Installed between a diaphragm seal and pressure gauge or transmitter, capillary tubing enables remote monitoring while protecting the instrument from extreme temperatures. Proper design and configuration minimizes the errors that capillary lines can introduce to pressure readings.
A diaphragm seal separates the process medium from the pressure measuring instrument, thereby protecting both from each other. These barriers are essential when the medium is aggressive and would damage the pressure gauge or transmitter, as well as to prevent the contamination of hygienic media.
In most applications, the pressure measuring instrument is attached directly to the diaphragm seal via a threaded, flanged, or sanitary connection. But in certain applications, a capillary tube (also called a capillary line) connects the two components.
Diaphragm seal system with capillary tube
Advantages of Using Capillary Tubes
There are two main reasons to opt for a capillary line rather than a direct process connection.
1. Greater accessibility
When the measurement point is too tight to fit a pressure gauge or transmitter, or if the location is too far or inaccessible for the gauge to be read easily, a fluid-filled capillary line allows the instrument to be installed away from the process connection. This is known as remote mounting.
2. Protection from extreme temperatures
High process temperatures is one of 8 common reasons for gauge failure. A capillary tube acts as a powerful cooling element. As heat travels through the line, it dissipates from the process medium before the pressure reaches the instrument. The longer the tube, the more heat is radiated.
Here is a practical example: As a rule of thumb for standard direct-mounted pressure systems, a diaphragm seal can protect the measuring instrument from temperatures between −40°F (−40°C) and 300°F (~150°C). Adding a heat-sink-type cooling element can boost the assembly’s temperature tolerance up to 500°F (260°C). However, by mounting a flexible capillary line between the diaphragm seal and the measuring instrument, the assembly can withstand temperatures up to 750°F (~400°C).
Note that a capillary connection measurement system requires a mounting kit or mounting bracket for the gauge or transmitter, as the instrument would not be a freestanding unit.
Three Considerations When Using Capillary Lines
Capillary tubes are excellent solutions in extreme conditions and tight and/or remote spaces, but there could be a dip in performance since they introduce variables to the pressure measuring system.
1. Response time
The longer the capillary, the farther the pressure has to travel to reach the instrument, thus increasing the response time.
2. Elevation or gravity effect
When using a capillary, it’s common for the diaphragm seal and the pressure instruments to be at two different levels. This difference in elevation and, thus, gravity can create false positive or false negative pressure readings. However, since the level difference is known, it can be compensated for during the calibration of the diaphragm seal assembly.
3. Temperature effect
Changes in temperature cause the fill liquid inside the capillary tube to either expand or contract. This change in fluid volume creates an error in the pressure reading as a function of the total volume of the entire pressure measuring system. There are two components to the temperature effect errors.
- Seal temperature effect – As the process and/or ambient temperature rises, the capillary fill fluid expands and creates internal pressure that result in false positive readings on the instrument.
Solution: The larger the diaphragm seal, the springier it is – and the more internal pressure it can absorb. Thus, any additional pressure from the expanded fill fluid is absorbed by the softer diaphragm rather than detected by the pressure instrument.
Head temperature effect – As the ambient temperature increases, the capillary fill fluid becomes less dense and less viscous. When the instrument is higher than the diaphragm, gravity can more easily pull the lighter-weight fluid away from the instrument, thus creating a false negative reading (less head pressure). Similarly, when the instrument is lower than the diaphragm, gravity will pull more fluid toward the instrument, thus creating a false positive reading (greater head pressure).
Solution: A smaller capillary inner diameter reduces ambient temperature effects, as there is less fluid volume to affect pressure readings. However, the narrower the inner diameter, the greater the friction of the fill fluid, which can increase the response time.
Proper configuration of the diaphragm seal system can minimize or even eliminate these temperature-related errors.
WIKA USA, Smart in Pressure Sensing
Located near the Houston Ship Channel, WIKA USA’s Diaphragm Seal Division has the expertise to design, install, and calibrate diaphragm seal systems for process industries ranging from oil and gas to food and beverages. Our technical staff and product specialists work with every customer to engineer unique assemblies for maximum precision and responsiveness. We also offer comprehensive repair services, including a 24/7 drop box in Pasadena, Texas. Contact us for more information about diaphragm seal products, services, or warranties.
For more information, contact WIKA USA.