Vortex shedding meters, how they work

The principal of vortex shedding can be seen in the curling motion of a flag waving in the breeze, or the eddies created by a fast moving stream as it flows around a rock or piling. The flag outlines the shape of the vortices created by the flag pole. In the water the eddy shows the forming of similar low pressure pockets downstream of the rock. Von Karman produced a formula describing the phenomena in 1911, and in the late 1960's the first vortex shedding meters appeared on the market.

When any fluid flows past an obstruction, downstream turbulence occurs. At low fluid velocities the turbulence is minor and the flow simply separates and recombines on the downstream side of the obstruction. As the fluid velocity increases the momentum of the stream prevents it from smoothly recombining downstream of the obstruction. This creates a low pressure region on the back side of the obstruction, resulting in a inward flow of the fluid. As the velocity increases, this effect is heightened with a increasing low pressure pocket causing the fluid to be pulled backwards in a swirling motion. As this occurs it relieves the low pressure pocket, leaving the swirl free to drift downstream. As this swirl separates it leaves a low pressure pocket on the opposite side of the obstruction, which causes the action to repeat, but this time the swirl is released from the opposite side. The result is series of swirls coming from opposite sides of the obstruction.

To put this into commonly used terminology, the swirls are "vortices", being "shed" from a "bluff body" (also known as the "shedder bar" or "shedder strut"). The center of the vortex path is called the von Karman Street.

The frequency of the vortices is directly proportional to the fluid velocity and is essentially linear. A drop off occurs at low fluid velocities, which may be linearized using electronics.


When to use the vortex meter, basic rules.

A. Gases need to be dense enough to allow the development of a strong enough pressure pulse to allow the pulse to be distinguished against background pipe noise. Helium and hydrogen are problem gases.

B. Fluids which have high viscosity's may not allow the vortex to form properly. The rule of thumb is to stay under 20 cP.

C. The meter is best on gases and fluids. This meter is excellent for dirty gases which can foul turbine meter bearings.

D. Consider this meter design for liquids, gases and steam. On gas/steam service our Endress & Hauser meter is suitable for flows to 250 FPS, on liquids it is good to 30 FPS.

E. As long as your fluid is within the limits of usage, the meter is not affected by changes in viscosity and density.

F. The Endress & Hauser meter creates low pressure losses.

G. It is assumed that you are using Sch. 40 steel pipe. If not, please advise the pipe and ID when asking for a quotation.


Installation and Maintenance

a. The meter may be installed in any position.

b. The meter works best with higher flow rates. It is common to buy a meter which is 1 line size under in order to increase fluid velocities to optimize the meter's operation. Straight piping ahead of the meter is required. Allow 15 diameters after a reducer and 20 diameters after a elbow. Allow 5 diameters downstream of the meter. Vortex meters can be overranged without damage.

c. The operation of a Vortex meter is based on it's mechanical shape and dimensions. There is no mechanical maintenance to be performed as long as physical damage has not occurred to the strut (shedder bar).

d. Do not allow piping gaskets to protrude into the pipe line ahead of the vortex meter. Protruding gaskets will create hydraulic noise which interferes with meter performance.