Flow Measurement

1

2

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

3

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)

4

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

5

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%

5

6

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

7

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

8

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)

9

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

11

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)

12

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

13

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

14

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%)

15

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

16

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

17

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

18

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.

19

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

20

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

21

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

22

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

23

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.

24

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

25

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

26

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