One of the most important accessories for any temperature-sensing element is a pressure-tight sheath known as a thermowell. This may be thought of as a thermally conductive protrusion into a process vessel or pipe that allows a temperature-sensitive instrument to detect process temperature without opening a hole in the vessel or pipe. Thermowells are critically important for installations where the temperature element (RTD, thermocouple, thermometer, etc.) must be replaceable without de-pressurizing the process.
Thermowells may be made from 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 other alloys) or ceramic materials. A simple
diagram showing a thermowell in use with a temperature gauge is shown here:
If the
temperature gauge is removed for maintenance or replacement, the thermowell
maintains pressure integrity of the pipe (no process fluid leaking out, and no
air leaking in):
Photographs
of a real (stainless steel) thermowell are shown here, the left-hand photo
showing the entire length of the thermowell, and the right-hand photo showing
the end where the temperature sensing device is inserted:
A
photo of a complete RTD assembly (connection head, RTD, and thermowell) appears
in the next photograph:
Another
photo shows an RTD installed in a thermowell on the side of a commercial
freezer, using a Rosemount model 3044C temperature transmitter to output a 4-20
mA signal to an operator display:
All thermowells,
no matter how well they may be installed, increase the first-order time lag of
the temperature sensor by virtue of their mass and specific heat value. It
should be intuitively obvious that one or more pounds of metal will not heat up
and cool down as fast as a few ounces’ worths of RTD or thermocouple, and therefore
that the presence of a thermowell will decrease the responsiveness of any
temperature sensing element. What is not so obvious is that such time lags, if
severe enough may compromise the stability of feedback control. The system receiving a “delayed” temperature measurement will not see the live temperature of the process in real-time due to this lag.
A
potential problem with thermowells is an incorrect installation of the temperature-sensing
element. The element must be inserted with full contact at the bottom of the
thermowell’s blind hole. If any air gap is allowed to exist between the end of
the temperature element and the bottom of the thermowell’s hole, this will add
a second-time lag to the measurement system. Some thermowells include a spring
clip in the bottom of the blind hole to help maintain constant metal-to-metal
contact between the sensing element and the thermowell wall.
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