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Vortex Shedding and Fluidic Flowmeters (Part 2 of 4) by David W Spitzer and Walt Boyes
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 stabilize 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.
Excepted from The Consumer Guide to Vortex Shedding and Fluidic Flowmeters
Damaged Flowmeter Causes Overbilling by David W Spitzer
Existing plant flowmeters that exhibit a (say) 5% error may not be that good, but it is not all that bad considering that most flowmeters in the plant are installed to operate the process and provide little or no economic benefit. In other words, they enable the plant to keep operating, even though they measure inaccurately. Accurate measurements would be better for other reasons such as when performing heat and material balances to better understand and optimize the process. Nonetheless, the plant will still operate with flowmeters that are not accurate.
On the other hand, the flowmeter becomes the “cash register” in custody transfer applications. Stated differently, a 5% flowmeter error becomes a 5% billing error. For this reason, flowmeters used for custody transfer are typically selected, engineered, installed, operated and maintained well. The goal is for these flowmeters to measure accurately and be trusted by both the seller and the buyer. Failure to achieve this goal can create animosity between the parties that can last for decades.
Despite best efforts, issues can occur such as the case of a plant where a damaged flowmeter caused its sewage flow invoice (calculated by bureaucrats) to increase substantially and then skyrocket almost thirtyfold. Needless to say, this was noticed immediately.
Achieving such a large amount of flow would have required everyone in the plant to flush toilets 24 hours per day while they were simultaneously running the showers. The obvious question was, “when would they work?” It took some time and gyrations to convince the authority to come up with an equitable plan to correct the previous bills.
This article originally appeared in P. I. Process Instrumentation magazine.
Potential Actions to Reconcile Flow Measurements by David W Spitzer
What potential actions can be taken if a problem is suspected because the measured flow leaving a reactor is higher than the measured flow entering the vessel?
Check the calibration of the inlet flowmeter.
Check the calibration of the outlet flowmeter.
Check the calibration of the inputs to the electronics providing the inlet and outlet flow measurements.
Presuming that you are knowledgeable in instrumentation, all of these answers provide direction to troubleshoot the flowmeters and resolve the problem. Answer A, Answer B and Answer C are correct.
Additional Complicating Factors
The suggested answers are the answers of a person with instrumentation knowledge.
There are times when you might want to consider looking outside the instrumentation box, especially when the instrumentation is known to be calibrated and operating properly. For example, there might be an additional connection providing flow into the reactor. The density of the incoming liquid may be different than the density of the liquid leaving the reactor so their measured volumes may be different. One of the flowmeters is a mass flowmeter, while the other flowmeter measures inferentially. Needless to say, there are many possibilities.
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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