Whether your industrial process involves challenging environments or everyday conditions, you will likely use a thermowell to prevent your temperature instrumentation from getting damaged. Without protection, it is easy for these sensors to become inoperable.
One challenge field operators face with a thermowell is calculating the stem length to properly fit a thermometer or electronic temperature sensor. The Ashcroft technical team is often asked how this is done, so we created this article as a point of reference for you. Continue reading to learn about the different types of thermowells, factors to consider when selecting the right thermowell for your application, how they connect to the process piping and more.
When you are done reading, you will also find additional resources that may interest you.
Types of Thermowells
Before we show you calculations, let's review what a thermowell is and why it matters. A thermowell is a protective housing for thermometers and sensors. It serves as a barrier between the sensor and the process media, safeguarding sensitive instruments from harsh environments while ensuring accurate temperature measurement.
Ashcroft offers a variety of thermowell styles to match virtually any temperature measurement instrument in your application. Each option is designed to protect thermometer stems and bulbs from corrosive media, system pressure and high-velocity flow, and enables you to remove your temperature instrument without the need to shut down your process.
Key factors to consider when selecting a thermowell.
Typically, the type of thermowell should be selected first to accommodate the application insertion length and to determine the instrument stem length. The right length for a thermowell largely depends on the diameter of the pipe or tube. One rule of thumb is to insert a thermowell anywhere from one-third to two-thirds of the way into the fluid stream.
Here are some other points to consider before fitting your thermowell to a temperature sensor:
1. Process connection type and size: This is the portion of the thermowell that connects with the vessel or pipe. The common types of connections include threaded style, flanged, socket well, Van Stone, weld-in and sanitary options. Make sure you choose the option that best fits your operation.
2. Insertion length (U length): This is the portion of the shank from the process connection (underside of the threads) to the tip of the shank which is inserted into the process area. For the most accurate reading, it is recommended that the entire sensitive portion of the bulb or sensor be immersed in the media. Ashcroft suggests:
- 33% - 66% insertion length of pipe diameter for liquid applications
- 50% - insertion length of pipe diameter for gas applications
3. Lagging length (T length): If applicable, lagging is used when the vessel or pipe where the thermowell will be inserted is insulated. It is the extra length between the process connection and the instrument connection of a thermowell, which is determined by the insulation thickness.
Figure 1. Standard and lagging configuration examples.
4. Sensor stem length: The thermowell stem length must match the temperature sensing device stem length. The best practice is to select the thermowell insertion length, calculate the stem length, and then select the measuring device stem length to match the thermowell stem length.
5. Bore of the thermowell: This is the inside cylindrical diameter of a thermowell designed to fit the stem or bulb of thermal instruments. This dimension is critical as the tolerance should provide easy installation of the instrument while ensuring a snug fit to minimize thermal lag. Typical bore diameters are 0.260˝ or 0.385˝.
6. Thermowell shank configuration: This is the shape of the portion of a thermowell that is inserted into the process. The tapered shank is recommended as opposed to a stepped or straight shank due to its superior strength and vibration resistance.
7. Material: Material selection depends on corrosion resistance and strength. It must be compatible with the process media, temperature, velocity, and the vessel or pipe material. Typical materials for these instruments are 316 stainless steel, 304 stainless steel, brass and carbon steel. If your application requires other materials, contact the factory.
8. Instrument Insertion Length: This is commonly referred to as the “S” dimension for thermometers or the "L" dimension for RTDs and Thermocouples. It is the length from the top of the mounting threads of a thermometer to the end of its stem.
Formulas for matching a threaded thermowell to your bimetal thermometer.
For this article, we are focusing on threaded and flange thermowells since they’re the most widely used. You will see two stem length calculations for a standard thermowell and a thermowell with a lag. The same process can be applied to the other thermowells.
Figure 2: Threaded thermowell and a bimetal thermometer example.
1. Calculation for Threaded Thermowell + Bimetal Thermometer for 8” Pipeline Application Without Insulation (Lag)
In the following application example, we are positioning the thermowell tip at 50% of the diameter or 4”. Lag is not necessary since the pipe is not insulated. The example below shows a threaded thermowell with 5.5” stem length will accommodate a bimetal thermometer with 5.5” stem length.
Figure 3. Threaded thermowell and bimetal thermometer example.
2. Calculation for Threaded Thermowell + Bimetal Thermometer for a 10” Pipeline Application With 3” Insulation (Lag)
In this next application example, we are positioning the thermowell tip at 50% of the diameter or 5”. A 3" lag length is required to expose the instrument connection on the thermowell.
Figure 4. Threaded thermowell with lag and bimetal thermometer example.
Formulas for matching a flanged thermowell to your RTD / thermocouple probe.
Now let's look at some flanged thermowell examples. The calculations below will show a standard flanged thermowell and a flanged thermowell with a lag.
Figure 5: Flanged thermowell and RTD/Thermocouple probe example.
1. Flanged Thermowell + RTD probe for 24” Pipeline Application Without Insulation (Lag)
In this application example, we are positioning the thermowell tip at 50% of the diameter or 12”. Lag is not necessary since the pipe is not insulated. The example below shows a flanged thermowell with 14” stem length to accommodate an RTD with a 14” (356 mm) L dimension.
Figure 6. Flanged thermowell and RTD probe example.
2. Flanged Thermowell + Thermocouple probe for 20” Pipeline Application With 3” Insulation (Lag)
In this application example, we are positioning the thermowell tip at 50% of the diameter or 10”. A lag length is required to expose the instrument connection on the thermowell. The example below shows a flanged thermowell with a 15” stem length to accommodate a thermocouple with a 15” (381 mm) L dimension.
Figure 7: Flanged thermowell with lag and thermocouple probe example.
Ready to learn more?
Calculating the correct thermowell stem length might seem complicated, but when you follow this systematic approach, we think you will be surprised that it is easier than you anticipated. If you still have questions, please reach out to our product experts. Until then, take a look at some other articles that may be of interest to you:
- How temperature is measured
- How Much Do Industrial Bimetal Thermometers Cost?
- When to use an RTD or Thermocouple
Or, download our guide to learn about RTD and Thermocouple Temperature Probes.
