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Vortex Shedding and Fluidic Flowmeters (Part 3 of 4) by David W Spitzer and Walt Boyes
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 vapor applications. These concepts should be applied to each application to understand the effect that operating conditions have on the minimum measurable flow rate.
A Tale of Two Sewage Districts by David W Spitzer
Did you ever prepare for a big event only to have the wind taken out of your sails before the event began? That is exactly what happened my first time testifying in court, but it is a long story that started about three or four decades ago. It is my understanding that some of the technical issues presented in the early years still remain unresolved and, if so, will likely remain unresolved until the existing contract expires and a new contract is negotiated.
This particular case involved a dispute regarding billing for the sewage generated in one sewage district that was subsequently treated in a sewage treatment plant located in and operated by a second sewage district that is adjacent to the first district. More specifically, this was a case involving measurements obtained from four flowmeters that deliver sewage from the first sewage district to the second sewage district, plus the measurement from a flowmeter that measures the total flow entering the water treatment plant. The total population of the two districts was approximately 10,000 people, so such issues that will be discussed could occur almost anywhere.
The first district’s system was designed to collect raw sewage, whereas the second district was a combined sewage system that transported not only raw sewage but also drainage water that included water gathered during wet weather events. As a side note, drainage water collected from street drains and the like could have been routed to a river without treatment if a separate drainage water piping system had been installed in the second district.
Read more next month about how the sewage flows generated by each sewage district are measured.
This article originally appeared in P. I. Process Instrumentation magazine.
Coriolis Mass Flowmeter Orientation for Liquid Applications (Part 1 of 3) by David W Spitzer
Which of the following orientations can be used to install a Coriolis mass flowmeter to measure the mass flow of a liquid in a horizontal pipe?
A. U-tube down
B. Inverted U-tube
C. U-tube horizontal (parallel to grade)
D. Flag position
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 flag position (Answer D) could be acceptable but would entail modification of the upstream and downstream piping to ensure that the flowmeter is full of liquid. Few Coriolis mass flowmeters are mounted horizontally (Answer C), so mounting the flowmeter with its U-tube down (Answer A) would be the practical answer.
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 straight single-tube Coriolis mass flowmeter mounted horizontally between rising upstream piping and falling downstream piping can effectively create and inverted U-tube that can accumulate gas in the flowmeter.
Conversely, a single path self-filling and self-draining Coriolis mass flowmeter that forms a loop, jumps up 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.
ABOUT SPITZER AND BOYES, LLC
In addition to over 40 years of experience as an instrument user, consultant and expert witness, David W Spitzer has written over 10 books and 500 articles about flow measurement, level measurement, instrumentation and process control. David teaches his flow measurement seminars in both English and Portuguese.
Spitzer and Boyes, LLC provides engineering, technical writing, training seminars, strategic marketing consulting and expert witness services worldwide.
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