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