Water Online

January 2017

Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.

Issue link: https://wateronline.epubxp.com/i/773139

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Page 23 of 39

PDs operate by isolating and counting known volumes of a fluid while feeding it through the meter. By counting the number of volumes that pass through the meter, a flow measurement can be obtained. There are many different PD designs, each using a different means of isolating and counting these volumes. These meters are highly accurate on the order of 0.5 percent over a 10:1 range of flow and do not require straight runs of pipe like other flow meters. As these meters collect the flow medium and move it, they are only used for small-diameter pipes, (e.g., 12 inches or less). PD devices have tight tolerances, which can lead to fouling if used in flows that contain suspended particles (100 microns or fewer). These particles can also lead to erosion of the finely machined parts, impacting accuracy. These meters can be constructed out of many materials including plastics and metals. Open-channel control structures can be used to determine flow rates by measuring the area of the flow stream and the head of fluid producing the flow. Weirs provide a simple means of measuring flow in open channels. A weir consists of a vertical plate or other obstruction placed across the open channel with a level or specially shaped opening or notch. This obstruction increases the water level behind the weir. When a fluid flows over the weir, its flow rate is a function of the water depth above the weir crest. Common weir constructions are the rectangular, v-notch, and broad-crested. Flume control structures are shaped, open-channel flow sections that force flow to accelerate. This acceleration is produced by reducing the cross-section of the flume. As the flow accelerates, it passes through the critical depth, which results in a unique water surface profile for a given discharge. This allows the use of a head versus discharge relationship for flow measurement. Flumes range in size from small (1 inch wide) to large structures (over 50 feet wide) and can accommodate a wide discharge range (50:1). The accuracy of control flumes is similar to that of weirs. The head loss in a flume control section is about one-fourth of that needed for a sharp-crested weir. In some long-throated flumes, the difference in head loss may be as low as one-tenth of what would be expected with a sharp- crested weir. However, control flumes are generally more expensive than weirs. Generated system curves can be created by correlating pump speed (if applicable) and system pressure to a known flow rate. The successful use of system curves relies upon obtaining enough pump speed and pressure versus flow data points and verifying the data points over time. This technique would only require the permanent installation of pressure taps and would only impact operations during the flow measurement and verification periods. By accurately measuring the flow, it is possible to determine flow rates under a wide range of flows and/or pump speeds. At locations with varying water levels (i.e., tidal water sources), measurements will need to be taken over a range of water heights to account for changes to the intake and wateronline.com n Water Innovations 21 INSTRUMENTATION Innovative Chemical Feed Solutions * * NEW PRODUCT * * Model 4180 Chlorine Dioxide Generator Safely and Accurately I ntroduce Chlorine Gas and Sodium Chlorite Simultaneously into Feed Water Stream - Feed Range of 0-500ppd - Accuracy of +/- 2% of Actual Feed - 4 Control Modes - 2.5hr of Battery Backup -Air Binding and Etching NOT Possible -Metering to Validate Actual Flow Rate -Automatically Regulates in Fixed and Variable Control Modes - Can be Remotely Monitored and Controlled by SCADA JCS Industries, Inc. I PO Box 90028 I Houston, TX 77290 Ph: 281-353-2100 I www.jcsindustries.us.com

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