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. Thermowell immersion length is the total length from the tip to the point of the fluid/media immerse. 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 551. 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. For example an RTD sensor which uses a Pt material that has 100 ohm resistant at 0 oC. 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. If this sensitive length isn’t immersed properly, then there will be a possible external temperature comes in through the un-immersed part and interfere the temperature sensing.
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. But in another part, they also mentioned their recommended immersion length is 60 mm in liquid and 75 mm in gas/vapor service.
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. In the middle of pipe, the fluid/media doesn’t in contact with pipe (external environment) so that the environment temperature not interfere the measurement.
So, what we can do now to determine the immersion length of the thermowell?
1. Ask to the vendor regarding their RTD characteristic such as the recommended immersion length and sensitive length of the probe.
2. Try to fulfill the project requirement regarding the immersion length of the thermowell.
3. Check the wake frequency and stress of the selected thermowell.
We will put the thermowell that served an RTD temperature sensor in the 8” pipe size for gas metering application. The given data from RTD vendor is minimum immersion length = 75 mm (3”) for gas/vapor application. We also have the project requirement that state the immersion length for 4-inch pipe and larger shall be approximately in the midway of the 1/3 to 2/3 pipe ID (Inside Diameter).
Now we will determine the 1/3 to 2/3 pipe ID area first. 1/3 of 8” is equal to 2.7” while 2/3 of 8” is equal to 5.3”. So the thermowell tip shall be laid out in between 2.7” to 5.3” or we can say the immersion length should be minimum 2.7” and maximum 5.3”. The minimum recommended immersion length of RTD from the vendor is 3”. Then if we decide to have 2.7” thermowell immersion length, it will be insufficient. If we specify the immersion length is 3” it will be ok since it laid up in between 2.7” to 5.3” and also it fulfill the vendor recommendation. The best immersion length will be in the middle of pipe which is 4”. Since this 4” is also laid up in between 2.7” to 5.3” and also fulfill the vendor recommendation + the tip (sensitive length of RTD) of the thermowell placed in the middle of the pipe.
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. But if not pass, then the length shall be extended up to maximum 2/3 of pipe ID.
Thermowell Insertion Length
The thermowell immersion length is the sum of thermowell immersion length + pipe connection length (usually uses nipoflange, see below picture for illustration). 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 150 mm or 6 inch in standard. As preliminary design, we can use this 6 inch extension to be summed up with the immersion length.
So in our example above, we can specify the preliminary thermowell insertion length is 6 inch + 4 inch or equal to 10 inch. 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. Since we have a requirement that the thermowell should be laid up from 1/3 to 2/3 of pipe ID, then the 3 inch will be the maximum 2/3 of pipe ID or correspond to 4.5 inch. The 4.5 inch pipe is rarely seen in the market, thus the minimum size of the pipe should be 6 inch for this case. 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. For example if we have a 2 inch pipe and we couldn’t swage it up to 6 inch, then the only option is install it in the 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. Because of the thermowell minimum size, in small pipe the pipe bore will be blocked and cross sectional area reduced dramatically up to 50% or event more. 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. We can use any thermowell length as long as it’s not more than 2/3 of pipe ID or ½ of vessel ID to get a better accuracy.
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. The thermowell tip shouldn’t place very close to such equipment. Another exception is the maximum length that it is recommended to not exceed ½ of vessel ID due to a very long thermowell will give nothing to us except an easy to broke thermowell.
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