Thermowell is provided to protect the temperature sensor from mechanical damage and corrosion.
An extremely sturdy design may increase the life of the sensor but may lead to a poor response. Similarly, a delicate design will have poor life but will improve the response time. Therefore, a proper balance needs to be struck.
For given process parameters, we can arrive at an optimum Thermowell design considering aspects such as temperature, pressure, fluid velocity and corrosion. Such designs will conform to ASTM PTC 19.3.
The Thermowell material can be brass, SS304, SS316, SS316L, SS310, Inconel® 600, Incoloy® 800, Monel®, Hastelloy® depending upon the process parameters and type of fluid.
Types of Thermowells
- Bar Stock Threaded (BT) (Process threads NPT, BSP or Metric)
- Bar Stock Flanged (BF) (Flanges as per ANSI, BS or DIN )
- Bar Stock Weld In (BW)
- Fabricated Threaded (FT)
- Fabricated Flanged (FF)
- Fabricated Weld In (FW)
Barstock Thermowell is normally offered up to an insertion length of 600mm.
Fabricated Thermowells are recommended above 600mm. If required, insertion length can be determined by performing wake frequency calculations, in accordance with PTC 19.3.
Welding (tig welding process) of the Thermowell is performed by professional and approved welders following practice laid down in the ASME code and weld joints can be tested up to 600 kg/cm.
Bore concentricity within 10% of wall thickness can be checked by radiography or ultrasonic method. Special material tests such as an ultrasonic test for flaw detection are also available.
Bar Stock Threaded Thermowell
- Bore in mm (d)
- Cold End Diameter in mm (D2)
- Hot End Diameter in mm (D1)
- Insertion Length in mm (U)
- Tip Thickness in mm (t)
- Extension Length in mm (T)
- Dia of Barstock in mm (D)
Bar Stock Weld-In Thermowell
Bar Stock Flanged Thermowell
Fabricated Weld-In Thermowell
Fabricated Threaded Thermowell
Fabricated Flanged Thermowell
Special Thermowells
Van Stone Type Thermowell
For high-pressure applications and where welding is to be avoided, Van Stone design thermowells are used. These are machined from single bar stock, sandwiched between the nozzle flange and cover (companion) flange.
The OD of the machined portion corresponding to the raised face portion of the flange. Wake frequency calculations (in accordance with PTC 19.3) are performed where ever data is made available and are essential in order to suggest appropriate dimensions of thermowell.
Welding Operation
Thermowells are often with welded design. Welding is the most critical operation as improper welding can cause the failure of welding in process in the form of corrosion & crack. This is particularly serious when thermowell is being used for critical applications such as Hydrogen, H2S or explosive media.
Nozzle suitable for Collar Design Thermowell
Engineers many times face a lot of problems in installing collar design thermowells at the site due to the following reasons:
a) Nozzle ID is inconsistent.
b) Some weld material penetrates inside Nozzle blocking entry of collar.
To solve these problems we supply collar design thermowells along with the matching nozzle. The engineer only has to weld the weldolet at the site.
Thermowell Material Selection Guide
Source: prisma-instruments
Read Next:
- Thermowell Insertion Length
- Calibration of Thermocouple
- Types of Thermocouples
- Compensation of Thermocouple
- Errors of Thermocouple
