How many tubes for a Coriolis flowmeter?

One is good, two is better, but why four?

05/12/2011


Dear Control Engineering: The recent story on large Coriolis flowmeters says the design uses four tubes. I’ve seen two-tube flowmeters, so is using more better? Will six or eight be next? It sounds like those fancy razor blades.

Understanding how Coriolis flowmeter technology works will help explain the concept, but it may not help with your shaving.

Recall if you will that Coriolis technology depends on sending your fluid through a vibrating tube. (Here’s a short tutorial as a refresher.) The mass of the flowing fluid changes the nature of the vibrations which provides data for the flow calculation. Single-tube Coriolis flowmeters have the problem that they are affected by ambient vibrations. The sensor has a hard time telling if it is seeing changes induced by the flow or it’s reading vibrations coming through the floor from that compressor down the aisle.

Adding a second tube helped designers reduce that effect by measuring the difference between the two tubes. This allows the device effectively to cancel ambient vibrations.

The move to four tubes is more complicated, so I put the question to Michael Nuber, who is E+H’s product manager for Coriolis flowmeters in Switzerland. He explains, “There are several advantages of the four tube design over conventional designs. First, it allows for better immunity towards external effects. The two-tube design is already a marked improvement, as one tube always moves in the opposite direction of the other, thus intrinsically stabilizing the measurement system. Each of the tubes is always impacted in the opposite way by external effects like pipeline vibrations.

“Having four tubes adds another dimension. Think of it this way: imagine the two tubes vibrating in one plane, for example along the x-axis. By adding two tubes for a total of four, and using what I call the ‘intelligent mechanical design,’ you also add another plane, the y-axis. That means that the whole measurement system is not only immune to external effects in one plane, but also in another plane that is angled at 90°. The x-y plane then allows for compensating virtually in any direction the external effect might come from.

“Second, having four tubes allows us better use of the circular area. In the same way that a high-performance car engine may have four valves instead of two for each cylinder, it allows us to use smaller measuring tubes. Those allow for reduced wall thicknesses, granting better density accuracy. In addition, the bending stiffness of the smaller tube is lower, either allowing for a higher sensitivity at the same overall length, or the same sensitivity while reducing the length of the sensor.”

Peter Welander, pwelander@cfemedia.com



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