How to Specify a Thermowell ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How to Specify a Thermowell

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Thermowells are available for light duty applications, high pressures, high temperatures and high velocity applications. They are used to meet many general service industry needs and are selected on the basis of pressure, temperature, flow, vibration and corrosion parameters. The basic thermowell types include: threaded, socket weld, weld in, flanged, sanitary and van stone. The threaded type is generally the least costly and most versatile. The schematic of a typical thermowell and all parts required to specify a thermowell for proper selection is shown below:

To properly specify and select a thermowell, the following parts should be well understood and correctly specified:

Process Connection Size
This is the size of the portion of the thermowell that connects with the vessel or pipe. It can be in the form of a thread size, flange size, pipe size, tri-clamp, etc.

Insertion Length
This is that portion of the shank from the process connection (underside of the threads or flanges) to the tip of the shank which is inserted into the process area. It is the portion called “U” in the schematic above. To obtain the most accurate reading with a thermowell, it is recommended that the entire sensitive portion of the bulb be immersed in the process media where temperature measurement is desired.

Lagging Extensions and Installation Wrench Flats
An allowance for lagging extension is specified when the vessel or pipe into which the thermowell is inserted is insulated. This is the extra length between the process connection and the instrument connection of a thermowell, which is determined by the insulation thickness. This is the portion referred to as “T” in our schematic above and could also be used on engineering drawings where applicable. The standard lagging extension allowance is 3 inches. Non standard extension allowance can be supplied by manufacturers if requested.

Shank Configuration
This is the shape of that portion of a thermowell that is inserted into the process. A thermowell shank can be straight, stepped or tapered. The tapered shank is recommended as opposed to a stepped or straight shank due to the superior strength and vibration resistance.

Stepped shank thermowells have a significantly reduced tip diameter improving heat transfer to the sensing element.It makes the sensor more responsive to the changes in the process temperature.

Straight shank thermowells are the original design shank style for
a protection tube or thermowell. This style would be the considered in a low pressure,low velocity application.

Process Connection Type
This is the portion of the thermowell which provides the connection with the vessel or pipe. These connections can be integral male threads, prepared surfaces for welding or brazing, flanges for mechanical pressure seals, etc.

Flanged thermowells can be in three distinct configurations namely flat face flanges, raised face flanges and ring joint flanges.

Flat face flanges are typically used when the mating flange is made from a casting. Flat face flanges should never be mated with a raised face flange.

Raised face flanges are most commonly found in process plant applications. The raised face has a texture to grip the gasket that is placed between the two flanges.

Ring joint flanges are used in high pressure and/or high temperature applications. This flange has a groove that accepts a metal ring gasket which once the bolts are tightened provides a metal to metal seal

Bore Diameter
This is the inside cylindrical diameter of a thermowell which is sized to accommodate the stem or bulb of thermal instruments e.g a bimetallic thermometer. This dimension can be critical as the tolerance should provide easy installation of the instrument as well as a snug fit to minimize thermal lag. Standard bore diameter is 0.260 or 0.385 inches

Material of Construction
This is the material with which the thermowell is constructed. Material selection is commonly based on corrosion resistance and strength. Selection should be compatible with process media, temperature and velocity as well as the material of the vessel or pipe to which it will be installed. Common materials include 316 stainless steel, 304 stainless steel, Monel 400, Hastelloy B, Tantalum and Aluminium 316 SS.

Instrument Insertion Length
This is commonly called the “S” dimension as shown in the schematics above. It is the length from the top of the mounting threads of a thermometer to the end of
its stem.

How to Specify a  Thermowell
A typical specification format for a thermowell is given in the table below:
Specification Description

Process connection size

Typical sizes include – ½’’, ¾’’, 1’’, 1 ¼’’, 1 ½’’, 2’’,3’’, 4’’

Insertion length or U dimension

Typical sizes include – 15/8’’, 2 ½’’, 4 ½’’, 7 ½’’, 10 ½’’, 13 ½’’, 16 ½’’, 19 ½’’, 22 ½’’

Thread size

For threaded thermowells, specify whether it is  ½’’ NPSM or ½’’ NPT

Lagging extension

Is thermowell With or Without lagging extension?

Shank style or configuration

Is thermowell shank to be tapered, straight, stepped ?

Process connection type

Is it NPT thread, Weld in, Flanged, Van stone, Socket-weld, limited space threaded, Ground joint or Sanitary?

Bore diameter

Is thermowell bore diameter 0.260’’ or 0.385’’?

Material of construction

It could be Carbon steel, 304 stainless steel, 316 stainless steel etc

Flange configuration

If using a flange configuration, specify whether it will be – Flat face, Raised face, Ring joint or Lap-joint

Pressure rating

Specify whether thermowell is to be class 150, 300, 600, 1500 or 2500

Special lagging length

Specify whether you require special lagging length for the thermowell

The list above is by no means exhaustive but is comprehensive enough to enable a thermowell to be selected and ordered accurately.