Science of Water : Concepts and Applications

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Water Hydraulics 81 and the depth of fl ow is measured, then fl ow area can be computed. From the relationship Q = AV, the discharge Q can be estimated. In preliminary fi eldwork, this simple procedure is useful in obtaining a ballpark estimate for the fl ow rate, but is not suitable for routine measurements. Example 3.16 Problem: A fl oating object is placed on the surface of water fl owing in a drainage ditch and is observed to travel a distance of 20 m downstream in 30 s. The ditch is 2 m wide and the average depth of fl ow is estimated to be 0.5 m. Estimate the discharge under these conditions. Solution: The fl ow velocity is computed as distance over time, or D V 20 m/30 s0.67 m/s T The channel area A = 2 m × 0.5 m = 1.0 m 2 The discharge Q = A × V = 1.0 m 2 × 0.66 m 2 = 0.66 m 3 /s BASIS OF TRADITIONAL FLOW MEASUREMENT Flow measurement can be based on fl ow rate or fl ow amount. Flow rate is measured in gallons per minute, million gallons per day, or cubic feet per second. Water/wastewater operations need fl ow rate meters to determine process variables within the treatment plant, in wastewater collection, and in potable water distribution. Typically, fl ow rate meters used are pressure differential meters, magnetic meters, and ultrasonic meters. Flow rate meters are designed for metering fl ow in closed pipe or open-channel fl ow. Flow amount is measured in either gallons or in cubic feet. Typically, a totalizer, which sums up the gallons or cubic feet that pass through the meter, is used. Most service meters are of this type. They are used in private, commercial, and industrial activities where the total amount of fl ow measured is used in determining customer billing. In wastewater treatment, where sampling operations are important, automatic composite sampling units—fl ow proportioned to grab a sample every so many gallons—are used. Totalizer meters can be the velocity (propeller or turbine), positive displacement, or compound types. In addition, weirs and fl umes are used extensively for measuring fl ow in wastewater treatment plants because they are not affected (to a degree) by dirty water or fl oating solids. FLOW MEASURING DEVICES In recent decades, fl ow measurement technology has evolved rapidly from the “old-fashioned way” of measuring fl ow to the use of simple practical measuring devices too much more sophisticated devices. Physical phenomena discovered centuries ago have been the starting point for many of the viable fl owmeter designs used today. Moreover, the recent technology explosion has enabled fl owmeters to handle many more applications than could have been imagined centuries ago. Before selecting a particular type of fl ow measurement device, Kawamura (2000) recommends consideration of several questions. 1. Is liquid or gas fl ow being measured? 2. Is the fl ow occurring in a pipe or in an open channel?
82 The Science of Water: Concepts and Applications 3. What is the magnitude of the fl ow rate? 4. What is the range of fl ow variation? 5. Is the liquid being measured clean, or does it contain suspended solids or air bubbles? 6. What is the accuracy requirement? 7. What is the allowable head loss by the fl owmeter? 8. Is the fl ow corrosive? 9. What types of fl owmeters are available to the region? 10. What types of postinstallation service are available to the area? DIFFERENTIAL PRESSURE FLOWMETERS Kawamura (2000) points out that for many years differential pressure fl owmeters have been the most widely applied fl ow-measuring device for water fl ow in pipes that require accurate measurement at reasonable cost. The differential pressure type of fl owmeter makes up the largest segment of the total fl ow-measurement devices currently being used. Differential pressure-producing meters currently on the market are the Venturi, Dall type, Herschel Venturi, universal Venturi, and venture inserts. The differential pressure-producing device has a fl ow restriction in the line that causes a differential pressure or “head” to be developed between the two measurement locations. Differential pressure fl owmeters are also known as head meters, and, of all the head meters, the orifi ce fl owmeter is the most widely applied device. The advantages of differential pressure fl owmeters include the following: • • • • • • • • • • Simple construction Relatively inexpensive No moving parts Transmitting instruments are external Low maintenance Wide application of fl owing fl uid; suitable for measuring both gas and liquid fl ow Ease of instrument and range selection Extensive product experience and performance database The disadvantages include the following: Flow rate is a nonlinear function of the differential pressure Low fl ow rate rangeability with normal instrumentation OPERATING PRINCIPLE Differential pressure fl owmeters operate on the principle of measuring pressure at two points in the fl ow, which provides an indication of the rate of fl ow that is passing by. The difference in pressures between the two measurement locations of the fl owmeter is the result of the change in fl ow velocities. Simply, there is a set relationship between the fl ow rate and volume, so the meter instrumentation automatically translates the differential pressure into a volume of fl ow. The volume of fl ow rate through the cross-sectional area is given by where Q = volumetric fl ow rate A = fl ow in the cross-sectional area V = average fl uid velocity Q = AV (average)
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Second Edition THE SCIENCE OF WATER
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Cover Image: Falling Spring at Fall
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Contents Preface ..................
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Contents ix Velocity Head .........
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Contents xi Specifi c Conductance .
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Contents xiii (5) Beetles (Order: C
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Contents xv Chlorine Residual Testi
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Contents xvii Water Filtration Calc
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To the Reader In reading this text,
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1 Introduction When color photograp
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Introduction 3 On second look, we s
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Introduction 5 As mentioned earlier
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Introduction 7 This scream of lost
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Introduction 9 Metcalf & Eddy, Inc.
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12 The Science of Water: Concepts a
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Page 76 and 77: Water Hydraulics 51 √ Important P
Page 78 and 79: Water Hydraulics 53 FIGURE 3.4 Thru
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Page 96 and 97: Water Hydraulics 71 The Darcy-Weisb
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Page 100 and 101: Water Hydraulics 75 In this section
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Page 108 and 109: Water Hydraulics 83 TYPES OF DIFFER
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Page 112 and 113: Water Hydraulics 87 Meter backpress
Page 114 and 115: Water Hydraulics 89 VELOCITY FLOWME
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Page 118 and 119: Water Hydraulics 93 Solution: 1200g
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132 The Science of Water: Concepts
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6 Water Ecology The subject of man
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Water Ecology 157 The environment i
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Water Ecology 159 FIGURE 6.2 Notone
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Water Ecology 161 H 2 O O 2 FIGURE
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Water Ecology 163 FeS FIGURE 6.6 Th
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Water Ecology 165 Algae FIGURE 6.8
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Water Ecology 167 2. Likewise, biom
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Water Ecology 169 Population densit
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Water Ecology 171 succession can be
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Water Ecology 173 dark winter month
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Water Ecology 175 In rain events wh
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Water Ecology 177 long profi le, cr
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Water Ecology 179 THE FLOODPLAIN A
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Water Ecology 181 It is important t
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Water Ecology 183 Specifi c Adaptat
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Water Ecology 185 serve to indicate
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Water Ecology 187 UNITS OF ORGANIZA
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Water Ecology 189 FIGURE 6.20 Stone
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Water Ecology 191 FIGURE 6.22 Midge
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Water Ecology 193 FIGURE 6.25 Riffl
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Water Ecology 195 They push their m
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Water Ecology 197 FIGURE 6.32 Damse
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Water Ecology 199 Darwin, C., 1998.
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10 Water Treatment Calculations Gup
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