How Vortex Shedding and Fluidic Flowmeters Work
When a fluid passes by an object or obstruction, oscillations can occur. Examples of these oscillations in nature include the whistling caused by wind blowing by the branches of trees, the swirls produced downstream of a rock in a rapidly flowing river, and the waving of a flag in the wind. Note that in all of these examples, when the flow is slowed, the oscillations stop. That is, the whistling stops when the wind dies down, the water flows calmly around the rock when the river is not flowing rapidly, and the flag does not wave in a mild breeze.
Fluidic flowmeters are flowmeters that generate oscillations as a result of flow. Vortex shedding flowmeters use a bluff body obstruction, whereas other fluidic flowmeters include designs based upon the Coanda effect and vortex precession. Increasing flow increases the frequency of oscillation. A sensor detects the oscillations and a transmitter generates a flow measurement signal.
The operating principles and geometries associated with vortex shedding, Coanda effect, and vortex precession flowmeters are markedly different. Vortex shedding flowmeters use a bluff body (shedder) that is typically approximately 20% of the inside diameter of the pipe located to symmetrically traverse the flowing stream to create vortices downstream of the shedder. The vortex frequency is directly proportional to the flow rate of the fluid. The shape of the shedder is typically designed to optimize a particular characteristic of the flowmeter such as manufacturability of the flowmeter, immunity to upstream disturbances, or perhaps improved linearity despite Reynolds number variations. Due to relatively similar shedder widths in the various vortex shedding flowmeter designs, the pressure drop associated with these flowmeters in water service is approximately 300 mbar at 5 m/sec (5 psid at 15 ft/sec).
Coanda effect fluidic flowmeters contain two feedback passages that alternately bring fluid back to the flowmeter inlet so as to alternately direct the flow to attach itself to one of the two internal “walls” of the flowmeter. The frequency that the flow alternates between the feedback passages is directly proportional to the flow rate of the fluid.
Vortex precession fluidic flowmeters impart rotation into the flowing fluid. This rotation causes a (cyclone-like) vortex to form and rotate around the centerline of the pipe (precession). The frequency of vortex precession is directly proportional to the flow rate of the fluid. Vortex precession fluidic flowmeters are relatively insensitive to hydraulic disturbances in the fluid, so they typically have relatively short upstream and downstream straight run requirements.
How to Use Vortex Shedding and Fluidic Flowmeters
Vortex shedding and fluidic flowmeters measure the velocity of liquids, gases and vapors in pipes, such as water, cryogenic liquids, boiler feed water, hydrocarbons, chemicals, air, nitrogen, industrial gases, and steam. Be careful in applications where flow measurement is required near the bottom of the flowmeter range because these flowmeters turn off at low flow rates. The velocity at which these flowmeters turn off is typically 0.3 m/sec (1 ft/sec) for liquids, but is typically higher for gases/vapors because a higher flow of the relatively low density gas/vapor is required to operate the sensing system. In addition, vortex shedding and fluidic flowmeters can become nonlinear and turn off as Reynolds number is reduced.
These flowmeters can be applied to sanitary, relatively clean, and corrosive liquids. Materials of construction are generally limited to stainless steel and Hastelloy C. These flowmeters are available from 0.25 inch to over 12 inches in size. These sizes reflect the availability of sufficient Reynolds number in small pipes, and the electronic limitations associated with low frequency vortices in large pipes.
General applications are found in the water, wastewater, mining, mineral processing, power, pulp and paper, chemical, and petrochemical industries. Mining and mineral process industry applications include process water flows.
Application Cautions for Vortex Shedding and Fluidic Flowmeters
Be careful not to operate the flowmeter below the minimum linear Reynolds number constraint because the measurement will not be accurate. Do not operate vortex shedding and fluidic flowmeters at low velocity or at low Reynolds numbers, because these flowmeters will turn off and measure zero flow.
Piping vibration can cause erratic and unreliable measurements with some vortex shedding flowmeter designs, especially at low flow rates. Be sure to support these flowmeters in a manner that reduces piping vibration. In addition, the flowmeter should fit snugly into the piping. Forcing the piping into place can adversely affect the operation of some designs because abnormal forces can affect the flowmeter sensing system.
In gas/vapor applications, it is advisable to orient the shedder in the horizontal plane so that the shedder is not impacted and potentially damaged by liquid droplets that may be present in the gas/vapor. Not withstanding the above, in high temperature applications, it is advisable to not orient the sensors at the top of the pipe where sensor damage is more likely due to the heat rising from the pipe.