The principle of operation for all Vortex Flow Meters rely on the Kármán vortex street (or a von Kármán vortex street), which is a repeating pattern of swirling vortices, caused by a process known as vortex shedding, which is responsible for the unsteady separation of flow of a fluid or gas around blunt bodies. A common visual example of the “Karman” vortex meter principle is clearly illustrated by a flag waving in the wind. As the air passes across the flag pole (The Blunt Body), vortices peel off and the flag is shaped by these pressure areas. You will notice that at low wind velocity the flag will move slowly from side to side. As the wind increases, the flag will start to flutter much faster, representing the increased frequency and intensity of these flag pole generated vortices as they pass by. Wind velocity can thus be determined by measuring the frequency of that flutter.
In all Vortex Flow Meters, a shedder bar or blunt body is inserted into the media (single phase Gas or Liquid). This is typically a solid piece of metal machined in a shape to maximize the strength of the vortex. As this vortex peels off, it temporarily causes a low-pressure area on one side, and then the other creating a frequency. As the fluid or gas velocity increases, the frequency also increases. This frequency is directly proportional to the fluid velocity. This point is where Vortex Flow Meters differ in design and operation. The measurement or pickup technology used to measure these vortices varies. Some manufactures use ultrasonic or pressure sensors flush mounted in the flow stream to pick up swirls or pressure created by the vortex. Others use piezoelectric crystals that sense torsion of the shedder bar as it is twisted by the vortices, and some have even used a wire operating at its own frequency that changes with the vortex frequency sensed. All of these pickup technologies have their advantages and disadvantages, which will not be discussed in this article.
Each Vortex Flow Meter is tested to determine the relationship between velocity/flow rate and Vortex frequency, which results in a meter K-factor expressed in “Pulses/unit volume” (Gallon, Liter, ft3, M3, etc). This volumetric relationship can then be converted to other engineering units and the Vortex Flow Meter converter can then retransmit this information with a 4-20 mA current signal or conditioned pulse. An optional LCD Indicator/Totalizer can display and totalize in whatever engineering units you prefer. For compressible mediums, such as gases and steam, the Vortex Flow Meter amplifier can correct for temperature and pressure with the MASS option, which includes either a temperature element in shedder bar or pressure sensor in the flow body, as well as look-up tables for ideal gases and steam. For mediums other than saturated steam, a pressure and temperature source must be provided. Outputs from the Vortex Flow Meter with the MASS option are compensated and linear for the compressible medium being measured.
There are two basic types of Vortex Flow meters, insertion and spool piece. Insertion Vortex Flow Meters can be used in line sizes as small as 2.0″ and up to 48″ depending on the application, and allow insertion and removal under process pressure. Spool piece Vortex Flow meters, typically wafer (up to 4.0″) and flanged meter bodies, are installed into the line as a section of piping similar to a valve.
Insertion Vortex Meters have a very small cross section measurement area and are very insertion depth critical as they measure a very small portion of flow stream and are very susceptible to flow profiling irregularities and measurement inaccuracies. They also rely on user programmable piping ID (internal Diameter) to calculate accurate cross sectional area values. Since there is generalized piping data pulled from tables with fairly wide tolerance specs, it may add additional inaccuracies outside flow testing data done under controlled environments. The cost of an Insertion Vortex Flow Meter installation becomes very attractive at 8.0″ and above.
Spool piece Vortex Flow Meters measure across the entire flow stream, and while still dependent on proper flow profiling, they are not susceptible to installation or insertion depth issues. Since the spool piece is part of the Vortex Flow Meter and is flow lab tested there are no inaccuracies from spool piece ID or cross sectional area calculations. Cost of installation becomes significant with 8.0″ and above, and may be excessive unless a precise and accurate measurement is required.