Thermowell Insertion and Immersion Length

What is Thermowell?

• A thermowell is an accessory for temperature sensors that consists of a closed end tube that provides mechanical and chemical protection to a temperature sensor that is measuring the temperature of a process.
• Thermowell is an industrial housing device for various temperature elements.
• Thermowell surrounds and safeguards a temperature sensor or elements such as thermocouples, thermistors, bimetal thermometers, or RTD probes against harsh conditions of the process fluid and protects damage from excessive pressure, material velocity, and corrosion.
• Thermowell helps to maintain the coherence of pressure limits within the system.
• Thermowell helps to increase the life span of temperature sensors.
• These thermowell allows and makes it easy to replace temperature sensor or probe without draining the system and minimize the chance of corrosion.
• Thermowell is directly exposed to process fluid or media in the process fluid line. Inside the Thermowell, temperature elements or gauges are installed with necessary fittings.
• Thermowell conveys the heat to the temperature element. Depending upon the type of element used, the temperature of the fluid in the pipeline can be measured easily.
• Thermowell may not be applicable or required in some cases would be bearing RTD, skin type thermocouple, air duct thermometer, etc.
• Thermowell is subjected to vibration when installed into the pipeline.
• This vibration in Thermowell is produced by vortices developed on either side of the Thermowell and leads to premature failure.

• Thermowells are designed and sized according to international standards ASME PTC 19.3 TW – 2016 code to avoid the effect of vibration and damage.
• There are two different terminologies for Thermowell’

1. Thermowell insertion length
2. Thermowell immersion length

Thermowell Insertion length.

• The insertion length of the thermowell is defined as the distance between the connection point, may be flange or threads, to the tip of the thermowell which is the point of fluid or media immersion.
• The insertion length of the Thermowell is represented by the standard symbol “U”.
• The insertion length “U” of the Thermowell must be always greater or equal to the immersion length “R” of a Thermowell.

U > R

• To achieve the highest possible accuracy and efficiency, the insertion length of the Thermowell must be long enough to allow the entire length of the sensitive portion of the probe or measuring device to extend into the fluid medium being measured.
• During liquid temperature measurement using a probe, the probe must extend into the solution the length of the sensitive part plus a minimum of a few millimetres.
• During the temperature measurement of gas or air using a probe, the probe must be immersed in the length of the sensitive segment of that probe plus three inches additional.
• Thermowell having shorter insertion length is used for thermocouples or thermistors having a short sensitive section.
• The sensitive section of various sensors such as bimetal thermometers, RTDs, and liquid-in-glass thermometers lies between 1 inch to 2 inches.
• Thermowell insertion length is not fixed for all but differs from one vendor to another vendor.

In above example the total probe length is 6.5 inches but the insertion length is 4 inches.

Thermowell Immersion length

• The immersion length of a Thermowell is defined as the actual distance between the Thermowell tip and the immersion point in the medium that is being measured.
• The immersion length of the Thermowell is represented by the standard symbol “R”.
• Generally, as per the thumb rule of Thermowell

1. The immersion length must be 10 times the diameter of the Thermowell.
2. Thermowell must be inserted into 11/3 (30%) or 1/3 (60%) depth of the fluid stream.

• Thermowell installed perpendicular to the pipe wall must at least have an immersion length of a minimum of 2 inches.
• These sensors must be immersed at least 2.5 inches in liquid to achieve better acceptable accuracy and efficiency.
• Some vendors define the tip of the Thermowell used for installation must lie between 1/3 (30%) or 1/3 (60%) of the pipe’s internal diameter, or 1/2 (50%) of the vessel’s internal diameter to achieve better temperature accuracy.
• This immersion length depends upon research and experiment performed because in worst cases this immersion is large than the specified sensitive length.

For example, some vendors specify the sensitive length of the sensor say RTD is 40mm, but they recommend immersion length must be 60 mm for liquid and 75 mm for gases or vapor service.

“U” – Immersion depth

“T” – Lag extension

“S” – Dimension of sensor

“B” – Sensitive portion of the sensor