Water Well Journal

March 2016

Water Well Journal

Issue link: http://read.dmtmag.com/i/643500

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Page 60 of 85

A s water conservation becomes more and more critical and strin- gent by regulatory bodies in states and around the world, old and new methods for tracking and register- ing water flows are becoming more common in fluid transfer applications. This is true whether it is from pump- ing stations, water distribution systems, or transmission pipelines. When I started in this business in 1974, the most common method of flow measurement was the use of flowmeters, specifically propeller, turbine, or differential pres- sure meters for larger flows and inline disc, turbine, or positive displacement for the smaller and residential flow monitoring. However, as time has passed so has the technology—as there have been many improvements and newer methods for registering and totalizing. In this two-part series we will first review the typical types of mechanical measuring devices used for flow measurement. Next month we will outline the many newer electromagnetic and ultrasonic methods in current use. Background There are two basic approaches taken for flow measurement using mechanical methods: velocity and displacement. Each one has its own subset of methods. The most common types of displace- ment methods include rotating disc and oscillating piston while velocity types include single- and multi-jet meters, propeller meters, and turbine meters. In addition to these styles there are also various non-mechanical methods, many introduced during the past decade. These recent additions to flow measure- ment include electromagnetic (mag me- ters) and ultrasonic meters as well as other types created for special uses. There are generally two separate flow patterns read and recorded by flow measuring devices: the instantaneous rate and the totalized flow. The instanta- neous rate is the per-unit volume pro- duced by the source of the flow (pump, pipeline). It is typically shown in vari- ous types of units: gallons per minute, gallons per hour, cubic feet per second, or in metric units such as liters per second or cubic meters per second or per hour. Totalizers are often shown in gallons, cubic feet, or cubic meters, but can also be in such unusual units such as acre-feet. Most flow measurement devices are used for metering cold water for water systems. But many of the meters de- scribed above can be modified for use in chemical, hot water, slurry, or other types of fluids. When used in a water system, the water purveyor is typically the owner and maintenance person charged with responsibility of the meter. On the other hand, the device may be owned by the entity receiving the fluid in order to better track usage and waste in certain cases. Residential Water Meters The most common type of water meter used in residential and small com- mercial applications is the velocity type, specifically the multi-jet style (Figure 1) and the small displacement style. The velocity meter is accurate in the low flows often seen in residential settings and the small sizes of ⅝ inch to 2 inches commonly used in these applications. Multi-jet meters use multiple ports surrounding an internal chamber to cre- ate multiple jets of water striking an im- peller. The rotation speed of the impeller varies with the velocity of water flow. Multi-jet meters are often equipped with an internal strainer to prevent clogging of the jet ports and are typically con- structed from bronze or thermoplastic resins. This type of meter is generally provided with either hose or National Pipe Threads (NPT) to enable connec- tion to the water provider and the customer's piping. The other style of meter used for res- idential and small commercial water metering is the displacement type. Displacement meters are also referred to as PD (positive displacement) meters and are usually available in oscillating piston or rotating disc styles. Either method depends on the water physically displacing the moving ele- ment in direct proportion to the amount of water passing through the meter at the time. The register assembly operates much as the velocity type, as the piston or disc moves a magnet that in turn drives the register. PD meters are also accurate within the low to moderate flow ranges typical of residential and small commercial users and are also commonly available in connection sizes of ⅝ inch up to 2 inches. Unlike velocity and other types of flowmeters, the PD style is typically limited to the lower to moderate flow rates as higher flow rates result in a high pressure drop through the meter. PD meters are also equipped with built-in strainers to prevent damage to the element and are available in bronze or thermoplastic bodies. Typically, a standard register used for both styles has a dial similar to a clock face with gradations situated around the perimeter (Figure 2). The rotating nee- dle usually displays the instantaneous rate of water consumption in gallons per minute or cubic feet with each rotation creating a turn of the totalizer register. Modern registers are normally driven by a magnetic coupling between a mag- ED BUTTS, PE, CPI ENGINEERING YOUR BUSINESS FLOW MEASUREMENT TECHNIQUES Part 1: Typical types of mechanical measuring devices 58 March 2016 WWJ waterwelljournal.com

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