How Water Vortex Shedding 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. Vortex shedding flowmeters use a bluff body obstruction. Increasing flow increases the frequency of oscillation. A sensor detects the oscillations and an electronic transmitter generates a flow measurement signal. 

How to Use Vortex Shedding Flowmeters

Vortex shedding flowmeters measure the velocity of liquids such as water, chemicals and corrosives in pipes. 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 1 ft/sec for liquids. In addition, vortex shedding can become nonlinear and turn off as Reynolds number is reduced.

These flowmeters can be applied relatively clean, and corrosive liquids and will tolerate a reasonable amount of particulates. Materials of construction are generally limited to brass, stainless steel or plastics. These flowmeters are available from 0.25 inch to over 6 inches in size. 

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.