By 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.
Photo courtesy of ABB
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.”
Photo courtesy of ABB
“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.
David W. Spitzer is a principal in Spitzer and Boyes, LLC, and can be reached at +1.845.623.1830 or www.spitzerandboyes.com.