How Coriolis Flow Meters Work
Functions of Some of the Most Accurate Flow Meters Available
Coriolis flow meters are one of the most, if not the most, accurate flow meters on the market, but how is this achieved? They are used in critical situations such as custody transfer as well as general process control. The operating principle of Coriolis flow meters is extremely simple, subject a moving flow to a force at ninety degrees to its path of movement and it will be deflected. This deflection will be proportional to the mass and velocity of the flow movement. If the moving flow is fluid inside a pipe you will be able to determine the mass flow rate of that fluid. Unfortunately the problem comes with how to make practical & accurate Coriolis meters based upon this principle.
Early Coriolis flow meters used a pair of looped tubes which were vibrated. As the fluid in such devices flows the vibration induces acceleration and deceleration of the fluid in the loops. This induces a differential vibration in the upstream and downstream arms of the flowmeter. Sensors measure the phase difference between these two arms and this phase difference is a measure of the mass flow. Some manufacturers still use this technique with great success.
Coriolis Flow Meters: Sensors positioned at positions A & B will have the outputs shown below
Coriolis Flow Sensors & Accurate Flow Meters
Measuring the mass flow is fine if you know the density of the fluid. However if the density is unknown or varies you may have another problem. Fortunately the natural frequency of the vibrating tubes is dependent upon the tubes mass and this will vary with fluid density so determining the density can also be achieved by measuring the natural frequency of the tubes. You now have both mass and volumetric measurement from a single sensor. One of the problems with this arrangement is the increased pressure drop because of the loop.
Another version of these Coriolis flow meters uses a straight tube. In this design the straight tube is vibrated in the middle so the flow on the upstream and downstream arms of the flowmeter are still subjected to differential accelerations causing the tube to “twist” with mass flow. This is a more compact system which often makes it a more popular solution.
The black line in the diagram above depicts the flexible part of the tube at one extreme of its vibration, the grey line is the other extreme. The red and blue lines show the tube distortion due to flow in the centrally vibrated tube. This twisting of the tube is seen differently at sensors A & B. The output sine waves are virtually identical to the twin tube system.
Unfortunately designing flowmeters with all this inherent technology is costly, Coriolis flow meters are consequently not cheap but do fulfill certain needs across a wide range of industries requiring accurate flow meters & flow system design.
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