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General Thermowell Information

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 tube).

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 (4) flanged.

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 ” 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 sensor.

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 user?

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 Thermowell material.

 

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