vol flow measurements

8/2/2019 Vol Flow Measurements

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Volume Flow Measurements


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Obstruction Meters

Orifice Meters

Venturi Meters

Flow Nozzles


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Flow through a Nozzle

2211

21

222111

21

vAvA

ibleincompressvAvA

Avmmm

1

1

1

1

v

A

m

2

2

2

2

v

A

m

Basic Equations:

a.) Continuity:

mass in = mass out

b.) Bernoullis Eqn.

Total pressure is

constant throughout


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pressuredynamicv2

1

pressuretotalPpressurestaticP

PPv2

1Pv

2

1

Pv2

1

.constessurePrTotalPBernoulli

2

0

02

2

221

2

11

2

0



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P2

A

A1

1YCAQ

FlowalReFor

IdealP2

A

A1

1AvAQ

RateFlow

P2

A

A1

1v

2

1

2

2

2

1

2

222

2

1

2

2



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P21

2

1

22

2

2

2

2

1

21

2

22

2

11

2

2221

12

1

2

1

2

1

2

1

2

1

when

A

Av

vA

Av

vvPPP



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Y = Compressibility Factor

=1 for incompressible flow

or when P


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Flow through a Venturi Meter

In a venturi, 0.95 < C < 0.98

Advantage:

Pressure recovery

Uses little power


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Flow through a Venturi Meter

C

Re

2 x 105

11

vd

eR

0.98

Based upon the conditions in the pipe

approaching the meter


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Back to the NozzleP1 P2

P

P1

P2


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Shorter and cheaper than venturi

But larger pressure drop.

Thus, more power lost in operating.

The Nozzle Flowmeter

C

0.86

0.98

103105

Re


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Flow Through an Orifice Meter


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P1

P2

P

P1

d D


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-Cheapest and Simplest-But biggest pressure drop and power lost (C~0.6 - 0.7)

-Side Note:

Pressure drop caused by friction and turbulence of shear

layer downstream of vena contracta

CM

A

A

C

2

1

21

1

0.6

0.85

b=d/D0.1 0.8

Re

100k

5000

10k


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Elbow Flowmeter


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Laminar

Flowmeter


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Pitot-Static Tubes


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Rotameter, variable-area-flowmeter

Force balanceDrag Force

Gravity

Buoyancy

(usually negligible)

cal

use

calusemm

Derived on next

slide


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D

MgA

D

MgAm

D

MgV

MgV

DF

VAm

2

2

22

2

2

For a fixed x-position, A is fixed. Then

cal

use

caluse

caluse

mm

mm

Rotameter

Equations


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Turbine Flow Meters


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Vortex meters operate on the principle that when a non-streamlined

object is placed in the middle of a flow stream, vortices are shed

alternately downstream of the object. The frequency of the vortex

shedding is directly proportional to the velocity of the liquidflowing in the pipeline.


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Magnetic flowmeter


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Based upon Faradays Law

The fluid is the conductor, must be electrically conductive.

E=BDVx10-8

E=voltage, volts

B=magnetic flux density, gauss

D= length of the conductor, cm

V=velocity of the conductor, cm/sec

Magnetic flowmeter


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Magnetic Flow

Meter


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Coriolis Mass Flow Meter


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Vibrating Flow TubeFluid Force is Reacting to Vibration

of Flow Tube

End View of Flow Tube

Showing Twist

Twist Angle

Twist Angle

Flow

Force

Flow Force

Flow

Flow


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Positive

Displacement

Meters


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vol flow measurements

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