(1.) flow measurement lecture

7/23/2019 (1.) Flow Measurement Lecture

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CHE

215

ProcessMeasurements

Winter

2014


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Many devices are available to measure flow.

The application depends on the physical properties of the

material to be measured, the precision required, cost,

and the type of signal required from the device.

Fluid

Flow

Measurement

Several types will be considered here:

1. Pitot tube

2. Venturi meter

3. Orifice meter

4. Flow nozzle

5. Variable area meter rotameter

6. Ultrasonic7. Thermal

8. Turbine

9. Magnetic10. Vortex

11.Weirs


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Advantages:

1. Cheap

2. Portable3. Velocity profile determination.

4. Very little power loss.

Disadvantages:

1. Average velocity not read directly.

2. Very low readings for gases.3. Can only be used for clean streams.

4. A computer signal not readily available.

Fluid

Flow

Measurement


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22

22

11

2

22

1 zg

gP

g

Vz

g

gP

g

V

cccc

cg

VVPP

2

21

22

21

=

Bernoulliequationforincompressiblefluid

1

221A

AVV

=

cg

A

AV

PP2

1

2

1

222

21

=

Fluid

Flow

Measurement

Therelationshipbetweenthefluidvelocityatpoints1and2

Fromtheequationofcontinuityforisothermalcondition(constant):

Substitution of Equation (3) in to Equation (2):

(1)

(2)

(3)

(4)

Thefluidvolumetricflowrateis(idealcondition)

21

21c

21

2

12

2 PPg2

A

A1

A=Q

(5)


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Under practical conditions (some friction loss),

a coefficient of discharge (or calibration coefficient), Co,which is a function of the ratio of the venturi throat

diameter to that of the pipe and the Reynolds number,

is added to the equation of the fluid volumetric flowrate

as below:

Fluid

Flow

Measurement

21

21c2

12

1

2

2o

PPg2

A

A1

AC=Q

LetM,velocityapproachfactor,2

1

21

1

=

A

A

21

21c2o

PPg2C=Q

AM

(6)

(7)

(8)

Co : discharge coefficient

Co is a function of the

diameter ratio () andReynolds number


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Fluid

Flow

Measurement

Venturi Tubecompressiblefluids:

Foragasflow(compressiblefluid)passingaventuri flowmeter.

Thevolumetricflowrate canbedetermined

2

1

21c2o

PPg2CY=Q

AM

WherethegasexpansionfactorYcanbeestimatedfromthe

followingequations

50

2

1

2

2

1

2

2

1

2

1

2

1

1

22

1

2

1

1

1

1

1

.

P

P

A

A

A

A

P

P

P

P

P

PY

=

: heat capacity ratio

r: P2/P1

Y: expansion factor

Where


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Fluid

Flow

Measurement

Venturi Tubecompressiblefluids


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(1.) flow measurement lecture

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