Review of Vortex Shedding and Fluidic Flowmeters

An excerpt from the report ‘The Consumer Guide to Vortex Shedding and Fluidic Flowmeters’, published by Spitzer and Boyes, reviews the techniques and technology behind these meters.

Vortex shedding and other fluidic effects are oscillations that occur when fluids pass by an object or obstruction.

Examples of these effects in nature include the whistling caused by wind blowing by the branches of trees, the swirls produced downstream of a rock in a rapidly flowing river, and the waving of a flag the wind.

Note that in all of these examples, when the flow is slowed, the phenomenon ceases.

That is, the whistling stops when the wind dies down, the water flows calmly around the rock when the river is not flowing rapidly, and the flag does not wave in a mild breeze.

Fluidic flowmeters are a class of flowmeters that generate oscillations as a result of flow.

The number of oscillations can be related to the rate of flow passing through the flowmeter.

Vortex shedding flowmeters are a specific type of fluidic flowmeter.

Other fluidic flowmeters include designs based upon the Coanda effect and vortex precession.

Vortex shedding flowmeters present the flow in a pipe with an obstruction in the general shape of a bluff body or strut.

At low flow rates, the fluid simply goes around the bluff body (or strut).

As velocity increases, alternate vortices are formed (shed) on each side of the bluff body (or strut) and travel downstream.

The number of vortices formed is proportional to the velocity of the fluid, such that doubling the flow will form twice as many vortices.

A variety of electronic and mechanical techniques can be used to sense the vortices.

The frequency of vortex formation is used to generate a flow measurement signal.

Bluff body vortex shedding flowmeter designs have shedder bars that have a width of approximately 20percent of the inside diameter of the pipe.

As a result, the pressure drops associated with these designs are similar.

It is advisable to use supplier information to determine the actual pressure drop across these flowmeters.

For estimation purposes, one rule of thumb is that the pressure drop from a water flow at 5 meters per second is approximately 400mbar differential (or approximately 15 feet per second and 5 pounds per square inch respectively).

Pressure drop varies as the square of the flow rate such that doubling the flow will result in four times the DP, differential pressure, across the flowmeter.

The relatively thin strut shedder designs reduce the loss of hydraulic energy across the flowmeter (pressure drop).

Reducing the pressure drop across the flowmeter can conserve hydraulic energy in some applications, such as when a pump or fan is controlled with a variable speed drive.

Note that, in many installations, installing a flowmeter with a lower pressure drop in place of a flowmeter with a higher pressure drop can cause the pressure drop to be transferred from the flowmeter to the control valve, and result in no energy savings.

Coanda effect fluidic flowmeters contain passages or other hydraulic mechanisms that allow a portion of the downstream fluid to be fed back near the inlet of its fluidic oscillator.

By impacting the incoming fluid, the feedback flow causes the main flow to preferentially attach itself to the opposite surface of the flowmeter.

This increases the opposite feedback flow and forces the main flow away from that surface.

This process repeats and causes flow in the feedback passages to oscillate in proportion to flow, such that doubling the flow will create twice as many oscillations.

A variety of electronic and mechanical techniques can be used to sense the feedback flow oscillations.

The frequency of feedback flow changes is used to generate a flow measurement signal.

In vortex precession fluidic flowmeters (often called swirl flowmeters), a static element is used to impart rotation to the incoming fluid and cause the fluid to form a vortex downstream that resembles a cyclone.

The downstream portion of the vortex rotates around the axial centerline of the pipe.

In other words, looking through the flowmeter in the downstream direction, the downstream portion of the vortex is rotating in a circle at the pipe wall.

A vortex breaker is installed at the outlet of the flowmeter body to stabilise the vortex and to keep it from propagating downstream where it can disturb the process or other hydraulic devices, such as control valves.

The speed with which the vortex rotates is proportional to the flow rate, such that doubling the flow will cause the vortex to rotate twice as many times.

A variety of electronic and mechanical techniques can be used to sense number of vortex rotations.

The frequency of vortex rotation is used to generate a flow measurement signal.

Fluidic flowmeters (vortex shedding, Coanda effect, and vortex precession) operate linearly within specific constraints.

These constraints are functions of fluid velocity and Reynolds number.

Both sets of these constraints must be satisfied for the flowmeter to operate properly.

In many vortex shedder and fluidic flowmeter designs, the fluid provides hydraulic energy to operate the sensing system.

When the fluid velocity is low, the fluid cannot provide the sensing system with sufficient hydraulic energy, so the flowmeter ceases to operate.

Therefore, when the fluid velocity falls below this minimum velocity constraint, the flowmeter will ‘turn off’.

More sensitive sensing system designs allow measurement of somewhat lower fluid velocities.

Fluid density can significantly affect the minimum velocity constraint.

For example, liquid applications typically allow measurement of velocities above approximately 0.3 meters per second (1 foot per second) of water.

Liquids with higher densities will operate the sensing system at lower flow rates, so the minimum velocity constraint is lower.

Conversely, lower density liquids will increase the minimum velocity constraint.

The concept that these flowmeters are not able to measure at low flow rates is important because in many applications, significant amounts of fluid at low flow rates can pass through the flowmeter without being measured.

Because the density of most liquids is in a relatively small range centered about the density of water, it is common to assume that the minimum velocity constraint for water (as determined by the supplier) is close to the actual minimum velocity constraint of the fluid.

Due to differences in sensing systems, it is advisable to consult supplier literature to determine the minimum velocity constraint.

In contrast, the minimum velocity constraint for free air applications can be over 2 meters per second (6.5 feet per second).

If the density of the air increases (i.e by being compressed), the minimum velocity constraint would be lower because the density of the compressed air is higher.

Due to variation in the sensitivity of sensing system designs, the minimum velocity constraint for gas applications should be determined using supplier literature.

In general, note that the minimum velocity constraint is dependent upon density — not specific gravity.

Notwithstanding this statement, when the effects of composition and liquid thermal expansion are neglected, density and specific gravity become essentially the same.

However, the specific gravity of a gas will remain the same even when changing pressure and/or temperature cause large changes to its density.

For example, free air and compressed air have a specific gravity of 1.00.

However, the density of free air will increase over ten-fold when it is compressed to 10 bar (approximately 145 pounds per square inch) gauge.

Stated differently, changes in fluid density affect the minimum velocity constraint of the flowmeter.

However, fluid density changes can be caused by changes in composition, temperature, and/or pressure, especially in gas and vapour applications.

These concepts should be applied to each application to understand the effect that operating conditions have on the minimum measurable flow rate.

The following examples show how fluid density can significantly affect the minimum velocity.

For free air flows at 0barg (atmospheric pressure) the density is 1.2Kg/m3, and the minimum velocity is 6.5m/s.

For air at 8bar the density is 11kg/m3, and so the minimum velocity is lower at 3.5m/s.

For water the density is 1000kg/m3, and the minimum velocity is 0.35m/s.

For a less dense liquid of density 500kg/m3, the minimum velocity is 0.50m/s.

REYNOLDS NUMBER.

Reynolds number is the dimensionless ratio of the inertial forces (associated with the momentum of the liquid flowing downstream) to the viscous forces (that tend to slow the fluid).

This ratio gives an indication of the hydraulic nature of the fluid flow.

Typically, fluid flows with similar Reynolds numbers exhibit similar hydraulic characteristics.

In general, it can be said that vortex shedding and fluidic flowmeters operate linearly at high Reynolds numbers.

When Reynolds number decreases below a certain value (depending upon flowmeter design and size), the flowmeter becomes nonlinear.

Decreasing the Reynolds number further will cause oscillations to cease and the flowmeter will turn off, ie no longer function at all.

This creates the possibility of three distinct regions of operation that are dependent upon Reynolds number — linear, nonlinear, and off.

For Vortex Shedders, the linear operating region is typically above a Reynolds number of approximately 10,000 to 20,000 (but can be higher in some designs and sizes of meter).

These Vortex flowmeters generally turn off below Reynolds numbers of between 3000 to 10,000.

For Fluidic flowmeters on the other hand, the linear range of operation can extend down to Reynolds numbers of approximately 500 or less.

Note that the factors that cause the low Reynolds number could be one (or more) of many, such as a lower than expected flow rate, a composition change that increases viscosity, or a temperature change that increases viscosity.

It is important to understand that fluid velocity and Reynolds number constraints are used to determine the conditions under which these flowmeters will operate, and when they will operate linearly.

Both constraints must be satisfied for proper operation.

For example, a vortex shedding flowmeter will not function in an application where Reynolds number is 1,000,000 when the fluid velocity is only 0.1 meter per second (0.3 feet per second) because its minimum velocity constraint is not met.

Similarly, a vortex shedding flowmeter will not function at a velocity of 2 meters per second (6.5 feet per second) when Reynolds number is 100 because its minimum Reynolds number constraint is not met.

Both velocity and Reynolds number constraints must be met for proper operation.

MULTIPLE VARIABLE MEASUREMENT.

There has been a trend to incorporate multiple process variable measurements into instruments.

Generally, this has occurred where additional measurements are necessary for proper operation of the flowmeter, such as when the raw measurement must be compensated for fluid temperature in order to perform within specifications.

However, even though purchasing multivariable instruments may be more expensive, this approach can reduce the number of piping penetrations and reduce installed cost as compared to purchasing and installing multiple devices.

Multivariable instruments are becoming more available where users in a market segment will pay a premium.

At least one vortex shedding flowmeter supplier has embedded a temperature measurement into the shedder to measure fluid temperature in the vortex shedding flowmeter.

Some suppliers are embedding flow computers into their transmitters to infer mass flow and density for use in gas and steam flow applications.

FURTHER READING This review has been excerpted from ‘The Consumer Guide to Vortex Shedding and Fluidic Flowmeters’, by Spitzer and Boyes, see http://www.spitzerandboyes.com/Product/vortex.htm.

This presents the first completely independent consumer guide to the world’s shedding and fluidic flowmeters, ranked by performance, by size, and by supplier.

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Tables include the types of flowmeters and selected features that are available from each supplier, along with their country of origin or source.

This guide is indispensible for designers, engineers, owners and end-users of shedding and fluidic flowmeters.

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