What is the Purpose of Thermowell?

Instrumentation Temperature
1

Thermowell is a solid tube like structure in which Temperature sensing element is inserted to keep it protected from process.

Currently recommended standard to be used for Thermowell wake frequency calculation are ASME PTC 19.3 TW2010

Thermowell

Generally if thermowell fails wake frequency test , meaning there is potential vibration problems following measure are recommended in particular order

  1. Try to use shorter length (but remember to check for minimum emersion criteria so that thermowell s dipped in process for measurement all time for continuous and correct reading)

  2. Increase shank diameter:- Since in wake frequency calculation depends upon diameter and decreases with increase in diameter. But do remember it will slightly increase the sensing time with thickness of thermowell.

  3. To install at elbow:- Since installing at elbow with thermowell Tip facing flow reduces the impact area and the impact area reduce to just the Tip of thermowell so no wake frequency calculation required.

  4. Use welded Thermowell:- Welded thermowell has more strength at support and reduces chances of vibration

  5. Increase flange size:- Increases strength at support.

  6. Use of Collar :- Use of collar shall be avoided as creates problem while installing thermowell to get collar exactly of diameter of nozzle inside diameter is difficult as if it is small it fail it purpose and if slightly big then thermowell wont insert and needed to be sanded rubbed at site.

Precaution while using Thermowell for dual phase service

  • If the content of one of the phase of fluid is less than 5 % it could be ignored

  • If content of any one of service is more than 5 % then the wake frequency calculation should be performed for both of the phase separately and accordingly thermowell needs to be selected.

Thermowell material needs to be selected as per service properties specialy if service is corrosive, erosive, or NACE service.

Pressure Temperature ratings need to be taken care for process connection and Flanged Connection

Generally recommended practice is

  • For Low Pressure/Temperature and service like Utilities and Water Screwed connection

  • For Medium Temperature Pressure service Flanged connection

  • For High Temperature Pressure welded connection is recommended

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