Temperature Measurement Thermowell Insertion and Immersion Length There are two different terminologies for thermowell, one is insertion length, and another one is immersion length. Thermowell insertion length is the total unsupported length from the edge of connection flange or threaded up to thermowell tip. Usually, for easiness, the border of the media that will immerse is the pipe or vessel wall see below picture for illustration. The problem is how we should specify the insertion length and the immersion length of this thermowell? Until this article being written, there is no specific requirement regarding the insertion length and immersion length of thermowell in relationship with pipe size.
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Temperature Measurement Thermowell Insertion and Immersion Length There are two different terminologies for thermowell, one is insertion length, and another one is immersion length. Thermowell insertion length is the total unsupported length from the edge of connection flange or threaded up to thermowell tip. Usually, for easiness, the border of the media that will immerse is the pipe or vessel wall see below picture for illustration.
The problem is how we should specify the insertion length and the immersion length of this thermowell? Until this article being written, there is no specific requirement regarding the insertion length and immersion length of thermowell in relationship with pipe size.
The only code and standard that specify the length criteria is only API But it only specify the minimum immersion length and the maximum distance between thermowell tip and the opposite pipe wall. But this minimum length should be questioned since after several checks it differs from one vendor recommendation to others.
Thermowell Immersion Length The idea behind the immersion length is the temperature sensor characteristic that will be used in the thermowell. Every temperature sensor has it owns characteristic and may vary from one vendor to others.
The RTD sensor has a probe to protect and isolate the Pt material from external environment. It also has some inch length of Pt material inside the probe. This length is called a sensitive length since in this length the Pt was placed. To get a reasonably accuracy this sensitive length must be completely immersed in the fluid or media that will be measured.
From this consideration, each vendor will have their own data regarding the sensitive length of the sensor both RTD and Thermocouple and also the minimum immersion length. Usually this immersion length is based on their research and experience. Because some times this immersion length is bigger than the specified sensitive length. For example some vendor of temperature sensor published catalogues that specify the sensitive length of their RTD is 40 mm. Another consideration is the question: where we must put the sensitive length of an RTD?
Of course it best to have an RTD sensitive length in the middle of pipe. So, what we can do now to determine the immersion length of the thermowell? Ask to the vendor regarding their RTD characteristic such as the recommended immersion length and sensitive length of the probe. Try to fulfill the project requirement regarding the immersion length of the thermowell. Check the wake frequency and stress of the selected thermowell.
So the thermowell tip shall be laid out in between 2. Then if we decide to have 2. Note: the preliminary thermowell immersion length shall be subject to thermowell wake frequency and stress calculation. If the selected length passes this calculation, then the dimension is ok. After we determine the immersion length of the thermowell, the last step is determining this pipe connection length to get the insertion length. In general, the standard length of the nipoflange available in the market is mm or 6 inch in standard.
As preliminary design, we can use this 6 inch extension to be summed up with the immersion length. Please note again that this thermowell insertion length is subject to pass the wake frequency and stress calculation. Minimum pipe size for thermowell Minimum pipe size for proper thermowell installation should be determined from the minimum immersion length of the RTD.
From our example above, the minimum immersion length recommend by the vendor is 3 inch. The 4. If the pipe is lower than 6 inch, it should be swaged up to 6 inch or as an alternative we can install the thermowell at pipe elbow. At elbow, we can install 3 inch thermowell at 2 inch pipe without any problem see below installation illustration. Other consideration of the prohibition of thermowell installed at small pipe is due to a blocked pipe bore.
So, for small pipe application, install the thermowell at elbow is the best way. Maximum length of thermowell Until now, there are no any codes and standard that limits the thermowell length except the manufacturer capabilities, wake frequency and stress calculation, and also project specification.
Special consideration should be given to the thermowell install at vessel. We can also consider the vessel just like the pipe but in a big size. The rule to select the immersion and insertion length is still the same but there are a few exceptions. The equipment and other device inside the vessel shall be studied carefully such as heater, mixer etc.
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In the process automation two measurement principles for temperature have asserted themselves as a standard RTD - Resistance temperature detectors: Measuring principle The RTD sensor changes the electrical resistance with a change in temperature. The sensor element most frequently used is a Pt TC - Thermocouples: Measuring principle A thermocouple is a component made of two different metals connected with each other at one end. An electrical potential thermoelectric force is caused due to the Seebeck effect at the open end if the connection and the free ends are exposed to different temperatures.
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Threaded Thermowells – Tapered