CE 547 - Flow Measurement and Screens

7/28/2019 CE 547 - Flow Measurement and Screens

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Flow Measurementand Screens

CE 547


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Flow Meters

Flow Meters: are devices used to measure theflow rate of a fluid

In Water, all types of flow meters can be used

In Wastewater, the choice is critical due to solidcontent:

Solids can be removed

Flow has enough energy to be self-cleaning


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Rectangular Weirs

Fully-contracted weir


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Suppressed weir: weir extends to the channelvertical sides


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P = weir height

H = head over the weir


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The energy equation between (1) and (2)

V = velocity (average) P = pressure y = height above bottom of channel Z = height of bottom above a datum

hl = head loss between (1) and (2) g = gravitational constant = specific weight of water

22222

11121

22ZyP

gVhZyP

gV

l


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In the figure:

Z1 = Z2 = 0

V2 >> V1P1 = P2 = atmospheric pressure

If hl was neglected, then:

y1 = H + P

y2 = yc + P

Substitute in the energy equation and change V2 to Vc

(critical velocity)

)(2

cc

yHZgV


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Specific energy equation states that:

The critical depth, yc, occurs at minimum specificenergy

Differentiate E with respect to y and equate to zero

Use ( Q = VA ) Q = flow rate

V = velocity

A = cross-sectional area

gVyE2

2


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A/T = hydraulic depth, D

D is simply equals to yc

dy

dAT

gD

V

TgA

V

TgA

V

gA

TQ

1

/1

2

3

2

c

c

gyV


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Substitute for yc in:

To get:

)(22 cc yHgV

gHVc 23

1


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If L = length of the weir, then

and

Use

cyLA

ccc

yLVAVQ

32385.0

23

1

1

HLgQ

then

VforgHV

and

yforgy

V

cc

c

c

c


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Remember

hl and V1 were neglected

y2 was assumed to be (yc = P)

L must be corrected depending upon whetherthe equation to be used for fully contracted or

suppressed weirs


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To make the equation more practical

For fully contracted weirs

P

HK

PHfor

HLgKQ

05.040.0

10

2 3

HLLweircontractedfully

2.0


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Example


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To measure the flow rate of wastewater, arectangular weir was used. The flow rate is 0.33

m3/s. Design the weir. The width of therectangular channel to be connected to the weir is2.0 m and the available head (H) is 0.2 m.

Solution

Use a fully suppressed weir and assume length,

L = 2.0 m


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Then, the dimensions of the weir are: L = 2.0 m

P = 0.6 m

mP

PP

HK

KK

HLgKQ

6.0

2.005.040.005.040.0417.0

792.0)2.0()2()81.9(233.0

2

3

3


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Triangular Weir (V-notch weir)


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For low flow rates, triangular weirs are moreaccurate than the rectangular ones.

The hydraulic profile in channels measured bytriangular weirs is exactly similar to that measuredby rectangular weirs

K is obtained from the Figure 3.4 and multipliedby (8/15) as a correction factor.

25

25

2

22tan

22

tan525

162

tan

HgKQ

HgQ

yA c


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Example


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Solve previous example for v-notch weir if:

Q = 0.33 m3/s

Channel width = 2.0 m

H = 0.2 m

Solution

16.4

2

tan

15

8

2.02

tan15

833.0

22

tan15

8

25

25

K

K

HgKQ


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From Figure 3.4 at H = 0.2 m

Values of [ K(8/15) tan (/2) ],in the table, is near 4.16

For > 90 , K = 0.58

then,

4.16 = 0.58 (8/15) tan (/2)tan (/2) = 13.45

so, = 171

K K(8/15) tan (/2)

90 0.583 0.31

60 0.588 0.18

45 0.592 0.13

20 0.609 0.06


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Trapezoidal Weirs

Flow is contracted in trapezoidalweirs

The equation for suppressed weirscan be used:

In this case = 28

32 HLgKQ


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Venturi Meters

Used to measure flow rate in pipes


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21

212

2

2

1

2

2211

21

2

22

2

11

2

1

2

4/4/

sin

22

PPgKAQ

D

d

where

PPgV

VdVD

VAVA

QQce

g

VP

g

VP

t


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Example


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Parshall Flumes


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Can be used with Parshall Flumes

Replace L with W (width of throat)

Replace H with Ha (water surface elevation above flume floor

level in the converging zone)

Then,

K can be obtained from Figure 3.7. Also Table 3.1 showsstandard Parshall flume dimensions.

32385.0 HLgQ

32 aHWgKQ


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Example


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Miscellaneous Flow Meters

Magnetic Flow Meter

(measures flow by producing

magnetic fields)


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What is a Magnetic Flow Meter?

A magnetic flow meter (magnetic flow meter) is avolumetric flow meter which does not have anymoving parts and is ideal for wastewater

applications or any dirty liquid which is conductiveor water based. Magnetic flow meters will generallynot work with hydrocarbons, distilled water andmany non-aqueous solutions). Magnetic flowmeters are also ideal for applications where lowpressure drop and low maintenance are required.


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Principle of Operation

The operation of a magnetic flowmeter or mag meter is

based upon Faraday's Law, which states that the voltageinduced across any conductor as it moves at right anglesthrough a magnetic field is proportional to the velocityof that conductor.

Faraday's Formula:

E is proportional to V B D where:

E = The voltage generated in a conductor

V = The velocity of the conductor

B = The magnetic field strength

D = The length of the conductor


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To apply this principle to flow measurement with a magneticflowmeter, it is necessary first to state that the fluid beingmeasured must be electrically conductive for the Faraday

principle to apply. As applied to the design of magneticflowmeters, Faraday's Law indicates that signal voltage (E) isdependent on the average liquid velocity (V) the magnetic fieldstrength (B) and the length of the conductor (D) (which in thisinstance is the distance between the electrodes).In the case of

wafer-style magnetic flowmeters, a magnetic field is establishedthroughout the entire cross-section of the flow tube (Figure 1). Ifthis magnetic field is considered as the measuring element of themagnetic flowmeter, it can be seen that the measuring element isexposed to the hydraulic conditions throughout the entire cross-section of the flowmeter. With insertion-style flowmeters, the

magnetic field radiates outward from the inserted probe (Figure2).


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Magnetic Meter Selection

The key questions which need to be answered before

selecting a magnetic flowmeter are: Is the fluid conductive or water based? Is the fluid or slurry abrasive? Do you require an integral display or remote display?

Do you require an analog output? What is the minimum and maximum flow rate for the

flow meter? What is the minimum and maximum process pressure? What is the minimum and maximum process

temperature? Is the fluid chemically compatible with the flow meter

wetted parts? What is the size of the pipe? Is the pipe always full?


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Turbine Flow Meters


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Rotameters or Variable Area Flow Meters

Variable area flow meters, orrotameters, use a tube andfloat to measure flow. As thefluid flows through the tube,

the float rises. Equilibriumwill be reached whenpressure and the buoyancy ofthe float counterbalancegravity. The float's height in

the tube is then used toreference a flow rate on acalibrated measurementreference.


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Important Information on Rotameters

The Variable-Area type flowmeter, or Rotameter, is oneof the most economical and reliable of flowmeasurement instruments. In various configurations itcan be designed to withstand high pres sures, corrosive

fluids, high temperatures, and is completelyindependent of factors influencing electronic meters.

They can be calibrated to measure nearly any gas orliquid, because their principles of operation are simple

and well understood. The flow indication is obtainedfrom a balance of the fluid forces underneath the float

with gravity.


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This is done using a uniformly tapered tube, a floatwhose diameter is nearly identical to the tube ID at theinlet, and a scale to correlate float height. The flow tube

is traditionally placed in a vertical position and fluidenters from the bottom, forcing the float up in the tubeuntil a sufficient annular opening exists between thefloat and tube to allow the total volume of fluid to flow

past the float. At this point the float is in an equilibriumposition and its height is proportional to the flow rate.


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With this in mind, many simple factors influencingrotameter performance are easily understood. Forexample, increasing the density and weight of the float

will require a higher flow rate to force the ball up to anyheight in the tube. In addition, it is easy to see that anychanges in the fluid caused by temperature or pressure

will affect the float's position. This is particularly true

for gases which are compressible, and are therefore,greatly affected by operating pressures. Studies over theyears have resulted in many


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Screening


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Is a unit operation that separates materials intodifferent sizes using screens

Bar Racks or Bar Screen (Fig 5.1)

Are composed of large bars spaced at 2580 mmapart

Used to exclude large particles

Used in water intakes at shores and wastewater

treatment plants Hand cleaned or mechanically cleaned


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Traveling Screens (Fig 5.2)

Used to remove smaller particles in watertreatment plants (following bar screens) such asleaves, small fish and other materials that pass the

bar screen.

Mi t i (Fi 5 3)


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Micro-strainer (Fig 5.3)

Made of very fine fabric or screen wound around a drum

75% of the drum is submerged Rotates at 5 to 45 rpm

Influent is introduced from the underside of the drum andexits into the outside

Strained materials (solids) are retained inside of the drum andremoved by jets of water through a trough inside the drum

Flow of influent is sometimes from the outside to the inside

Used to remove high concentrations of algae (effluent from

stabilization ponds) or treatment of effluents from biologicaltreatment processes

The pore size of micro-strainers range between 2060 m

Material used in micro-strainers include stainless steel andpolyester


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Head Loss in Bar Racks

Apply Bernoulli equation(Fig 5.2)

P = pressure

V = velocity (V1 = approachvelocity)

h = elevation head

g = acceleration due to gravity

2

2

221

2

11

22hg

VPhg

VP


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Approach velocity should be maintained at self-

cleaning velocity ( 0.76 m/s)

Since, P1 = P2 = atmospheric pressure

Then, From continuity equation

122

2

2

1 2 hhgVV

1

22

1

2

12222

1

221

1

2

1

2

A

A

hgA

A

A

hhgAVAQ

thus

A

VAV

B lli i f i i l fl


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Bernoulli equation assumes frictionless flow, to correctfor this, a coefficient of discharge must be added to theequation, thus:

Solve for h

Cd is determined experimentally or a value of 0.84 may beused. As the screen clogs, the value of A2 will decrease.

1

22

1

2

A

A

hgACQ d

22

2

1

22

2

1

AgC

A

AQ

hd


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Head Loss in Micro-strainers

The flow turns at right angle (90) as it enters the openingsof the micro-strainer cloth. Therefore, the approach

velocity (V1) is equal to ZERO. Thus:

Similarly, Cd can be determined experimentally or a valueof 0.60 can be used. The above equation can be applied toscreens where the approach velocity is negligible.

2

2

2

2

2 AgC

Qh

d

D i P t d C it i f


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Design Parameters and Criteria for

Bar Screens

Parameter MechanicallyCleaned ManuallyCleaned

Bar Size

Width, mm

Thickness, mm

520

20

80520

20

80

Bar Clear Spacing, mm 2050 1580Slope from Vertical, degree 3045 030Approach Velocity, m/s 0.30.6 0.61.0


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CE 547 - Flow Measurement and Screens

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