How to select correct thermowell for temperature measurement application?


#1

Selection Of Thermowell

You need to find out answers for 3 questions for selecting correct thermowell .

1.What type of connection you are making to the process ?

Most common type of thermowell connections are

a)Threaded
Applications of Threaded Thermowell-
It is used where frequent removal is not required.
It is commonly used in small pipes or vessels and Non-Corrosive media.

b)Flanged
It is used applications where frequent removal is required.
Common applications - Large Pipes,High Pressure,High Corrosion

c)Socket Welded
These thermowells are welded directly to pipe or tank and it provide a high quality connection.
Since they are welded it cannot be removed easily.It should be used only where access is not required and corrosion is not an issue.
Common application - High Temperature,High Pressure,Non-corrosive media

d)Sanitary
These thermowell has a smooth surface which is easy for cleaning.This prevents contamination of the process.
Common application - Diary,Food processing,Pharmaceuticals
Thermowell Connection type -Threaded-Flanged-Welded-Sanitary

2.What is the best material of construction for the application?

Consider the following factors for selecting the correct material.

a) Temperature Limits of the material

Temperature Limits of Thermowell Material

b)Chemical Compatibility with the media

Thermowell Materials Used

c)Compatibility with the process piping material

3.What is the best stem profile for the application?

a) Tapered
Tapered Stem profile have good response time and good strength.
Consideration - High Drag Force

b) Straight
Advantages - High Strength
Disadvantages - Slow response time,High Drag Force

c) Stepped
Advantages - Low Drag Force,Fast Response Time,
Disadvantages - Lower Strength,

Thermowell Stem Profile Types

Additional Consideration for thermowell Failures
1.High Drag Force
2.Excessive Static Pressure
3.High Temperature
4.Corrosion
5.Fluid-induced vibration