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Types of Ultrasonic Flowmeter s (Part 1 of 2)by David W Spitzer and Walt Boyes
Variation in certain characteristics of ultrasonic 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 Ultrasonic Flowmeters
Most ultrasonic flowmeters are designed for use in liquid service. These flowmeters can be applied to many industrial applications.
Portable Ultrasonic Flowmeters
Portable ultrasonic flowmeters are typically housed in a case that is approximately the size of a small suitcase. Their electronics are larger and more rugged than handheld ultrasonic flowmeters, and may sometimes actually be a standard ultrasonic flow transmitter housed in a portable case. Sensors are attached to pipes using temporary wiring.
Handheld Ultrasonic Flowmeters
Handheld ultrasonic flowmeters are small lightweight instruments that available for field testing and surveying. Their transmitters are typically larger than a large hand calculator and can be easily transported.
Clamp-on Ultrasonic Flowmeters
Many ultrasonic flowmeters designs use sensors that are clamped onto the pipe. The concept of clamping a device onto a pipe to determine the flow inside the pipe has a certain appeal. This not only reduces piping changes, but also allows flow measurement using an instrument that has no wetted parts.
Excerpted from The Consumer Guide to Ultrasonic and Correlation Flowmeters.
Field Calibration of Magnetic Flowmeters: Case Studyby David W Spitzer
Instrumentation engineers and technicians often deal with flow measurement on a daily basis. However, interest in flow measurement often lies far beyond to include accountants and lawyers because in many applications, flowmeters function as cash registers that can result in billing that totals millions of dollars per year. Ensuring that these flowmeters are functioning accurately is not necessarily an easy task in many applications.
The water we use in our daily lives might appear to be inexpensive at a cost of a few dollars per thousand gallons. In this context, a flow measurement error of one percent would seem negligible. However, the value of water flowing annually in the water distribution system for a major population center can easily be on the order of hundreds of millions of dollars per year. Therefore, a one percent flow measurement error for a flowmeter used to bill $100 million per year represents an error of $1 million. These types of potential errors tend to attract the interest of accountants who want to ensure that the cash registers are functioning accurately, and lawyers who might want to question billings to recoup lost revenue or correct over-billing.
In addition, flowmeters are important for maintaining safe and efficient system operations, such as measuring the flow of water pumped from wells and the water drawn from reservoirs. Influent and effluent flow measurements are important to ensure conformance with environmental regulations.
Calibration is used to ensure that flowmeters (and other instruments) are functioning accurately. However, both wet and dry calibrations have their problems. Wet calibrations that check the entire flow measurement system are generally not an option in most applications because challenging the installed flowmeter with a known volume of water is impractical --- especially in large pipes in remote locations. Even removing the flowmeter to perform a wet calibration in a flow laboratory can be expensive and time-consuming. Further, it does not take the installed piping into account.
On the other hand, dry calibrations verify the operation of only part of the flow measurement system. In the case of magnetic flowmeters, a traditional calibrator checks the transmitter by verifying that a given (millivolt) input produces the correct flow rate. What if the wiring is faulty? What if the coil has changed its characteristic? What if the flow tube leaked and shorted an electrode? Dry calibrations do not necessarily check for these problems.
Greg Livelli, Product Manager for Magnetic Flowmeters at ABB (Warminster, Penn.) says, "that the CalMaster magnetic flowmeter calibration tool is a portable PC-based system that performs a pre-programmed verification process of the entire flowmeter to include the flowmeter primary, liner, electrodes, interconnection cabling, and transmitter. This process entails comparing the measured flowmeter characteristics with characteristics that were generated when the flowmeter was manufactured."
"The verification process takes about 15 minutes and occurs without interrupting the flow measurement. This is a key benefit to the user because it allows the user ultimate operational flexibility in when the test can be performed. A certificate of calibration is generated if the test results are within acceptable limits. In addition, a history of test results is stored to allow predictive diagnostics to identify problems before failure occurs. This allows the user to conduct preventative maintenance and prevent costly downtime."
Many water authorities struggle with these issues including the City of Mesa Water Division (Mesa, Ariz.). Jim McCarter, Controls Engineering Specialist, says that, "ensuring accurate flow measurement systems is a prime concern. We have about 10 magnetic flowmeters that are used for custody transfer of water and wastewater that result in billings totaling millions of dollars. Needless to say, ensuring that these flowmeters are functioning accurately is important not only to the City of Mesa, but also to the City of Phoenix that treats sewage from the City of Mesa."
Jim found that "traditional dry calibration of our magnetic flowmeters did not adequately verify that the magnetic flowmeter was functioning accurately. Simulators could check the analog signals and totalizer, but could not check the wiring and the condition of the magnetic flowmeter primary. On the other hand, inline wet testing was not an option and removing the flowmeter to perform testing in a flow laboratory was simply too expensive. Removal was also not practical because it required that the flowmeter be taken out of service for a significant period of time. The ABB CalMaster was selected as a compromise because it performs a dry calibration plus it verifies that the magnetic flowmeter characteristics have not changed since the flowmeter was manufactured. We currently use the CalMaster to verify our custody transfer magnetic flowmeters that have accuracy specifications of 0.5 percent of rate."
"About 5 years ago, we chose the ABB CalMaster because ABB was the only supplier that was willing to train our people to perform the verification. Having our people perform the verification was important because it was expensive to have factory people come to our site to perform the testing. Now we can verify any magnetic flowmeter whenever we suspect a problem. Both Endress+Hauser and Krohne currently offer acceptable equipment that exhibits essentially the same functionality. Given the alternatives, these systems generally offer the best option for verifying magnetic flowmeter operation."
Jim says that there is a "general trend towards magnetic flowmeters and away from using mechanical flowmeters that are more prone to maintenance problems. However, Jim cites problems with magnetic flowmeters including the coating of the flow tube in wastewater service when ferrous chloride is used for odor control. This created a significant maintenance problem. A multi-path ultrasonic flowmeter has been operating maintenance-free for approximately 18 months in a critical wastewater application."
Overall, CalMaster and other similar diagnostic tools do not constitute a wet calibration of the flowmeters. However, they are superior to a traditional dry calibration and represent a pragmatic approach to assessing whether flowmeter characteristics have appreciably changed since the magnetic flowmeter was manufactured.
This article originally appeared in Flow Control magazine.
Air Bubble Troublesby David W Spitzer
Impulse tubing that is not
completely full can cause measurement error. Air bubbles can form when the
impulse tubing can trap air when empty. In one installation, the transmitter
taps in a 2 bar steam flow measurement system are located at the same elevation
as the flowmeter taps. The impulse tubing is routed down 25 cm (10 inches) from
the taps and then up to the transmitter to form a condensate seal. What is the
approximate pressure measurement error if the air is not bled out of one of the
impulse tubes during calibration?
25 cm. of water column
12.5 cm. of water column
8.3 cm. of water column
5 cm. of water column
The first order of business is to
determine whether the pressure unit (bar) is an absolute pressure unit or a
gauge pressure unit. Some engineers were educated to always treat bar as an
absolute pressure unit whereas common industry practice is to use bar as a
gauge pressure unit. For the purposes of this problem, let's assume that bar is
a gauge pressure unit. In addition, one atmosphere is 1.01325 bar absolute.
However, 1 bar absolute will be used in this problem to simplify the
The bubble will be 25 cm high
before the transmitter is put in service. Putting the transmitter into service
will compress the bubble and reduce its size. Therefore, Answer A is not
The size of the bubble is
inversely related to its absolute pressure. The absolute pressure is not 2 bar,
so Answer B is not correct.
The absolute pressure of the compressed bubble (3 bar
absolute) is calculated by adding one atmosphere (approximately 1 bar) to the
gauge pressure (2 bar). This will compress the bubble to approximately
one-third (1/3) of its original size at atmospheric pressure (0 bar) so its
height can be calculated to be approximately 8.3 cm. Answer C is the correct
answer. This corresponds to almost 0.3 percent (8.3 / (3 * 1000)) of the
Additional Complicating Factors
The transmitter and the flowmeter taps are
typically not located at the same elevation and the installed impulse tubing
routing is typical more complicated. Both of these complicating factors should
be considered in the analysis (when applicable).
This article originally appeared in Control 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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