Vortex flow meters work by placing a strategic obstruction in the path of the flowing media. This obstruction is commonly called a bluff body. As the gas or liquid passes by, vortices are created. They form on either side and break away in an alternating pattern.
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A good example of this type of behavior in everyday life is a flag that is attached to a flagpole. As the flowing wind contacts the flagpole, the flagpole itself creates the obstruction that causes the flag to billow in the wind in an alternating wave. Another example is a rock in the middle of a river. It is plain to see, via the eddy behind the rock and the subsequent flow, that vortices are being created by the rock.
In the formed vortices, pressure decreases when a vortex is formed and increases when it is shed. This is true on both sides of the bluff body and causes pressure pulsations. The frequency of the pressure pulsations is directly proportional to the flow rate. A sensing element picks up the shifts from side to side behind the bluff body, amplifies the signal, and converts it to a 4-20 mA signal.
I saw a post from Greg McMillan, Tip #11: The Good, the Bad, and the Ugly of Vortex Flowmeters- http://automation.isa.org//01/tip-11-the-good-the-bad-and-the-ugly-of-vortex-flowmeters/
In the post he noted that vortex technology can be an excellent choice for a wide variety of flow measurement applications.
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I think that vortex technology is particularly well-suited for steam applications, where vortex technology simplifies installation and eliminates maintenance issues with impulse lines.
I also think that it is important to note that vortex technology has been greatly improved since the initial development of the technology. For example, the Rosemount Vortex overcomes traditional limitations around plugging and vibration. The Rosemount vortex has a gasket-free meter body and contains no ports or crevices which lead to plugging in other manufacturer's vortex designs. Addtionally, the development of Rosemount's Adaptive Digital Signal Processing virtually eliminates issues with vibration.
On another note, I think the issue with low flow cut-off in vortex technology is overstated. Commonly, the low flow cut-off for a vortex meter can be set as low as 5,000 Reynolds number, nearly the accepted lower limit for turbulent flow. Other technologies, such as differential pressure flowmeters, require turbulent flow for the technology to accurately measure flow. Would you rather have a meter that reported no flow when it was out of its accurate operating range, or have the meter show a flow rate even if it was grossly inaccurate? What do you think?