About Coriolis Meters

The Coriolis Effect was discovered by the physicist Gustave Gaspard de Coriolis in the 1830s, and is described as “the inertial force exerted on an object as a result of movement relative to a rotating frame of reference.” This science has been applied to many technologies: hydraulics, machine performance, missiles, ergonomics, ocean and atmospheric circulation and, of course, mass flow metering.

The use of the Coriolis Effect as a technique for liquid and gas mass flow measurement was firmly established over 25 years ago. Since then, a number of different designs of Coriolis Effect meters have been produced.  With the tremendous advances made in electronic signal processing technology in recent years, the application base for Coriolis mass flow meters has expanded dramatically and the term “Coriolis meter” is now intimately associated with high accuracy and reliable performance in a flow measurement instrument.

How Coriolis Meters Work

There are quite a few different Coriolis flow meter designs available. Each has it’s own measurement tube arrangement but all work to essentially the same principle – the Coriolis Effect. Coriolis meters are primarily Mass Flow Meters and they can measure mass very accurately. When used for volumetric measurement, mass measurement is combined with a density value to provide a calculated volume flow rate

Mass Flow Rate Measurement

Coriolis flowmeters work by measuring the amount of twist (or deflection) imposed upon an oscillating tube by the inertia of a fluid flowing through that tube.  The amount of twist imposed upon the tube is in proportion to the mass flowrate of the fluid flowing through. Rheonik Coriolis meters have two tubes that are driven to vibrate in opposition with each other to simulate rotation (first in one direction and then the reverse) although the geometric arrangement of this rotation varies by meter mechanism design.

Electromagnetic sensors are mounted across the gap between the meter tubes, one towards the meter inlet and one towards the meter outlet. As the coils oscillate, these sensors create an electrical sinusoidal output wave.

As a Rheonik Coriolis flow meter oscillates with fluid stationary in the meter tubes (no-flow condition), the inlet side and outlet side sine waves are 180 degrees out of phase with each other.

When fluid is moving through the meter (flowing condition), the tubes twist in proportion to mass flowrate. The extent of this twist is detected by the inlet and outlet sensors in the form of a phase shift (time difference) between the sine waves generated by the two sensors. The time difference created by this phase shift is measured and used to derive the mass flowrate of the fluid.

It could be said that primarily, Coriolis meters are very accurate time measurement devices.  Of course, this is not the end of the story.  As temperature changes, so do the properties of the metal in the mechanism and this changes the relationship between time and mass.  Proprietary algorithms are used to compensate for these changes and keep the measurement accurate and reliable.

Density

Many meters in the Rheonik range are available with process density output as a secondary measurement.  Calculation of density is derived from the resonant oscillation frequency of the meter mechanism plus flowing fluid.  As the density of the fluid becomes heavier, the measured resonant frequency of the meter will become lower and vice versa for a lighter fluid.

Volumetric Flow Rate

Volumetric flow is calculated by multiplying the mass flow rate by the density.

Volume = Mass × Density

The Rheonik Omega Tube Design

The Rheonik Omega Tube meter mechanism consists of three distinct sections:

The Measurement Section

Pickup Coils – provide signals to the transmitter

Measurement Tubes – the part of the meter where the measurement takes place

The Meter Drive System

Mass Bars – provide stability and support to the measurement tube oscillation

Drive Coils – provide power to maintain oscillations at constant amplitude

Torsion Rods – help energize and guide oscillation

The Meter Connection System

In/Out Section – decouples measurement tubes from process line stress and misalignment

Each section plays a specific role in ensuring that the meter performs at a high level when measuring flow. The following information briefly describes the role of the various components within the meter and how they interact to ensure accurate and reliable flow measurement, even in difficult conditions.

The Details

Large Active Measurement Area

The entire top half of the omega loops in the meter are dedicated to the measurement. Oscillating the meter results in large movement of the measurement plane and consequently high signal amplitude that gives unrivaled sensitivity and a highly advantageous signal to noise ratio.   Careful design has ensured that the active measurement section is located away from the meter process connections where line stresses and vibration do not present any problems to the measurement. The semi-circular shape is also highly resistant to deformation from process pressure, a potential source of inaccuracy and instability with some other Coriolis meter designs.

Mass Weight Assisted Movement

Coriolis meters measure the time difference between two electromagnetic sensors attached to their mechanism. Resolution of measurement depends upon there being a significant time difference between the two – i.e. the greater the meter movement, the more significant the time difference seen. One of the features of the Omega Coriolis meter is the mass bar assisted mechanism. These mass bars provide significant weight to the mechanism, forcing it to travel further than more traditional designs and as a consequence, time difference measurements can be an order of magnitude or more greater than other designs. This equates to a much higher resolution of measurement in the meter and this higher resolution means greater confidence in the reported flow rate, even with heavy wall, high pressure tube variants.

Torsional Movement

The torsional movement of the meters in the Rheonik range clearly distinguishes them from other Coriolis meter designs and the patented Omega tube arrangement is the key to the robustness of this measurement technique.

With torsional drive, energy is delivered to the mechanism movement smoothly and continuously rather than abruptly as in “push-pull” drive systems. As the mechanism rotates in one direction, the torsion rod is “loaded” and gently arrests movement until full travel is reached. The mechanism then rotates in the other direction as the torsion rod unloads, gathering momentum as it goes. As it passes its center of rotation, the torsion rod is fully unloaded and starts to load up again. The mechanism begins to slow until its eventual stop at the extent of its “swing”. The cycle then begins again. External power from the drive coils is applied as needed to ensure full travel of the mechanism is always reached.

This continuous drive method limits the effect of external vibration on the meter mechanism as it positively drives through. External vibration is most often in one plane (i.e. horizontal or vertical) and almost never rotational. Consequently, Rheonik meters see very little effect from vibration and give an accurate, stable output where other meters struggle to operate. The drive is mechanically stable – no software processing is needed for noise and drive inconsistency issues.

Yet another benefit of this highly efficient drive system is its exceptionally low power usage. The amount of energy needed to maintain the mechanism oscillation at its optimum level is so low, all Rheonik meters are certified intrinsically safe and can be used in any hazardous area.

Resilience to Installation Issues

The tubes in an Omega Tube Design Meter are deliberately grouped towards the center of the meter body to minimize the effect of any pipeline or connection misalignment on the meter mechanism. The correct geometry of the meter mechanism is critical if the meter is to perform to specifications and any misalignment of the process connections will put stress on the meter body and potentially create a measurement bias by changing the relationship of the measurement tubes to each other. In extreme cases, the meter can be seriously effected.

The Rheonik Omega tube design gathers the tubes towards the center of the meter body so that any external forces caused by pipeline misalignment are minimized or eliminated; the measurement tubes become effectively decoupled from the pipeline and the meter therefore performs as expected. Of course, good installation practices should always be followed when installing any Coriolis meter in line.

Increased Wall Thickness

The unique torsional drive movement of the Omega tube design allows the Rheonik range to have the thickest tube walls of any Coriolis principle meter currently available. This is advantageous in high pressure applications and where corrosion or the effects of abrasion are possible. The rotary oscillation motion eliminates bending forces that might lead to failure at welded or brazed joints; it is these bending forces that are one of the limiting factors on tube wall thickness in conventional Coriolis meter designs. There is no requirement for secondary pressure containment housings on Rheonik meters as internal tubing has sufficient wall thickness to provide an operating pressure rating at least equal to that of the connected pipework. The thicker wall tubes make the meter piping impervious to changing pressure conditions giving the best possible stability of measurement in dynamic conditions.

Omega Tube Design Advantages

Summary

Decoupled Measurement Section

  • Semi-circular shape unaffected by pressure changes
  • Unaffected by pipeline stresses from misalignment

Balanced Torsional Drive System

  • Supports meter mechanism
  • Low energy requirements – intrinsically safe
  • Drives through external vibrations and “noise”
  • Resilience to aeration and flow disturbances

Thick Wall Tube Measurement Tubes

  • Allows high pressure applications – over 20000psi in small meters
  • Abrasion and corrosion allowance
  • No requirement for meter body secondary pressure containment