Vortex Flow Meter - Operating Principle
How does a Vortex meter work?
Vortex flow meters exploit 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 allows 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.
The Vortex Meter
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?
Vortex meters are generally look like one of the three shown:
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, similiar to the way a thermowell is inserted.
Vortex flow meters 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 temperature ranges, 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 flowmeters have no moving parts, therefore unlike some other types of 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 can lead to permanent pressure drops of around 10psi.
Vortex meters are readily available from many manufacturers and are available as SIL rated instruments. See our page on Safety Integrity Levels for further information on SIL ratings, and their importance.
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 characterize 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. See our guide on Vortex Meter Installation for further discussion on best practice for installation.
Cost will depend on the specification of the meter 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 Flow Measurement and Vortex Meter Installation:
For those who want to delve further into the world of flow measurement, then the following may be of interest: