How to Select Thermowells
Introduction
Thermowells are used to shield thermocouple
elements against mechanical
damage and corrosion. Many variations
are available in a variety of materials
to meet individual job specifications.
The chemical and physical properties
of all standard bar stock materials are
rigidly controlled. All bar stock wells
are drilled by the gun drilling process.
Use of specially designed and constructed
measuring equipment enables
standard guaranteed bore concentricity
to be within ± 10% of wall thickness.
Internal threads are within 1/2 turn of
standard plug gauge. External threads
are within 1/4 turn of standard ring
gauge.
If required by purchase order, your thermowells
undergo an internal hydrostatic
test as a final precaution against pressure
failure. Test pressures and duration
are determined by the customer.
Radiograph and other tests can be performed
and results furnished upon
request.
A variety of alloys suitable for every
thermowell requirement is available.
Thermowell material should be selected
for ability to withstand the process environments,
high thermal conductivity
and low porosity to gases.
Choosing Bore Sizes for Maximum
Flexibility
Where several types of temperature
measuring instruments are used, the
selection of a standard bore diameter
can provide greater efficiency and flexibility
of use. The same well can
accommodate either thermocouple,
resistance thermometer, bi-metal thermometer
or test thermometer. The
bore sizes of wells shown on this website
accommodate the most commonly
used temperature sensing elements.
For example:
.260 Diameter Bore:
- Bi-Metal Thermometers (1/4" stem)
- Thermocouples (#14 Awg)
- 1/4" & 3/8" O.D. Cerampak Thermocouples & RTD's
- Liquid-in-glass Test Thermometers (unarmored)
- Other elements having .252 max. O.D.
.385 Diameter Bore:
- Bi-metal Thermometers (3/8" stem)
- Thermocouples (#14 Gauge)
- 1/4" & 3/8" O.D. Cerampak Thermocouples
& 3/8" RTD's (Use .260 Bore for 1/4" RTD's)
- Liquid-in-glass Test Thermometers (armored)
- Other elements having .377 max. O.D.
When to Use Tapered or Straight Thermowells
Tapered thermowells provide greater
strength without sacrificing sensitivity.
Because of its higher strength-to-weight
ratio, the tapered thermowell provides
greater resistance to high frequency vibrations
than straight thermowells. This permits
reliable operation at high fluid velocities.
Thus, for higher fluid velocities, the
tapered well should be chosen; for lower
fluid velocities, the straight well. When
choosing wells, refer to the velocity rating
charts and other design information.
Choosing the Material
A most important factor in selecting thermowell
material is to determine the corrosive
conditions to which the well will be
exposed. Recommended materials for various
services are given in the Thermowell
Material Guide. The high
mirror polish given to all wells enhances its
corrosion resistance
capability.
Occasionally, the material consideration is
one of strength rather than corrosion. For
example, a stainless steel well may be
required for high pressure water service,
where a brass well might have been satisfactory
from a corrosion viewpoint. It will
be helpful to consult the pressure-temperature
ratings given for each well type.
Choosing the Proper Connection
In this website you will find standardized
wells of threaded, flanged (ASA and Van
Stone), and socket weld types with standard
bore sizes. A provision for customer
specifying design parameters or "weld-in"
thermowells is also provided.
Threaded wells are made in readily weldable
material. Standard flanged wells
(other than Van Stone) have flanges welded
front and back with "V" or "J" groove
design. Full penetration double welded
flanges are also available.
The double-welded construction eliminates
possible crevice corrosion and stress
cracking. Heat treated to NACE specs is
available.
Socket weld of wells are especially simple
to install. They fit ASA standard socket
weld couplings or flanges to produce a
clean, tight installation.
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