Furness also notes, The condition of the flowmeter element can have a significant effect on the measured flow rate.
He found that surface buildup can affect the Venturi flowmeters installed in some water systems by as much as 0.2% to 0.3% per year. In one such installation in the U.K., a Venturi that was in service almost 100 years was documented to be measuring 17% higher than the actual flow.
Furness is also quick to point out that the details of the pressure tap can introduce unacceptable errors. In one application in the U.S., pressure tap changes resulted in a flow measurement shift of approximately 3%. This may seem small, but it is approximately 6 times more than some suppliers claimed accuracy of the Venturi. In a fiscal metering system, such an error would be unacceptable and likely grounds for lawsuits.
Note the extremely disturbed flow profile, which makes measurement hard to do.
Juergen Amann, instrumentation and telecommunications specialist at the City of Tampa, Fla., works at a wastewater treatment plant that uses many magnetic flowmeters. It is difficult or impossible to remove many of the flowmeters for service due to their size and location in the system Amann says. In some installations, he says, such as when the flowmeter cannot be removed from service, flowmeter performance is checked using other flow measurements that can also be in error. Expanded diagnostics that are available with some magnetic flowmeters allow at least some assurance that the flowmeter primary has not shifted its calibration. However, this is not a substitute for removing the flowmeter from service and performing a wet flow calibration.
Other flow devices need calibration, too, and may be better fits for plant requirements. Venturi flowmeters are passive mechanical devices where inlet and throat diameter measurements are used as the basis for calculating flow rate. This gives these flowmeters an inherent reliability and repeatability with few sources of measurement error and limits on their magnitudes. Furthermore, redundant taps can be used for custody transfer applications where it is desirable to independently check transmitter measurements, says Dave Wyatt, president of Wyatt Engineering, Lincoln, R.I., who designs and manufactures Venturi flowmeters for the water and wastewater industries.
The ratio of throat diameter to inlet diameter (beta ratio) provides significant design flexibility by reducing upstream straight run requirements, adds Wyatt, because Venturi flowmeters are less sensitive to upstream piping as the beta decreases. Tailoring your flowmeter to your installation, rather than the other way around, is a significant bonus for engineers and plant managers of older water treatment facilities where sufficient straight run may not be available.
Wyatt warns users that square root extraction should be taken either in the transmitter or in the control room, but not in both places. In addition, Turndown can be increased by stacking transmitters (i.e., low range and high range) and being sure to size the Venturi to maintain sufficient Reynolds number, he says.
|FIGURE 4: WHAT IT REALLY LOOKS LIKE|
This is the actual flow shown in Fig.3 so you can really see the disturbance.
Pennington adds, Chemical composition and concentrations are subject to change in industrial wastewater streams due to process and production changes. Magnetic flowmeters are prevalent in these applications because they are available with compatible materials of construction and are not affected by changes in sound speed. On a lesser note, there is a trend to use flowmeters in place of pump strokes as a measure of flow so as to reduce chemical consumption.
One of the biggest problem areas in water and wastewater flow measurement is the measurement of flow in open conduits and partially filled pipes. Generally found in wastewater collection systems (sewers), stormwater systems and industrial outfalls, these flows have been problematic since Robert Mannings formula was created in the 19th century. Generally, open channel flows are measured when possible through the use of a primary device (a weir or a flume) but in many cases, a primary device cannot be installed because of application conditions. In those cases, several types of devices to produce rude approximations of flow rate have been designed, including dippers, area/velocity flowmeters, varieties of compound meters and a magnetic flowmeter variant that actually measures flow when the pipe is not full. Most of these devices, unfortunately, have been found to have potential error in excess of 40% of rate. Included in this number are nearly all of the portable solutions to the problem, so many users bite the bullet and dig up the street and do it over properly.
Water and wastewater flowmeters sometimes have much in common, but often are considerably different. Flowmeter accuracy is often more important than the application would initially indicate. Many existing flowmeters have not been installed according to manufacturer recommendations, so they are apt to exhibit significant flow measurement errors. Further, recent research has shown that upstream flow profile effects in large pipes are larger than previously anticipated. In general, the user is cautioned to pay attention to flowmeter detailsespecially when measurements are used to generate invoices valued at millions of dollars per year.
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