Mass flow meters are the types of flow meters mainly used in mass-related processes which accurate flow measurement is the prerequisite. Coriolis mass flow meter is the most widely used mass flow meter in the market, based on Coriolis effect. Coriolis flow meters are considered accurate mass meters because they directly measure mass flow. Mass does not change, no adjustment is necessary to vary the characteristics of the fluid, therefore, the Coriolis counter works linearly.
The Coriolis Principle
G.G. Coriolis, a French engineer, who was the first to notice that all bodies moving on the Earth’s surface tend to move sideways due to the planet’s eastward rotation. In the northern hemisphere, the deviation is to the right of the motion; in the southern hemisphere, the deviation is to the left. This drift plays a major role in both ocean tidal activity and global weather. As a point on the equator draws a larger circle than a point closer to the poles per day, a body travels towards one or the other. The pole will point east because it maintains its highest rotational speed (eastward) as it passes over the Earth’s slower-rotating surface. This drift is called the Coriolis force.
When a fluid circulates in a tube and undergoes a Coriolis acceleration by mechanical introduction of an apparent rotation in the tube, the amount of deflection force generated by the Coriolis inertia effect will be a function of the mass flow rate of the fluid. If a pipe is rotated around a point while the liquid is flowing through it (toward or away from the center of rotation), this fluid will generate an inertial force which will be perpendicular to the direction of flow.
The most common type of units are u-shaped flow tube, sensor assembly, and an electronics unit. In this metering unit, the liquid passes through a U-shaped tube which vibrates in an angular harmonic oscillation. Coriolis forces deform the tube and a different vibration component is added to the already oscillating tube. This additional vibration element causes a phase shift or twist in some parts of the pipes. This resulting phase shift directly proportional to the mass flow of the liquid is measured using sensors. Finally, this measured information is then transferred to the electronic unit, where it is transformed into a voltage proportional to the mass flow.
A Coriolis flow meter is shown in the following figure.
Principle of Operation
The flow is guided through the U-shaped tube. When an exciting force is applied to the tube, it will cause vibration. Then the fluid flowing through the tube will twist the tube due to the Coriolis acceleration acting in opposite directions on either side of the applied force. This action causes the tube to twist. As the tube descends during the second half of the vibration cycle, it twists in the opposite direction. This twist results in a phase difference (delay) between the input side and the output side, and this phase difference is directly affected by the mass passing through the tube.
An advantage of Coriolis flowmeters is that it directly measures the mass flow rate, eliminating the need to compensate for changes in temperature, viscosity and pressure conditions.
A new single straight tube design is available for measuring some dirty and abrasive liquids that could clog the older U-shaped design.
Fc = 2 * m * w * V
m = mass (kg)
w = angular velocity (rad/s) V = velocity (m/s)
‘U’ and ‘Ω’ shaped coriolis meters cause pressure drop as the flow increases, resulting in permanent pressure loss. As a result, liquids near boiling point can introduce cavitation when the pressure across the meter drops below the vapor pressure of the fluid. The bubbles collapse when the pressure returns above vapor pressure.
Cavitation will cause the instrument to malfunction.
The tube cross section is usually smaller than the line tube cross section to increase velocity (v) to create higher Coriolis forces. The latter also applies to Coriolis meters with straight measuring tubes. Therefore, the pressure drop will increase accordingly.
Coriolis mass flow meters will accurately measure mass flow, true density, and the nature of the fluid as long as the flow is single-phase and the liquid is homogeneous. They are often accurate for custody transfer and critical reactor feed (ratio) control and also in streams where significant variation in a fluid composition will occur, which otherwise could not be measured.
A weak point is their requirement for zero adjustment under operating conditions. Zero adjustments are easily disturbed by the meter’s not entirely stress-free installation. The importance of zero adjustments is also demonstrated in the formula for the uncertainty in the reading of the meter, expressed as:
+/- [(zero stability/ flow rate)*100]% +/- x%
Where x varies between 0.1 and 0.3 depending on meter design.
BCST Group, founded in 2000, specializes in the research, development, and production of valve automation and process instruments. The range of our products includes low-cost cast models for easy installation to high-end units capable of withstanding both high temperatures and high pressures. BCST can provide you with the tools you need to revolutionize your business processes and get more done.
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