Water. Desalination + reuse
Issue link: https://fhpublishing.uberflip.com/i/513849
May-June 2015 | Desalination & Water Reuse | 33 | Clean away: wastewater from some 1.6 million people fowing into the Nuremberg treatment works in Germany is measured by electromagnetic fowmeters with accuracy determined at Khrone's calibration facilities. TECHNOLOGY Manufacturer Velocity (fps) Rig max fow rate (gpm) Percentage of full scale (125 mgd) A 3.50 44380 51.13 B 4.16 52840 60.88 C* 10.62 134727 155.22 D** 10.62 134727 155.22 E 1.99 25230 29.07 F 3.47 44029 50.72 Table 1. Manufacturers' maximum available calibration fow rates for a 72in meter. C* Krohne altometer calibration facility, Dordrecht, The Netherlands D** Krohne /KMTS calibration facility Shanghai, China Table 2 shows the largest diameter meter for which each manufacturer has accreditation of calibration, along with the velocity that can be achieved. Manufacturer Rig maximum fow rate (gpm) Max. diameter (ins) Max. calibrated velocity (fps) Velocity (fps) in 84 ins meter A 44,380 86 2.45 2.57 B 52,840 96 2.34 3.06 C* 134,727 120 3.82 7.80 D** 134,727 120 3.82 7.80 E 25,230 84 1.46 1.46 F 44,029 84 2.55 2.55 Table 2. Manufacturers' maximum accredited meter sizes and fow rates. THE CHaLLENGE Of LarGE diamETErs Poor accuracy can have a material cost in large installations. For example, anticipated inaccuracies in large diameter magnetic fow meters can lead to discrepancies of more than 150,000 l each day. A magnetic fow meter measures the velocity of a conductive fuid product passing through a magnetic feld. As it travels through this feld, the fuid's velocity creates a proportional induced voltage at two electrodes in the meter. The volumetric fow rate is determined by multiplying the fuid velocity by the cross-sectional area of the measured section. Most of the time, with large diameters, the meter's size is selected to operate at about 10 feet a second (about 3 m/s) for the maximum volumetric fow rate. Magnetic fow meter accuracy is normally stated as a function of measured value or rate of fow. For example, a small meter with a typical stated accuracy of 0.5% measuring a fow rate of 1,000 l/m can be reasonably expected to be within 5 l/m. However, a large diameter meter with that same stated accuracy measuring a fow rate of 500 Ml/d is expected to be within 2.5 Ml/d or a 175,000 l a minute error. Furthermore, failure to calibrate at the maximum fow rate can add to error. Calibration of a small-diameter meter can readily be done at the expected maximum velocity and hence with a high confdence in the measurement throughout the full performance range. So were a large- diameter meter measured at fow velocities of 2 - 4 m/s, a fow rig with the ability to