Straight Tube Coriolis Flow Meter Working Principle
When a particle located in a tube that rotates with P as a fixed point (rotation center) moves toward or away from the rotation center, an inertial force will be generated. The principle is as follows:
In the figure, the particle with mass δm moves to the right in the pipe at a constant speed υ, while the pipe rotates around a fixed point P with an angular velocity ω.
At this time, this particle will obtain two acceleration components:
1. Normal acceleration αr (centripetal acceleration), its magnitude is equal to ω²r, and its direction is toward point P.
2. The tangential acceleration αt (Coriolis acceleration) has a magnitude equal to 2ωυ and the direction is perpendicular to αr.
The force generated by tangential acceleration is called Coriolis force, and its magnitude is equal to Fc=2ωυδm.
In the figure, the fluid δm=ρA×ΔX,
Therefore the Coriolis force can be expressed as:
ΔFc=2ωυ×δm=2ω×υ×ρ×A×ΔX=2ω×δqm×ΔX
In the formula, A is the internal cross-sectional area of the pipe
δqm=δdm/dt=υρA
For a specific rotating pipe, its frequency characteristics are certain, and ΔFc only depends on δqm. Therefore, mass flow can be measured by directly or indirectly measuring the Coriolis force. Coriolis principle mass flowmeter works according to the above principle.
The actual flow sensor does not implement rotational motion, but pipe vibration instead. The principle is shown in the figure below.
Both ends of a curved pipe are fixed, and a vibration force is applied to the pipe at the middle position of the two fixed points (according to the resonant frequency of the pipe), causing it to vibrate at its natural frequency ω with the fixed point as the axis.
When there is no fluid flow in the pipe, the pipe is only affected by external vibration force, and the two halves of the pipe vibrate in the same direction without phase difference.
When there is fluid flow, it is affected by the Coriolis force Fc of the medium particle flowing in the pipe (the Coriolis forces F1 and F2 in the two halves of the pipe are equal in magnitude and opposite in direction (Figure 1.2)), and the two halves of the pipe press in opposite directions. The direction twists, resulting in a phase difference (Figure 1.3, Figure 1.4). This phase difference is proportional to the mass flow rate.
The design of the sensor is to convert the measurement of the Coriolis force into the measurement of the phase difference on both sides of the vibrating tube. This is the working principle of the Coriolis mass flow meter.
What Is Micro Motion Flow Meter?
Micro motion, a subsidiary of Emerson, was born in Boulder, Colorado, USA. In 1977, Micro Motion truly applied the Coriolis principle to the field of industrial measurement and invented the world’s first practical Coriolis mass flow meter, setting a new standard for fluid measurement.
Designed to meet a variety of industry needs, Coriolis flow meters provide the most accurate, repeatable mass flow measurement for liquids, gases or slurries. Simply put, it uses a Coriolis force measurement principle to directly measure mass, thereby maximizing mass accuracy.
Emerson Micromotion mass flowmeter is the main user of Coriolis, and it is second to none in terms of quantity and stability.
Does a Coriolis flow meter requirements straight run?
Coriolis flow meter installation requirements are not high and there are no special requirements for the upstream and downstream straight pipe sections. This is mainly because the Coriolis flow meter has no Reynolds number limit. They are also insensitive to velocity profile distortions and eddies.
The mass flow meter sensor should be installed so that the sensor flow direction mark is consistent with the fluid flow direction; when measuring liquid flow, the fluid flow direction should be from bottom to top as much as possible. At the same time, avoid installing the instrument at the highest point of the pipeline to prevent the accumulation of pipeline gas from affecting the normal operation of the instrument. .
