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

In general, 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. 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 could be higher in some designs and sizes. These flowmeters generally turn off below Reynolds numbers of 3000 to 10,000. The linear range of operation for fluidic flowmeters can extend to Reynolds numbers of approximately 500 or less. Note that the cause of 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.

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.

Excerpted from The Consumer Guide to Vortex Shedding and Fluidic Flowmeters.

It would be desirable to measure the flow from each sewage district in order to appropriately allocate the cost of operating the sewage treatment plant between the two districts. The amount of sewage generated by the first sewage district can be calculated by summing the flow measurements obtained by the four sewage flowmeters. However, these four sewage flowmeters emptied into the second district’s system at different locations, so the amount of sewage produced by the second sewage district alone could not be measured.

The total plant flowmeter measured the sewage from the first sewage district plus the sewage from the second sewage district plus the co-mingled drainage water from the storm drains in the second sewage district. Sewage flow normally varies throughout the day --- increasing in the morning when people wake up, decreasing somewhat during the day when people are at work, increasing in the evening when people are home and then decreasing during the overnight hours when people are sleeping.

As a practical matter, sewage systems are not completely tight, so both systems will exhibit a certain amount of inflow and infiltration (I&I) during normal operation and (more so) during wet weather events — especially in the second sewage district system that is designed to collect drainage water. Therefore, normal sewage flow variation throughout the day plus relatively large amounts of drainage water that are periodically generated by the second sewage district require that the total plant flowmeter operate over an extremely wide range of flow rates.

Read more next month about how the sewage treatment plant operating costs are allocated.

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

 A. U-tube down

B. Inverted U-tube

C. Horizontal (parallel to grade)

D. Flag position

Commentary

Coriolis mass flowmeters in liquid service must be completely full of liquid to measure accurately. The inverted U-tube orientation (Answer B) could accumulate gas and should not be used for liquid applications.

The U-tube down orientation (Answer A) and horizontal orientation (Answer C) could be acceptable but would entail modification of the upstream and downstream piping. The flag position (Answer D) would typically be most practical.

Additional Complicating Factors

Not all Coriolis mass flowmeters have U-tube geometry, and some of these geometries can allow gas to accumulate in the flowmeter.

For example, a Coriolis mass flowmeter with delta-shaped tubes can accumulate gas in the downstream (upper) portion of the tubes when mounted in a flag position (Answer D), so the upstream and downstream piping should be modified to mount the flowmeter with its delta-shaped tubes down (Answer A).

Similarly, a single-path self-filling and self-draining Coriolis mass flowmeter that forms a loop, jumps up (sideways if mounted in flag position) and then forms another loop must be installed in the horizontal plane (Answer C) to remove all gas from the system because any other orientation can allow gas to accumulate in the flowmeter.

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