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Vortex Shedding and Fluidic Flowmeter Applications (Part 2 of 4) by David W Spitzer and Walt Boyes
Vortex shedding and fluidic flowmeters can be used to measure the mass flow of gases and vapors. These flowmeters measure the fluid velocity that is used to infer the mass flow in conjunction with other measurements, such as pressure and temperature. Because the density of gases and vapors is usually significantly lower than that of liquids, the ability to generate sufficient hydraulic energy to operate the sensing system can be limited. Therefore, vortex shedding and fluidic flowmeters in gas and vapor applications generally have higher turn off velocities, so they may not measure when the flowing stream is at low velocity, at low pressure, or at high temperature.
Vortex shedding and fluidic flowmeters measure the velocity of the fluid. The measurement is linear with velocity and can exhibit a large turndown when high fluid velocity is acceptable.
Variation in certain characteristics of vortex shedding and fluidic flowmeters has allowed this technology to be applied to many processes. The following sections define flowmeter design categories and how they fit the needs of their intended applications.
Liquid Vortex Shedding and Fluidic Flowmeters
Most vortex shedding and fluidic flowmeters measure liquid flows, however some designs were developed specifically and exclusively for gas flow measurement.
Gas Vortex Shedding and Fluidic Flowmeters
Although the majority of vortex shedding and fluidic flowmeters were designed to measure liquid flow, many can be applied to measure the flow of gases. Vortex shedding and fluidic flowmeters measure gas velocity, so pressure and temperature compensation may be necessary to accurately infer mass flow measurement. Because temperature and pressure can vary significantly in actual operation, failure to compensate vortex shedding and fluidic flowmeter measurements can introduce significant measurement errors in many applications.
Steam Vortex Shedding and Fluidic Flowmeters
Some vortex shedding and fluidic flowmeters can be used to measure steam flow. These flowmeters are designed for gas (vapor) flows at high temperatures. The temperature ratings of these flowmeters must exceed the operating steam temperature (taking superheat into account) to avoid damaging the flowmeter. The temperature ratings of practical designs vary from approximately 250 degC to over 400 degC.
Excepted from The Consumer Guide to Vortex Shedding and Fluidic Flowmeters.
The More Experts, The Better by David W Spitzer
Previous articles described the sewage collection systems for two adjacent sewage districts where the flow measurements used to allocate expenses and bill one of the sewage districts flowmeters was questioned in court.
In general, companies hire experts to investigate what happened, explain what occurred to the company, explain why a particular party is at fault and provide expert testimony in court - all in terms that can be easily understood by a lay person.
Good experts are not inexpensive (double negative) so companies tend to hire as few experts as possible (usually one) on a given case and trust that that expert can address all aspects of the case.
As such, experts are generally trusted sources of information about the technical aspects of a case but they sometimes can become enamored by being the primary source of information, education and opinions provided to the client. However, a good expert will openly recognize when an important aspect of the case is beyond his/her expert knowledge and proactively requests that the client retain an additional expert to focus on that particular aspect. In my experience as an expert witness, working with other experts has led to challenging (in a positive way) and rewarding experiences that yielded results that complemented my findings and provided a more solid case for the client.
For example, benefiting from a second expert occurred on another case involving a flowmeter in a water treatment system for a miniature water park at a hotel where a flowmeter that allegedly failed was the primary focus of the litigation. Interestingly, combining my flowmeter expertise with the expertise of an experienced swimming pool engineer (who designed the swimming pool at the White House, among many others) yielded insights that neither of us could obtain separately.
This approach is not much different than involving additional people with different expertise to solve an engineering problem in a plant.
Read more next month about the sewage districts.
This article originally appeared in P. I. Process Instrumentation magazine.
Calculating Fill Time for a Tank by David W Spitzer
A chemical intermediate with a specific gravity of 1.00 will be diverted to an empty flat-bottom tank with a diameter of 6 feet that is 12 feet high at a flow of 4 gallons per minute. Approximately how long will it take to fill the tank?
The tank volume can be calculated using the given dimensions as (3.14 x 3 x 3 x 12) or 339 cubic feet. The volume can then be converted to gallons as (339 x 7.48 gallons/cubic foot) or 2,537 gallons, from which the fill time can be calculated as (2537 / 4) or 634 minutes or 10.6 hours.
The flow will vary somewhat, so the tank will take approximately 10 hours to fill (Answer C). However, it may be pragmatic to start checking the tank level after (say) six or eight hours to reduce the probability of creating an overflow condition due to a higher than normal flow rate.
Additional Complicating Factors
The working volume of the tank will likely be somewhat less than the calculated value because the tank should not be filled to the overflow pipe, thus providing a margin of safety to avoid overflowing the tank.
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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