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Jan 23,2020
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On many occasions in processing industries, it is not possible or desirable to insert a temperature sensor—that is, a thermocouple, RTD, bimetallic thermometer, glass stem thermometer, or filled system bulb—directly into the process material. A protective enclosure, called a “thermowell” or “well,” must be used to protect the sensor from the process. In addition to protecting the sensor, the thermowell permits temperature sensor removal during processing operations.

Materials Thermowell
Thermowells generally consist of Type 304 or Type 316 stainless steel (ss). Wells of this material are usable to approximately 1200°F. The Type 304 or Type 316 ss wells are common in most typical industry applications. Some of the other well-type materials are carbon steel, brass, and ceramic. When selecting a thermowell, be sure to refer to a chemical compatibility chart to see if the well material is compatible with the application’s fluid. The brass well is suitable for air and
water applications.

The ceramic well is usable in temperatures above 1200°F. Ceramic wells are fragile and require careful installation. Ceramic wells are not used in very high pressure applications. When a process is starting up, the well must be warmed up to operating temperatures slowly in order not to crack or fracture the well.

Three factors affect the choice of material:

Type of corrosive environment to which the thermowell will be exposed
Temperature and pressure limits of the material
Compatibility with the process piping material to ensure solid, noncorroding welds and junctions

Mounting Methods
Thermowells can be mounted in one of three methods, which results in three different construction designs. Thermowells can be threaded, welded, or bolted (flanged style) onto the process pipe.

Flanged type thermowell
Flanged thermowells are bolted onto a pipe or tank and can be easily removed or installed. Flanged thermowells are used in corrosive environments as well as in high-velocity, high-temperature, or high-pressure applications. Flanged thermowells are the most expensive type of thermowell.

Threaded thermowell
Threaded thermowells are the weakest type of thermowell and may leak.

Welded thermowell
Welded thermowells are permanently welded onto the process pipe or tank. Welded thermowells are the strongest type of thermowell and are used with fluids of high velocity, high temperature, or high pressures. Welded thermowells are necessary for applications that require a leak-proof seal.

Thermowell Failure
Thermowells can fail for many reasons, such as high drag forces, static pressure, and corrosion. The von Karman effect can also cause thermowell failure. Fluid flowing around a thermowell forms a turbulent wake called the von Karman trail.

This is a  dangerous phenomena that can occur in a high velocity application is the potential catastrophic results of vibrational effects upon a thermowell.  The turbulent wake has a specific frequency that is the result of the thermowell’s diameter and fluid’s velocity. If the natural frequency of the well should match the frequency of the turbulent wake, the thermowell will vibrate until it is destroyed. The thermowell tends to break below the mounting in these cases. 
Response Time of thermowell

Thermowells have a significant effect on a temperature sensor’s time response. Thermowell time constants   are so large that the temperature sensor’s time constants become only a small factor. The thermowell time constant becomes the major contributing factor to the overall time constant for the entire thermowell/sensor/transmitter system.

If rapid speed of response is desirable in a typical industrial application:

• Use a thermowell that has the minimum wall thickness permitted by operating conditions.

• Install thermocouples and RTDs with spring-loaded assemblies so that the tip of the sensor seats at the bottom of the well. This approach improves thermal conductivity

• Use stepped thermowells rather than tapered or straight thermowells as stepped thermowell have smallest tip diameter and thus have a faster response time.