Software allows designers of single straight-tube meters to ensure the response of the Coriolis signal is not affected by changes in the density or mounting options. The sensor element, which is relatively immune to environmental changes, takes into account that the Coriolis forces are changing with fluid density at the appropriate vibration levels.

Each design has merit

There are similarities between dual, curved-tube and single, straight-tube meters, but also some significant differences. The key differences are how they behave to mounting effects, fluid density changes, and temperature changes.

The dual curved-tube meter is much more immune to meter-mounting variations. The two tubes are essentially identical. When they vibrate in opposition to each other, they are extremely well-balanced. All the energy is contained within these two tube sets, and nothing is transmitted out to the environment. The tubes are "unaware" of the environment, because everything is maintained within the sensing elements themselves.

The same attributes make the dual curved-tube meter reliable despite fluid density changes. Because these two tubes are identical, when the fluid density changes within them, they respond identically.

These tubes extend from the pipeline and are relatively free to expand. Therefore, as the temperature changes, they grow in length, unrestrained, with no stresses or loads generated through expansion. A single measurement compensates for the effect of temperature on the modulus of the material.

In contrast, single straight-tube meter mounting presents challenges. The single-tube system was designed to be symmetric from inlet to outlet, but during vibration, the flow tube and reference tube don't vibrate to the same extent, so the single-tube meter is not inherently balanced. The reference part was designed to maintain that balance, but it responds to vibration as if no vibration leaked out the ends.

As fluid density changes, it only changes in a single tube, not in the reference tube. A great deal of research was invested to understand the dynamics and make these meters stable as fluid density changes. The design of the reference tube is vital for stable vibrations throughout the entire density range.

The most challenging design aspect of single straight-tube meters is dealing with temperature changes. Its case is fairly rigid, so it holds pipeline loads. It also holds the tube very rigidly. A fluid temperature change will change the tube, but the case will not allow it to expand.

Best use

The differences between curved tubes and straight tubes will impact user applications. Every plant or facility has a variety of needs for applications and measurement points, which often require significantly different specifications. One example would be a material balance, in which many different streams are combined. The output should be very consistent, but the error tends to increase as more streams are added together. In this case, users will want very high accuracy, because the errors will multiply throughout the measurement points.

To solve this problem, the user could choose a dual curved-tube meter because of its ability to deliver highly accurate and repeatable measurements. When blending or combining different materials, if operators are changing the amounts of materials that they are combining, they might add them at different flow rates. The dual curved-tube meter's increased accuracy also gives it a higher rangeability, from 100 percent of flow to 2 or 3 percent of flow, with very little degradation of the measurement.

In some cases, when operators are just blending and filling, a mass measurement is required but it doesn't need to have the same accuracy as in the material balance situation.

Despite the dual meter's superiority, some users will want to select the single straight-tube meter. If a volumetric meter is being replaced, there might be a limited amount of space to install a new meter between closely spaced pipe flanges. The compactness of the straight tube might make it preferable. The straight tube is also easy to make highly sanitary. 

 

Chuck Stack is director of sensor technology for Micro Motion, Boulder, Colorado.