Developing Coriolis with a difference

The dual curved tube Coriolis meter has solved many of the problems that plagued early models but this design does not meet the needs of all users. In some applications, process considerations demand a single straight tube meter

Customer expectations and design requirements for single straight tube Coriolis meters are the same as for curved tube meters. In both cases, the meter must have no special installation requirements and it must perform accurately over a wide range of process conditions. But the geometry and configuration for the single straight tube and the dual curved tube Coriolis meters are very different and as a result the technical challenges are also different.

Much of the field robustness of dual, curved tube Coriolis meters is inherent in the design. The meter must have no special installation demands and it is important that there is symmetry of design, both from inlet-to-outlet and from tube-to-tube. Under normal operating conditions the tubes are driven in opposition to each other at resonance by a coil and magnet located at the tubes’ midpoints. The coil and magnet velocity sensors are located equidistant upstream and downstream from the tube midpoints to measure relative velocity between the two tubes.

The meter symmetry acts as a mechanical filter, rejecting motion that is common to both tubes, such as externally induced vibration. Furthermore, since the two tubes are essentially identical, change in fluid density does not affect either symmetry or mechanical filtering. Also consider the case where a mechanical load, such as a force or a moment, is applied by the pipeline to the flanges of the sensor. These loads have relatively little influence on vibration of the tubes and hence also little effect on the meter performance. The fact that the tubes are curved has another advantage: the tubes are relatively free to expand with changes in fluid temperature. As a result, change in fluid temperature does not impart stress into the tubes and therefore has minimal effect on performance.

The straight tube design is symmetric from inlet-to-outlet and about the plane of vibration. The vibration itself, however, is not symmetric. The flow tube and the concentric reference tube vibrate in opposition to each other as shown. This lack of symmetry in the plane of vibration means that the benefits of mechanical filtering will not be as readily achievable as for curved tubes. Design of the single, straight tube Coriolis meters must provide mechanical filtering despite the design’s inherent lack of symmetry.

Another important difference between straight tube and curved tube designs is that in the former case the measuring tube is coaxial with the process pipeline. As a result, pipeline loads can be transferred directly into the measuring tube. To prevent this from affecting measurement performance, a rigid case is placed around the flow tube to carry pipeline load. The rigid case, however, can in turn introduce problems itself when fluid temperature changes. The case constrains the expansion and contraction of the tube due to temperature changes. This gives rise to thermal stresses in the flow tube, which can negatively influence both meter performance and reliability. The design of a single, straight tube Coriolis meter must effectively accommodate temperature changes in order to be acceptable for use under even relatively mild field conditions.

In today’s world, the kind of structural design required can be accurately and comprehensively modelled on computers. Most of the major characteristics of Coriolis meters can be analysed, predicted, and optimised before units are ever built, and such a modelling approach is the one Micro Motion pursued in the development of a single, straight tube Coriolis meter. This work specifically addressed both the necessary matter of achieving good reference performance and the equally important and more difficult problem of developing a design that would be robust when exposed to real-world field conditions.