Vortex Flow Meter Operating Principle
Vortex meters, especially the "smart" type are a popular choice in the process industry. Since their introduction by Eastech in 1969, through their adoption by Yogokawa in 1972 and Emmerson Rosemount in 1994 they have become increasing accepted by instrument engineers due to their competitive price, ease of installation, and their ability to handle a wide range of process conditions.
Multivariable vortex meters have an inbuilt RTD temperature sensor and pressure transducer. Using these inputs they can compute mass flow, as well as measuring volumetric flow making them useful for measuring steam and gas flows.
How does a Vortex Flow Meter Work?
A vortex flow meter's working principle exploits the phenomenon of "Kármán vortex streets". A Kármán vortex street is a repeating pattern of swirling vortices, and in a vortex flow meter the rate of pressure oscillations caused by these vortices is measured and correlates to the fluid velocity. A simple calculation using fluid velocity and cross sectional area of the pipe allow the volumetric flow rate to be determined. Pressure and/or temperature compensation can be used to allows mass flow measurements to be made.
The Von Kármán Effect
The Von Kármá effect, states that when a bluff body (i.e. a non streamlined body) is placed in the path of a fast-flowing stream, the fluid alternately separates from the object on its two downstream sides causing a repeating pattern of swirling vortices to be generated, aka a Kármán vortex street.
It is for this reason that this type of flow meter is sometimes referred to as a "vortex shedding flow meter".
Vortex Meter Construction
In a vortex flow meter, the bluff body is known as a "shredder bar". The shedder bar is shaped to allow process fluid to separate and generate vortices around the back side of the shedder bar. Sensors, generally piezoelectric or capacitance-type, located in or just behind the shredder bar are used to detect the pressure oscillation caused by the vortices leaving the shredder bar.
The sensors respond to the pressure oscillation with a signal which has the same frequency as the oscillation pressure, and the frequency of the pressure oscillations is directly proportional to the fluid velocity.
What does a Vortex Meter Look Like?
There are many vortex flow meter manufacturers and each manufacturer's flow meter will look different. However in general a vortex flowmeter will look like one of the three shown below.
Wafer type - the meter is clamped between two flanges in the pipe,
Flanged type - the meter is bolted inline using its own flanges and those in the pipe,
Insertion type - the meter is inserted into the pipe, similar to the way a thermowell is inserted.
Each of the above have an integral transmitter - the blue head - and therefore are often called a vortex flow transmitter.
Vortex Flow Meter Turndown Ratio
The turndown ratio (i.e. the ratio of maximum flow rate to minimum flow rate) of most vortex meters will be somewhere between 10 and 15, depending on the viscosity of the fluid being metered. Vortex flow meters measuring the flow of a high viscosity fluid will have a lower turndown ratio than when measuring flow of a low viscosity fluid.
Vortex Flow Transmitter Applications
Vortex flow transmitters are well suited for a variety of applications and can be used on liquids or gasses. They can also be used over a wide range of temperatures, from cryogenic liquids to superheated steam. They are best suited to continuous flow measurement and not usually recommended for batching or other intermittent flow applications.
Vortex Meter Advantages
Vortex flow meters have no moving parts, therefore unlike some other types of flow meter, e.g. turbine meters they do not require bearings to be lubricated or replaced.
Their ability to be mounted at any angle makes them a popular choice in congested plant locations. See our guide on Vortex Meter Installation for further discussion on best practice for installation.
The permanent pressure loss through a vortex meter is about half that of an orifice plate, i.e. only a few psi. However, the size of meter selected is often "one size down" from that of the pipe e.g. a 4" meter in 6" line, so that a sufficiently high Reynolds number is achieved. This practice of changing flow meter line size can lead to permanent pressure drops of around 10psi.
Vortex Flow Measurement Limitations
Vortex meters do not like low flow, or slow moving fluids. In these situations the Reynolds number may be too low to allow vortices to form. It is for this reason that they are not usually recommended for batching or other intermittent flow applications.
They like relatively clean fluids. Sludge or slurry may coat the shredder bar disrupting the formation of vortices therefore they are not recommended in these applications.
They require straight lengths of pipe both upstream and downstream of the flowmeter to properly characterise the flow. Although the straight length requirements for a vortex meter are lower than required for many flow instruments this still could be prohibitive in some applications.
The cost of a vortex meter will depend on the specification, with pipe size, material of construction, insertion or flanged or wafer, all impacting on cost. However, for smaller sizes the installed cost of vortex meters is competitive with that of orifice meters.
The following pages on Control and Instrumentation.com give more detail on the basics of flow measurement:
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For those who want to read further about the theory of flow measurement and the differing types of flow instrumentation, then the following books from Amazon will be of interest: