Choosing the Right Flow Meter – Vortex

-Robert Roda, Danbury, CT

When it comes to choosing a flow meter engineers are spoiled with choices, making selection of the appropriate meter a daunting task simply due to the sheer number of possible types of measurement devices: differential-pressure, thermal mass, turbine, and vortex meters being among the most common. Vortex meters, however, offer some unique advantages and should be considered if they fit your needs.

A vortex meter works through the mechanism of vortex shedding: a bluff body (or shedder bar) is inserted into the flow stream causing vortices of alternating direction to form in the process flow. The rate at which these vortices form is proportional to the velocity of the fluid: as fluid velocity increases, the frequency of vortex formation increases.

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Immediately downstream of the shedder bar is a pressure sensor that measures the frequency of the vortex formation as a series of pressure pulses. These pressure pulses are used to calculate the volumetric flow rate of the fluid. For applications requiring mass flow measurement, temperature and/or pressure measurement can be integrated into the vortex meter to calculate mass flow.

Vortex meters offer many advantages over other technologies: They have high accuracy over a wide range with exceptional turndown, some manufacturers offer 20:1 or better turndown ratio; vortex meters have no moving parts so suffer from significantly less wear than other meters, particularly turbine meters, reducing maintenance and failure rates; there are no impulse lines or manifolds required allowing for easier, lower-cost installation. In addition, no impulse lines means no draining, filling, venting, or more importantly leaking!

There are applications for which this technology is not a good fit. Vortex meters require ample straight pipe both upstream and downstream of the meters, sometimes as high as twenty pipe-diameters upstream and ten downstream depending on manufacturer. Pipe size can be limiting as well, as Vortex meters will not be appropriate for small (less than ½ inch) or large (greater than 12 inch) pipe. Additionally, Vortex meters require a high velocity fluid with a Reynolds number above approximately 100, and as such may not be suitable for very low flow rates or fluids with high viscosity.

If there is enough straight pipe distance with a fast-moving liquid or gas across a wide range of temperature and pressure, then a Vortex meter may be a perfect fit. If you are choosing a flow meter and value low maintenance, low-cost installation, high reliability, and high turndown over a wide flow range, consider a Vortex meter.

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