DAVID W SPITZER'S E-ZINE (January 2022)

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 20 percent 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 400 mbar 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 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.

Excerpted from The Consumer Guide to Vortex and Fluidic Flowmeters

It is likely that you have flowmeters in your plant that you feel do not measure accurately. You may not know why, but you can just feel it. That’s nice, but it is much more compelling to back this up with analysis based on information using other plant measurements. Performing a heat and material balance around the flowmeter can go a long way to confirming your feeling or not.

The economic benefit of fixing an inaccurate flowmeter can be significant in some applications such as when feeding non-recoverable raw materials into a process. In other applications, significant operational benefits can accrue when a precise amount of material must be metered to maintain product quality.

While necessary to operate the process, the majority of flowmeters in plants do not generate significant economic or operational benefits. Flowmeters that do generate benefits are typically selected, engineered, installed, operated and maintained well. Such was the installation of a flowmeter at a reactor discharge that was used to determine its yield. The flowmeter did not add to the bottom line but it let you know when there was a problem so you could act quickly to reduce the economic and production losses associated with the problem.

This article originally appeared in P. I. Process Instrumentation magazine.

1. Coriolis mass
2. Differential pressure
3. Magnetic
4. Positive displacement
5. Target
6. Thermal
7. Turbine
8. Ultrasonic
9. Variable area
10. Vortex shedding

Insertion flowmeters infer the flow in a pipe by measuring flow at one (or more) strategic locations in the flowing stream. The cost of insertion flowmeters of a given technology is similar for most size lines so these flowmeters are often applied to large pipes where full-bore flowmeters would be much more expensive plus involve shutting down the line for installation.

Coriolis mass (Answer A), positive displacement (Answer D) and variable area (Answer I) are not available as insertion flowmeters.

Additional Complicating Factors

Insertion flowmeters are generally much more susceptible to velocity profile disturbances as compared to full-bore flowmeters, so careful consideration should be given to the upstream piping configuration and straight run. 

This article originally appeared in P. I. Process Instrumentation magazine.