flow measurement 1. 2 objective to determine chemical dosage, air supply into the aeration basins,...
TRANSCRIPT
Flow Measurement
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Flow Measurement
ObjectiveTo determine chemical dosage, air supply into the aeration basins, sludge volume to return into the biological reactors, to provide daily flow records required by regulatory agencies, and to evaluate infiltration/inflow during wet weather
LocationsWithin an interceptor or manholeAt the head of the plantDownstream of bar screen, grit channel, or primary
sedimentationIn the force main of pumping stationBefore the outfall
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Flow Measurement - continued
Basic types of measurement· Differential pressure producers· Direct discharge measurement· Positive volume displacement measurement· Flow velocity-area measurement
Flow meters Venturi type meter, orifice meter, propeller type meter,
magnetic flow meter, ultrasonic flow meter, vortex meter, rotameter (variable-area meter), flumes, weirs, ect.
Liquid chemical flowMeasured by positive displacement pumps (or
rotameters)
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Flow Measurement - continued
Selection Criteria Type of application: open channel/closed conduits Proper sizing: range of flow Fluid composition: compatibility, solids, passage Accuracy (±%) and repeatability Headloss or hydraulic head available Installation requirements: straight length,
accessibility, disconnection method Operating environment: explosion proof, resistance
to moisture and corrosive gases, temp. range Ease of maintenance: provision for flushing/rodding Cost Type and accessibility of the conduit
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Flow Metering Devices in Wastewater Treatment Facilities
Raw Primary SecondaryPrimary Return ThickenedMixedProcess
Metering device WW effluent effluent sludge sludge sludge liquorwater
For open channelsHead/area Flume x x x
x Weir x x
xOther Magnetic (insert type)
xFor closed conduitsHead/pressure Flow tube xa xa x xa xa xa,b x
Orificex
Pitot tubex
Rotameter x Venturi xa xa x xa xa xa xMoving fluid effects Magnetic (tube type)_ x x x x x x
x Ultrasonic (doppler) x x x xc
Ultrasonic (transmission) x xx
Vortex shedding x xx
Positive displacement Propeller
x Turbine x
x
a Flushing or diaphragm sealed connections recommendedb Use with in-line reciprocating pumps not recommendedc Solids content < 4%
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Venturi Type Flow Meter· Measure differential pressure· Consists of a converging section, a throat, and a
diverging recovery section· The difference in two heads is analyzed by electrical or
electromechanical instruments· Accuracy: ±1%; range: 4:1· Take considerable space (L/D = 5~20)· Cannot be altered for measuring pressure beyond a
maximum velocity
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Flow Nozzle Meter· Measure differential pressure· A Venturi meter without the diverging recovery section· Less expensive than Venturi meter but higher headloss· Accuracy: < ±1%; range: 4:1
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Orifice Meter· Measure differential pressure· Easy to install and fabricate· Advantages: least expensive of all differential pressure
devices and good accuracy (±1%)· Disadvantages: least efficient, high headloss, easy
clogging, and narrow range of flows (4:1)
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Electromagnetic Meter· Faraday’s law: a voltage
produced by passing a conductor through a magnetic field is proportional to the velocity of the conductor (wastewater)· Advantages: good accuracy
(±1~2%), capable of measuring large range of flows (10:1), no headloss, and unaffected by temperature, conductivity, viscosity, turbulance, and suspended solids· Disadvantages: high initial cost
and need for trained personnel to handle routine O&M
1010
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Turbine Meter· Use a rotating element
(turbine) · A wide range of fluid
applications covering from water to oils, solvents to acids· Limited to pipes running
full, under pressure, and liquids low in suspended solids· Excellent accuracy
(±0.25%) and a good range of flows (10:1)
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Acoustic Meter· Use sound waves to measure
the flow rates· Sonic meter or ultrasonic
meter depending on whether the sound waves are in or above audible frequency range· Determine the liquid levels,
area, and actual velocity· Advantages: low headloss,
excellent accuracy (2~3%), usable in any pipe size, no fouling with solids, and wide flow ranges (10:1)· Disadvantages: High initial
cost and need for trained personnel to handle routine O&M
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Parshall Flume· Consists of a converging
section, a throat, and a diverging section· Self-cleaning and small
headloss· Converts depth readings to
discharge using a calibration curve
· Less accurate (±5~10%)· Range: 10:1 ~ 75:1
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Palmer-Bowlus Flume
· Creates a change in the flow pattern by decreasing the width of the channel without changing its slope.· Installed in a sewer at a manhole which causes the back-up
of the water in the channel. By measuring the upstream depth, the discharge is read from a calibration curve.· Lower headloss than the Parshall flume· Less accurate (±5~10%)
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Weirs (Rectangular, Cipolletti, Triangular, or V-Notch)
· The head over the weir is measured by a float, hook gauge, or level sensor · Measure the flow in open channels
Accuracy: ±5%; Range: 500:1· Advantages: relatively
accurate, simple to install, and inexpensive
· Disadvantages: large amounts of headloss and settling of solids upstream of the
weir and more maintenance
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Ultrasonic Meter· Measured based on the time
required for an ultrasonic pulse to diagonally traverse a pipe or channel against the liquid flow.· Clamp-on types measure flow
through the pipe without any wetted parts, ensuring that corrosion and other effects from the fluid will not deteriorate the sensors. · Accuracy: ± 1% for a flow
velocity ranging from 1 to 106 ft/sec. Should be free of particles and air bubbles.
http://www.sensorsmag.com/articles/1097/flow1097/main.shtml
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Vortex Meter
· The frequency at which the vortices are generated is proportional to the velocity of the liquid flow.
·Accuracy: ± 1% for a flow range of 12 to 1.·Headloss: two times the
velocity head
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Rotameters· Consist of glass tube
containing a freely moving float.
·May be used for both gas and liquid flow measurement
· Read or measured visually only
·May be applied for very low flow rates, 0.1~140 gph for water and 1~520 scfm for air.
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Selection Guide (1)
Flow Meter
RecommendedService Turndown
TypicalPressure
Loss
TypicalAccuracy
Required upstreampipe, Ф
Effects from
changing viscosity?
Turbine Clean, viscous liquids 20 to 1 High +/- 0.25%
of rate 5 to 10 High
PositiveDisplacement
Clean, viscous liquids 10 to 1 High +/- 0.5% of
rate None High
Electromagnetic(Mag-Meter)
Clean, dirty, viscous, conductive liquids and slurries
40 to 1 None +/- 0.5% of rate 5 None
Variable Area (VA, Rota-
meter)
Clean, dirty, viscous liquids 10 to 1 Medium +/- 1 to
10% FS None Medium
Thermal Mass Flow (TMF)
Clean dirty viscous liquids some
slurries10 to 1 Low +/- 1% FS None None
Coriolis Mass Meter
Clean, dirty. viscous liquids, some
slurries10 to 1 Low +/- 0.5% of
rate None None
Orifice Plate Clean, dirty, liquids some slurries 4 to 1 Some +/- 2 to 4%
FS 10 to 20 High
FS=full scale http://www.buygpi.com/selectionguide.aspx
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Selection Guide (2)
Flow Meter
RecommendedService Turndown
TypicalPressure
Loss
TypicalAccuracy
Required Upstreampipe, Ф
Effects from
changing viscosity?
Pitot tube Clean liquids 3 to 1 Very low+/- 3 to 5%
FS 20 to 30 Low
Ultrasonic(Doppler)
Dirty, viscous, liquids and slurries 10 to 1 None +/- 5% FS 5 to 30 None
Ultrasonic(Transit Time)
Clean, viscous, liquids some dirty liquids
(depending on brand)40 to 1 None +/- 1 to 3%
FS10 None
Venturi Some slurries but clean, dirty liquids with high viscosity
4 to 1 A little +/- 1% FS 5 to 18 High
Vortex Clean, dirty liquids 10 to 1 Medium+/- 1% of
rate 10 to 20 Medium
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Flow SensorsSensor Range Accuracy Advantages Disadvantages
Orifice 3.5:1 2-4% of full spanLow costExtensive industrial practice
High pressure lossPlugging with slurries
Venturi 3.5:1 1% of full spanLower pressure loss than orifice
Slurries do not plug
High costLine under 15 cm
Flow nozzle 3.5:1 2% full spanGood for slurry serviceIntermediate pressure loss
Higher cost than orifice plateLimited pipe sizes
Elbow meter 3:15-10% of full
spanLow pressure loss Very poor accuracy
Annubar(Pitot tube)
3:10.5-1.5% of full
spanLow pressure lossLarge pipe diameters
Poor performance with dirty or sticky fluids
Turbine 20:10.25% of
measurementWide rangeabilityGood accuracy
High costStrainer needed, especially for slurries
Vortex shedding
10:11% of
measurement
Wide rangeabilityInsensitive to variations in density, temperature, pressure, and viscosity
Expensive
Positive displacement
10:1 or greater
0.5% of measurement
High reangeabilityGood accuracy
High pressure dropDamaged by flow surge or solids
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Checklist for Design ofFlow-Measuring Device
· Characteristics of the liquid (SS, density, temp., pressure, etc.)· Expected flow range (max. and min.)· Accuracy desired· Any constraints imposed by regulatory agencies· Location of flow measurement device and piping system
(force main, sewer, manhole, channel, or treatment unit)· Atmosphere of installation (indoors, outdoors, corrosive, hot,
cold, wet, dry, etc.)· Headloss constraints· Type of secondary elements (level sensors, pressure sensors,
transmitters, and recorders)· Space limitations and size of device· Compatibility with other flow measurement devices if already
in operation at the existing portion of the treatment facility· Equipment manufacturers and equipment selection guide
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Design ExampleConditions 92-cm (36-inch) force main Max. flow: 1.321; min. flow: 0.152 m3/sec Measurement error: < 0.75% at all flows Headloss: < 15% of the meter readings at all flows Capable of measuring flows of solids bearing liquid Reasonable costSelect a Venturi meterDesign equation
Use Bernoulli energy equation for two sections of pipe with the assumption that the headloss is negligible and the elevations of the pipe centerline are the same.
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Governing Equations Bernoulli’s equation
[Pressure head]+[Elevation head]+[Velocity head]
where P = pressure, m; ρ = density, kg/m3; z = elevation, m; v = velocity (m/sec), and g = 9.8 m/sec2.
Continuity equation
Q = v1 A1 = v2 A2
where A = cross-sectional area.
0 0
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Design Example - continued
where Q = pipe flow, m3/sec;C1 = velocity, friction, or discharge coefficient
h = piezometric head difference, m;A1 = force main cross-sectional area, m2;A2 = throat cross-sectional area, m2; andD1 and D2 = diameter of the pipe and the throat, m.
Standard Venturi meterTube beta ratio (throat /force main ): 1/3~1/2K = 1.0062 (1/3 beta ratio), 1.0328 (1/2 beta ratio)C1 = 0.97~0.99; normally provided by the manufacturer
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Design Example - continued
Develop calibration equation:Assume C1 = 0.985
= 0.7489 h m3/sech = (Q/0.7489)2
At Qmax, h = 3.111 m; at Qmin, h = 0.041 m
Headloss calculationsK = 0.14 for angles of divergence of 5°
hL/h = 0.147 < 0.15; thus acceptable