How Do I Order A Thermowell? JMS is a custom
manufacturer of thermowells. We not only stock common thermowell
designs but can also manufacture and ship thermowells on a same
day or next day basis even when the required thermowell design
is custom. Where the order is not on a Swifty basis, standard
delivery of non-stock thermowells is 2 weeks ARO. Here are some
handy links to thermowell specifications so that you can build a
JMS part # for the thermowell that you need:
No matter whether you require an Inconel
thermowell, a Hastelloy thermowell, a 316/316L SS, 304/304L SS,
Monel, Titanium, or plain old Cabon Steel thermowell, JMS
maintains a broad inventory of thermowell bar stock material to
enable rapid thermowell construction.
What is a thermowell? The American
Society for Testing and Materials (ASTM) has defined the term
thermowell as follows:
Thermowell, n. –
a closed-end reentrant tube designed for insertion of a
temperature-sensing element, and provided with means for a
pressure-tight attachment to a vessel. (See also protecting
Vol. 14.03, E 344 – 02 § 3.1 (2007).
In this instance, the ASTM definition of thermowell leaves a bit to be
desired. Thermowells are typically constructed of solid
drilled-out bar stock and are designed to protect a temperature
sensor from flow, high pressure and harsh environments.
Thermowells encase and protect temperature sensors from the
harmful effects of the processes into which they are immersed
without substantially insulating the temperature sensor
(thermocouple, RTD, etc.) from the temperature of the process.
What are the most common types of thermowells?
Thermowells are commonly classified according to their connection to a process.
The most common types of thermowells are (1) threaded,
(2) socket weld,
(3) weld-in, and
As the names imply, a threaded thermowell is screwed into the process either
directly into the wall of a tapped pipe or into a thermowell
threadolet. A socket weld thermowell is typically welded into a
weldolet socket, but the thermowell may be welded directly into
the pipe wall. A weld in thermowell is welded directly into the
process vessel or piping. A flanged thermowell has a flange
collar which is attached to a mating flange on a pipe nozzle.
What are the components of thermowells?
Typically a thermowell consists of (1) a process connection, (2) shank
construction, (3) a “Q dimension”, (4) bore size, (5) immersion
(“U) length, and (6) lagging extension (“T”) length.
Thermowell process connections:Thermowells are inserted into and
connected into a process in a pressure tight manner. The most
common process connections for thermowells include threaded,
socket weld, and flanged connections.
Thermowell shank construction: The most common shank constructions for
thermowells are (1) straight, (2) step, and (3) tapered. A
straight shank Thermowell is the same size all along the
immersion length of the Thermowell. A step shank Thermowell has
an outer diameter of ½” at the end of the thermowell immersion
length to provide a quicker response time. In a tapered
Thermowell the outside diameter of the Thermowell decreases
gradually along the immersion length of the Thermowell. A heavy
duty tapered thermowell is typically used for high velocity
applications due to the specification of a tapered thermowell
shank in the old ASME PTC 19.3 (1974) thermowell standard.
However, where the nozzle inside diameter is a design constraint
the straight shank thermowell design is often the most resistant
to velocity induced resonance.
Thermowell root dimension (Q): The “Q” dimension of a thermowell is
the thickest part of the shank of the thermowell that is on the
hot side of the process connection or flange. The size of a
thermowell Q dimension is, of course, related to the bore size
of the thermowell and the process connection size.
Thermowell Bore size: The inside diameter of a Thermowell. Standard
Thermowell bore sizes are .260” and .385”. These sizes are
intended to accept a quarter or three eights inch diameter
Thermowell Immersion (“U”) Length: Thermowell immersion lengths are
often called the “U” length. The U length is the measurement of
the Thermowell from the bottom of the process connection to the
tip of the Thermowell. The U length establishes the length of
the Thermowell that is actually in the process being measured.
Thermowell Lagging Extension (“T”) Length: The lagging extension of a
thermowell is often referred to as the thermowell’s “T” length.
The lagging extension or T length is located on the cold side of
the process connection and is usually an extension of the hex
length of the Thermowell. Typically, the T length enables the
probe and thermowell to extend through insulation or walls.
What thermowell criteria are typically important to a thermowell
There are three Thermowell criteria that are particularly important when
selecting a Thermowell: (1) immersion length, (2) potential for
vibration, and (3) material.
Thermowell Immersion Length: Thermowells encase temperature sensors.
It is important to remember that thermowells are meant to assist
in providing reliable temperature measurements. Accordingly,
the Thermowell U dimension is vital to the accuracy of a
temperature reading. Typically, the minimum U dimension of a
Thermowell into a liquid process is a length equal to five times
the outer diameter of the Thermowell. For a process involving
gas or air, the minimum U dimension of a Thermowell is equal to
ten times the outer diameter of the Thermowell.
Potential for vibration of the thermowell: When
thermowells fail, they sometimes fail due to the effects of
vibration. The common source of vibration is the flow of media
in the part of the process where temperature is being measured.
As the media of a process flows by the thermowell it forms a
turbulent wake that causes vibration in proportion to the
diameter of the well and the flow of the fluid. In order to
minimize and avoid vibration, the thermowell must have
sufficient stiffness so that the wake frequency will never equal
the natural frequency of the well itself. Many users prefer a
tapered thermowell design as this design provides greater
stiffness without sacrificing the temperature sensitivity of a
straight thermowell. The ASME PT 19.3TW committee has developed
a standard (ASME/ANSI PTC 19.3TW-2010 Thermowells) that
stipulates the design criteria for thermowells. JMS's free
SwiftyCalc Thermowell Design Software can be used to quickly
evaluate your thermowell design applying the current ASME
Thermowell 19.3TW standard. Helpful reference:
Do Your Thermowells Meet the ASME Standard, Flow Control
Magazine (August 2012)?
Thermowell material: Selection of the proper thermowell material for
the process is also important to preventing thermowell failure.
Thermowell material is usually chosen based upon a consideration
of the temperature of the process into which the thermowell is
being immersed, the corrosion conditions of that process, the
material of construction of the piping, vessel or other
structure into which the thermowell is being installed and the
possibility of erosive conditions. It is important that you
are aware of these factors before selecting a particular