What is a Thermowell? Flanged and Threaded Thermowells

What is a Thermowell?

Thermowells, sometimes called pockets or thermo-pockets, are a specialised protection tube or vessel designed to hold and to protect a thermocouple or rtd temperatue sensor against attack from the chemicals or harmful atmosphere of a process by encasing the temperature probe in a protective sheath.

They maintain the integrity of the process and also allow for temperature sensors to be removed for calibration or replacement without needing to drain the system. By providing a protective environment, they also reduce the risks of contamination and degradataion of the sensor thereby improving its longevity.

Styles of Thermowell Sheath

Thermowell sheaths are generally available either straight (parallel), which has the same diameter of sheath along the length of the thermowell. This basic style provides the sensor with protection against corrosion and contamination.
Threaded Thermowell with a Straight Sheath

Stepped thermowells have a larger diameter at the top (near the process connection) which is then stepped down to a smaller diameter near the tip. This allows for a faster response to changes in temperature in the process.
Threaded Thermowell with a Stepped Sheath

Tapered thermowells are robust thermopockets which have a sheath diameter that gradually reduces over its length. As well as providing a fast response to temperature changes, tapered thermowells are often used where the process is running at a high velocity or flow rate as the installation of this design tends to reduce the vibration of the sensor.
Threaded Thermowell with a Tapered Sheath

Process Connection Types for Thermowells

Flanged Thermowells

We have two main types of process connection: either threaded or flanged to be welded. Flanges are manufactured in accordance with internationally recognised standards such as ANSI, DIN or JIS and are available in 1/2", 1+1/2" and 2" diameter with face types of either Flat, Raised, Ring Type Joint or Tri Clamp).

Thermowells with a Thread Connection

Our threaded thermowells are generally available with a 1/2", 3/4" or 1" BSPT process thread, although other thread sizes such as NPT may be specified if required. Socket weld connections are also available.

Insertion Length (Immersion Length)

The immersion length is the distance from the thread (below the thread start for a tapered thread type e.g. BSPT or below the hex for a parallel thread type) or the distance from the flange face to the tip of the thermowell.

If a thermocouple is used, then the length of the thermowell can be shorter than if an rtd sensor is used, due to the impact immersion depth has on temperature measurement for rtd sensors compared to thermocouples which are tip sensitive.

Whichever sensor is used the insertion length must be long enough to allow the temperature sensitive section of the sensor to extend fully into the process being measured.

We can also supply thermowells with a lag extension if required. Our engineeers are on hand to assist you and make recommendations based on a drawing or the type of sensor being used and the temperature range of the process. Further information is available in our Thermowell Datasheet.

Testing and Certification

We offer and range of testing and certification for all our thermowells including: ASME PTC 19.3 TW-2010 (wake frequency calculation), pressure testing, MTC's, X-Ray, dye penetration, full penetration weld and NACE MR0175 (hardness test). Please contact us for further information.