Water Well Journal
Issue link: https://read.dmtmag.com/i/655200
F low measurement devices, also known as flowmeters, were the subject of our column last month. The first part of a two-part series outlined the concept of mechanical flowmeters in which a mechanical action creates the force needed to indicate and register flow. We'll switch gears this month and cover flowmeters that depend on an electronic method of reading flow. Most of the electronic systems we'll discuss can be adapted and used in SCADA (supervisory control and data acquisition) systems. As such, they are quite popular for use with potable water and wastewater systems. Many of the principles we will discuss generally apply to liquids such as water flowing within a closed system (a pipeline). But we will also cover the many types of meters used for gases as well as flowmeters used in open systems, usually referred to as open-channel flow. Flowmeter Units Flowmeters are devices that measure the amount of a liq- uid, gas, or vapor passing through them. In discussing fluid flow, gases and liquids are both defined as fluids. Both gas and liquid flow can be measured in volumetric or mass (weight) flow rates, such as in liters per second or kilograms per second. These measurements are related to the material's density. The density of a liquid such as water is almost always inde- pendent of conditions. Not so for gases. The densities of gases depend greatly upon the pressure, temperature, and to a lesser extent, composition. For liquids, various units are used—depending upon the application and industry—but might include liters per second and bushels per minute. When describing open-channel flows, the registering unit can be in cubic feet or meters per second. With irrigation applications, acre-feet per day is used. Electronic Flow Measurement Electronic flowmeters typically consist of a flow measure- ment device, a transducer, and transmitter. The transducer senses the fluid passing through the primary flow-measuring device. The transmitter produces a usable flow signal from the raw transducer signal. These components are often combined, so that the actual flowmeter may be one or more physical devices. Flow measurement can be described by using various rela- tionships of the velocity (V) and the area (A) in flow. The equation Q = A × V means the volume of fluid passing through a flowmeter (Q) is equal to the cross-sectional area of the pipe (A) times the average velocity of the fluid (V). The equation W = R × Q means the mass flow of fluid passing through a flowmeter (W) is equal to the fluid density (R) times the volume of fluid (Q). Volumetric flowmeters directly measure the volume of fluid passing through the flowmeter. The only flowmeter tech- nology measuring volume directly is the positive displace- ment flowmeter. Velocity flowmeters use techniques that measure the veloc- ity of the flowing stream to determine the volumetric flow. Examples of flowmeter technologies measuring velocity include propeller, magnetic, turbine, ultrasonic, and vortex shedding and fluidic flowmeters. Mass flowmeters use techniques that measure the mass flow of the flowing stream. Examples of flowmeter technolo- gies measuring mass flow include Coriolis mass and thermal flowmeters. Inferential flowmeters do not measure volume, velocity, or mass but rather measure flow by inferring its value from other measured parameters such as pressure. Examples of flowme- ter technologies measuring by inferring include differential pressure, V cone, target, and variable area flowmeters. Flow computers are used to compensate flow measure- ments for actual conditions such as pressure, temperature, vis- cosity, and composition—particularly when used with gases. Other flowmeter technologies include flowmeters that measure liquid flowing in an open channel, and insertion flowmeters that measure the partial flow at one location in a pipe and use this measurement to infer the flow in the entire pipe. Insertion flowmeters often use a flow computer to com- pensate for hydraulic effects. Common Flowmeter Outputs Although a propeller flowmeter is a mechanical type of meter as described in last month's column, various electronic methods are usually available to attach to the meter to trans- mit the instantaneous flow rate and totalized flow to SCADA devices and remote locations. These devices vary from one ED BUTTS, PE, CPI ENGINEERING YOUR BUSINESS FLOW MEASUREMENT TECHNIQUES Part 2: Electronic methods of reading flow waterwelljournal.com 36 April 2016 WWJ The density of a liquid such as water is almost always independent of conditions . The densities of gases depend greatly upon the pressure, temperature, and to a lesser extent, composition.