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General Thermowell Information |
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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. [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?
The most common types of thermowells are (1) threaded, (2) socket weld, and
(3) flanged welded. Thermowells are classified according to their connection to a process. For example, a threaded Thermowell is
screwed into the process. A socket weld Thermowell is welded into a weldalet and a weld in Thermowell is welded directly into the
process. A flanged Thermowell has a flange collar which is attached to a mating flange.
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.
1. 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.
2. 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.
3. Q Dimension: 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.
4. 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 sensor.
5. 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.
6. 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 user?
There are three Thermowell criteria that are particularly
important when selecting a Thermowell: (1) immersion length, (2) potential for vibration, and (3) material.
1. 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.
2. 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 19.3 committee has developed a standard that
stipulates the design criteria for thermowells.
3. 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 and the possibility of erosive conditions. It is important that you are aware
of these conditions before selecting a particular Thermowell material. |
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