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Thermowell

thermowell-detail

Thermowells provide protection for temperature probes against unfavourable operating conditions such as corrosive media, physical impact (e.g. clinker in  furnaces) and high pressure gas or liquid. Their use also permits quick and easy probe interchanging without the need to “open-up” the process.

Thermowells take many different forms and utilise a variety of materials (usually stainless steels); there is a wide variety of thread or flange fittings depending on the requirements of the installation. They can either be drilled from solid material for the highest pressure integrity or they can take the form of a “thermopocket” fabricated from tubing and hexagonal bushes or flanges; the latter construction allows for longer immersion lengths.

Thermowells transfer heat from the process to the installed sensor but “thermal inertia” is introduced. Any temperature change in the process will take longer to affect the sensor than if the thermowell were absent; sensor response times are thus increased. This factor must be considered when specifying a thermowell; except when thermal equilibrium exists, a temperature measurement will probably be inaccurate to some extent.

 

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Optimum bore is an important parameter since physical contact between the inner wall of the thermowell and the 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.

Process connections are usually threaded or flanged but thermowells can be welded into position.

  1. 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.
     

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  2. 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.

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  3. 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.

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Lagging extensions are provided on thermowells (or even directly on probe assemblies) for use on lagged processes. A lagging extension distances the  terminal head 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 which are made of solid barstock of various heat and corrosion alloys by drilling usually preferred over the welded protection tubes for critical applications where high mechanical strength and longer service life required. If the alloy bar material is correctly selected and designed properly, the Thermowell last long againts corrosive, high temperature, mechanical shock and vibration result from high velocity of fluids.

 

Ordering Info Thermowell