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.
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.
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.