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.

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By Philip S. Stacy W hat is your flow rate? Seems like a simple question; however, the answer may be quite complex. A lot of variables contribute to whether or not measuring flow is easy or nearly impossible. Variables consist of, but are not limited to, flow conveyance material and size, fluid type, fluid "cleanliness," acceptable level of uncertainty, and expected range of flow rates and velocities. Assessing these variables is critical before selecting and installing a measurement technology. To assist with initial selection and evaluation, the following information provides the reader with an overall list, and preliminary evaluation, of flow measurement technologies. Flow Measurement Technologies The following is a comprehensive list of available technologies that can be used to monitor flows, regardless of system operation or components, along with brief descriptions of each technology group. This list provides the basis of a technology evaluation, with technologies organized into five different groups by their modes of operation: velocity meters, differential pressure technologies, other closed-conduit devices, open-channel control structures, and generated system curves. ○ Velocity meters • Turbine meters • Propeller meters • Vortex meters • Magnetic flow meters • Ultrasonic flow meters • Calorimetric meters ○ Differential pressure technologies • Elbow meters • Orifice plates • Flow nozzles • Venturi meters • Flow tubes • Target meters • Pitot Tubes ○ Other closed-conduit flow meters • Mass flow meters • Positive displacement meters ○ Open-channel control structures • Weirs • Control flumes ○ Generated system curves • Dye dilution • Current meter flow measurement Velocity meters are flow meters that measure the velocity of a flow which, when multiplied by a known flow area and velocity profile, can be correlated to a volumetric flow rate. For these meters to provide accurate results, they must be placed in locations with uniform flow. Velocity meters can be intrusive or nonintrusive. Intrusive meters may increase the pressure loss in a pipe and are also prone to fouling as they are located within the flow. Nonintrusive velocity meters are typically mounted to the outside of a pipe, but in some cases they may require the installation of sensors and conduits along the pipe walls within the pipe. Differential pressure flow meters are the most common devices for flow measurement used today. They operate on the basic principle that an increase in the velocity of flow is accompanied by a decrease in the pressure of the fluid under consideration. The pressure drop across the meter is proportional to the square of the flow rate. The flow rate across these meters is obtained by measuring the pressure differential, extracting the square root, and multiplying this by an area and a meter coefficient. The simplest form of a differential pressure-type meter consists of a pressure-detecting element located in the flow path, operating in conjunction with a measuring unit. Other closed-conduit flow measuring devices include mass flow meters and positive displacement (PD) meters. Mass flow meters, also known as inertial flow or coriolis flow meters, are devices that measure flow as a mass per unit time, unlike other flow meters that measure volume per unit time. To determine the volumetric flow rate, the mass flow rate is divided by the fluid density. If the density of the fluid changes over time or if there are entrained air bubbles, converting the mass flow rate to a volumetric flow rate may not provide accurate results. 20 wateronline.com n Water Innovations Selecting A Flow Measurement Technology What's the right flow measurement method for your operation? A leading independent lab breaks down the options and considerations. Installed magnetic flow meter

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