Introduction to Thermowells
Materials—The Longevity Factor
In general, the thermowell material to be chosen for an installation
is governed mainly by the corrosion conditions the well will
face. The high polish given to all stainless and Monel wells
provides maximum corrosion resistance. Occasionally, the primary
consideration is one of strength rather than corrosion resistance.
For example, a stainless steel well may be required for high
pressure water service instead of a brass well which would
be satisfactory from a corrosion standpoint. Consult the pressure-temperature
ratings given for each well type.
Wells are also available in special grades of stainless steel,
Chrome-molybdenum steel, Silicone bronze, Hastelloy B & C,
Nickel, Titanium, and Monel. Tantalum jackets are also available.
Connection—The Installation Factor
In these pages, you will find standard bore wells of
threaded, flanged (A.S.A. and Van Stone), and socket weld
types. All threaded wells are made in easily welded or brazed
materials. This is important for installations requiring seal
welding or brazing. The pipe thread provides mechanical strength,
and the weld or braze provides the seal.
Flanged wells (other than Van Stone type) consist of a bar
stock well which is solidly welded to a top quality flange.
Standard construction uses a primary “J” groove weld and a
bevel groove secondary weld. The underside weld is machined
to produce a clean fillet. This double welded construction
eliminates the possibility of crevice corrosion since no open
joints are exposed from either inside or outside the installation.
Socket weld wells are simple to install—simply weld them into
place. These wells fit A.S.A. standard socket weld couplings
or flanges. The resulting installation is clean and tight.
Insertion Length—The Accuracy Factor
The distance from the tip of the well to the underside of
the thread or other connection means is the insertion length
(designated as “U”). For best accuracy, this length should
be great enough to permit the entire temperature-sensitive
part of the element to project into the medium being measured.
A properly installed element will project into a liquid a
distant equal to its sensitive length plus at least one inch.
In air or gas, the element should be immersed its sensitive
length plus at least three inches.
Thermocouples and thermistors have short sensitive lengths.
They can be used with shorter insertion length thermowells.
Bimetal thermometers, resistance thermometers and liquid-in-glass
thermometers have sensitive portions between one and two inches
long. Therefore, the minimum standard insertion length of
2 1/2" (63.5 mm) must be entirely immersed in liquid for proper
Filled system thermometer bulbs may have sensitive portions
from one to several inches in length. Determine the sensitive
length of the bulb before choosing an insertion length. Above
all, be sure that dead length, i.e., that required to pass
through walls, pipe fittings, etc., is taken into account
when choosing the necessary well insertion length.
Bore Size—The Interchangeability Factor
Almost any installation uses several types of temperature
measuring sensors. The selection of a standard bore diameter
can produce extreme flexibility within the plant. The same
well can accommodate either thermocouple, resistance thermometer,
bimetal thermometer, or test thermometer.
The bore sizes of wells shown in this handbook cover the most
commonly used temperature-sensing elements as follows.
0.260" (6.6 mm) Bimetal Thermometers 1/4" (6.4 mm)
Liquid-in-glass Test Thermometers (unarmored)
Other elements having 0.250" maximum diameter
0.385" (9.8 mm) Thermocouples (#14 Gage)
Liquid-in-glass Test Thermometers (armored)
Other elements having 0.35" (9.5 mm) maximum diameter
Tapered or Straight Wells— The Velocity Rating Factor
Tapered shank wells provide greater stiffness with the same
sensitivity. The higher strength-to-weight ratio gives these
wells a higher natural frequency than the equivalent length
straight-shank wells, thus permitting operation at higher
fluid velocity. Refer to “Velocity Ratings of Wells.”
Velocity Ratings of Wells
Well failures, in many cases, are not due to the effects of
pressure and temperature alone. Inadequate strength of well
can be due to improper choice of wall thickness or material.
NEWPORT assumes no responsibility for the failure of a thermowell
except as stated in the NEWPORT warranty found in the front
of the Temperature Measurement Handbook.
Less familiar, and more dangerous, are the vibrational effects
to which wells are subjected. Fluid, flowing by the well,
forms a turbulent wake (called the Von Karman Trail) which
has a definite frequency based on the diameter of the well
and the velocity of the fluid. It is important that the well
have sufficient stiffness so that the wake frequency will
never equal the natural frequency of the well itself. If the
natural frequency of the well were to coincide with the wake
frequency, the well would vibrate to destruction and break
off in the piping.
Recommended maximum velocity ratings can be found for every
standard well length and material cataloged. To reduce the
complexity of presenting this information, the ratings given
are based on operating temperatures of 1000°F (537°C) for
wells made of Carbon Steel (C-1018), A.I.S.I. 304, & A.I.S.I.
316. Values for brass wells are based on 350°F (177°C) operation.
Limits for Monel wells are based on 900°F (482°C) service.
Slightly higher velocity may be possible at lower temperatures.
Where single values appear in the velocity tables, these may
be considered safe for water, steam, air or gas. For shorter
insertion lengths, consideration is given to the velocity
pressure effect of water flowing at higher velocities. The
values in parentheses, therefore, represent safe values for
water flow, while the unbracketed values can be used for steam,
air, gas and similar density fluids.
It should be pointed out that the values given are intended
primarily as a guide. To be safe, check each well with your
own calculation. If you have operating conditions requiring
special well designs, our Engineering Staff is available to
assist you in all cases. NEWPORT assumes no responsibility
for the failure of a well except for its repair or replacement.
See NEWPORT’s Warranty.
When ordering FLANGED thermowells, be sure to specify the
flange fully. USE A SKETCH or DRAWING OF THE FLANGE and specify
FLANGE MATERIAL. FOR BREVITY, USE “SS” for stainless steel,
“CS” for carbon steel, “FF” for flat face flanges, and “RF”
for raised face flanges. TO BE SURE THE CORRECT FLANGE IS
ORDERED, CONTACT NEWPORT’S APPLICATION ENGINEERING DEPARTMENT
FOR ASSISTANCE IN ORDERING FLANGED THERMOWELLS.