressure applications are typically machined from metal bars. For lower pressure environments, you can use smaller thermowells welded from tubes.

There are several categories of thermowell, depending on the shape of the cell. Straight wells are the same diameter down the entire length and can protect against corrosion and erosion. Stepped thermowells switch from ¾ of an inch down to half an inch; the reduced surface area at the measuring end allows smoother velocities and faster temperature response for sensing devices.

Tapered thermowells have a diameter that decreases gradually over the length of the insertion length. They offer superior strength with no fragile joins or breaks, and they have the same quick response time due to the smaller surface area at the measuring end; the strength means they’re useful for high velocity applications. Straight thermocouples have been observed to break or fail under flow-induced vibrations, but tapered thermowells can survive this.

The thermowells can be used with a variety of different sensors - RTDs, thermistors or thermocouple heads by means of several different connection types. Some of the most common are:

Threaded connections are made of materials that can be welded or brazed and provide additional strength. In food and medical applications, though these connections can involve contamination, so they use weld-in connections instead. O-ring connections use an O-ring to seal inside a sleeve welded to a tank. The double weld construction of the ANSI B16.5 flanged thermowell seals open joints throughout the device to protect against corrosive substances.

Standard bore sizes are available, and they can enables a single thermowell to be used for a thermocouple, RTD, bimetal thermometer or test thermometer, across a range of different applications. These standard bore sizes can be used with the most commonly-used devices, and include:

The right material is important for lifetime of a thermowell; you need to consider the temperatures, flow-rates, and the chemicals that the thermowell will be exposed to. Certain corrosives can be even more damaging at higher temperatures and concentrations, and particles suspended in the fluid can cause damage and frictional impacts. For these considerations, a wide range of materials have been deployed in thermowells, including:

Carbon steels have advantages; they don’t corrode easily – but they can only be used at low temperatures and pressures. Stainless steel is the most common – it can withstand high T and p but resists corrosion, as well as being cost-effective. Chromium/molybdenum provides even more strength and is a stainless steel for use in high-pressure vessels; molybdenum resists corrosion easily. Haynes alloy is comprised of cobalt, nickel, chromium and tungsten. It is most often used for sulphidizing, carburizing and chlorine containing environments.

The distance from the point of connection for the thermowell to the tip of the thermowell is the insertion length; this needs to be long enough to allow the entire temperature-measuring portion to be immersed in the substance being measured. For liquids, an additional inch needs to be inserted; for gas substances, an additional three inches is advised. The temperature sensing part is typically small, so the thermowell can have a shorter insertion length. The temperature sensing section for RTDs, liquid in glass thermometers, and bimetallic substances are usually between one and two inches, so this leads to an immersion of 2.5 inches in liquid.

Regardless of the sensor you use, it will deteriorate over time due to exposure to pressure, heat, and velocity from fluid flow; in harsh industrial environments, performance and structural integrity of the sensors can be affected; for example, corrosion can be a problem in metals. Thermocouple wires have very small diameters, which means changes in the shape of the metal with prolonged exposure to heat can be important. To redress these problems, thermowells are used to protect the sensor and prevent measurement drifts or the destruction of the sensor. Periodic calibration can verify the accuracy of the sensors. Dry block probe calibrators offer NIST traceable calibration for thermistor, thermocouple and RTD probes. Non-contact sensors like thermal cameras, IR devices, pyrometers and so on can be used to calibrate these systems. The accuracy is quite low at 1%, but the process is highly repeatable, so it’s good for calibration. An accredited AS17025 calibration lab will ensure the methods used are NIST traceable and more reliable than in-house calibration attempts.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.

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