Thermowells

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The process environment where temperature monitoring is required is often not only hot, but also pressurized and possibly chemically corrosive or radioactive. To facilitate removal of the temperature sensors (RTD and Thermocouple), for examination or replacement and to provide mechanical protection, the sensors are usually mounted inside thermal wells widely referred to as a thermowell in the industrial environment
What is a Thermowell?

A thermowell is basically a hollow metal tube with one end sealed. It is usually mounted permanently in a vessel or pipe work. The sensor is inserted into it and makes contact with the sealed end. 
A simple diagram showing a thermowell in use with a temperature gauge is shown below:

    
A major disadvantage of thermowells is their long response time because heat must be transferred through the well to the sensor; Minimizing the air space between the sensor and the well, can however decrease this thermal lag. 

A thermowell acts as a barrier between a process medium and the sensing element of a temperature measuring device. It protects against corrosive process media, as well as media contained under pressure or flowing at a high velocity. A thermowell also allows the sensing element to be removed from the application while maintaining a closed system. Thermowells may be made out of any material that is thermally conductive, pressure-tight, and not chemically reactive with the process.

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A Thermowell must satisfy the following functions while in service:
(a) Position the temperature sensitive sensor tip in the process
(b) Protect the temperature sensor and
(c) Seal the process areas from the environment. 

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Thermowells are principally used with Thermocouples, RTDs(Resistance Temperature Detectors) and Bimetallic Thermometers in applications where it is necessary to measure temperature at high pressure (above 75 psig) or in hostile environments. Thermowells are machined from solid barstock. Safe working pressures depend on the well material, operating temperature and the velocity of the flowing medium.

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Optimum Bore
The optimum bore is an important parameter that should be considered when selecting a Thermowell since physical contact between the inner wall of the thermowell and the temperature probe is essential for thermal coupling. In the case of a thermocouple which is tip sensing it is important to ensure that the probe is fully seated (in contact with the tip of the thermowell). For Pt100 sensors which are stem sensing the difference between the probe outside diameter and bore must be kept to an absolute minimum. Response times can be optimised by means of a tapered or stepped-down well which presents a lower thermal mass near the probe tip


Thermowell Process Connections
Various process connection configurations are used when installing a Thermowell. The more popular ones used are: 
(a) Threaded connections
Parallel or tapered (gas tight) threads make for convenient installation into a welded-in fitting directly into the process. Such a connection is suitable for smaller diameter wells which are not likely to be changed frequently (e.g. where corrosion rates are low). A hexagon is used at the top of the well for ease of fitting. An extended hexagon length can be used to allow for insulation thickness. Typical thread sizes are 1⁄8” BSP (T), 1⁄2 ” BSP (T) or 20mm
The diagram below shows a tapered threaded thermowell:
(b) Flanged Connections
Flanged connections are preferable if there is a need for more frequent well replacement such as high corrosion rates. The flange bolts to a mating flange mounted on the process. Such a technique is more appropriate for large pipe diameters and for high pressure applications. Flanges are usually of 2 to 3 inches in diameter. The diagram below shows a flanged parallel well:


(c) Welded Connections
Welded connections can be used when the process is not corrosive and routine removal is not required. High integrity is achieved and this technique is suitable for high temperature and high pressure applications such as steam lines. Removal of a welded-in well usually involves considerable effort and time. The diagram below shows a weld-in Thermowell:

Lagging Extensions
Lagging extensions are provided on thermowells or even directly on temperature probe assemblies for use on lagged processes. A lagging extension distances the terminal head of the sensor from the immersion part of the assembly to allow for the depth of lagging (thermal insulation). This technique is useful in allowing the head, perhaps with an integral transmitter, to reside in a cooler ambient temperature region rather than adjacent to the much hotter process. Lagging extensions take various forms depending on overall probe or well construction, fitting method and type of termination.

Thermowell Fittings
Installing temperature sensor assemblies into thermowells or directly into the process requires the use of some kind of brass or stainless steel fitting.  Fittings include various threaded unions, bayonet caps (and adapters) and flanges.

Adjustable compression fittings are used directly on probes to achieve the required insertion length in the process and to ensure the proper seating of probes into thermowells. Adjustable flanges can similarly be used to secure the sensor assembly into the
process.

Bayonet caps provide a method of quick fitting into suitable adapters located in the process; this technique is widely used in plastics machinery.

Bushes and hexagon plugs are used when adjustment or removal is a lesser consideration. The choice of fitting may be dictated by the need for pressure integrity or by physical size constraints. Compression fittings and threaded bushes can be supplied with tapered threads to achieve a pressure-tight connection

Thermowells are commonly used in two installation formats: tapered thermowells and straight thermowells. Tapered and straight thermowells are shown below:


Tapered thermowells are used in many process applications and provide greater strength, faster response times and more resistance to vibration than straight wells. The taper provides a higher natural frequency which permits use at higher fluid velocities. The reduced tip on a straight well improves response time when it is used with a length sensitive sensor such as an RTD or Bimetallic Thermometer.
                       
Thermowells are more likely to fail from vibratory stress than from the effects of temperature and pressure. ASME calculations can be used to determine if the selected thermowell dimensions are adequate to withstand the specified service conditions of temperature, pressure, velocity and vibration.
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