What are Thermowells?
Thermowells are one of the most important accessories used in temperature measurement. A temperature sensor is placed in the flow stream to measure the temperature can be affected by pressure, flow and vibration. To reduce those effects temperature sensors are put into a thermowell and the thermowell is installed into the process channel:
Thermowells are critically important for installations where the temperature element (RTD, thermocouple, thermometer, etc.) must be replaceable without de-pressurizing the process. This may be thought of as a thermally conductive protrusion into a process vessel or pipe allowing a temperature-sensitive instrument to detect process temperature without opening a hole in the vessel or pipe. The temperature sensor can be mount and release at any time without shut down the flow.
Thermowells may be made out of any material that is thermally conductive, pressure-tight, and not chemically reactive with the process. Most thermowells are formed out of either metal (stainless steel or another alloy) or ceramic materials.
There are different types of thermowells classified based on different criteria:
Selection based on the connection:
There are commonly three types of thermocouples with different connection
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Threaded
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Socket welded
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Flanges
Threaded:
In threaded thermowell the connection is made by the thread connection. The threaded wells are used where there is no need for frequent replacement. The well is threaded into the smaller pipe with non-corrosive fluid, it can be used for a long time.
Socket welded:
The well is welded directly into the pipe, which provides a very high-quality connection. Since they are welded, they cannot be removed easily.
Used in very high pressure and high-temperature applications. They are only be used where corrosion is not initiated.
Flange:
Flange thermowells are used where frequent removal of thermowell is required. 
Flanges are connected to the pipeline as shown in the above figure. Used for large pipeline with high corrosion, high-pressure applications.
Stem profile selection:
Tapered well has small inclination at the bottom of the well, which has good response time and good strength
A stepped bar well have step in the rod, sometimes referred to as reduced tip, wells are usually ¾” diameter and step down to ½” diameter to improve time response and low drag force
A straight bar well have high strength, it is a stright rod. These wells have less time response compared to other two wells.
Material selection
Material selection can be as simple as choosing the same material as the tank or pipe where it is to be installed. This works most of the time but there are exceptions. In the case of corrosion or erosion an object placed into a flow is more susceptible to those effects and a different material will be necessary to provide longer life. There are corrosion charts available from thermowell manufacturers that list the preferred material for chemicals at different conditions.
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