flow measurement pdf

Flow MeasurementsTypes of flow encountered:

• Clean or dirty

• Wet or dry

• Hazardous/corrosive or safe

• Single or multi phase

• Laminar of turbulent

• Varying pressure

Contents provided in this presentation are for reference purpose & the detailed theory to be read/written from respective text book/Class Notes

• Varying pressure

• Varying temperature

• Flow rates from least to highest

Types of flow measurements:

A. Primary or quantity meters

B. Positive displacement meters

C. Secondary of rate meters

D. Special methods

Flow MeasurementsA. Primary or quantity meters:

Signifies the amount of fluid in terms of mass or volume

that flows past a given point in a definite period of time.

• Simple

• Economical

• Accurate

• Routinely used in the calibration of other flow measuring

devices


devices

1. Volumetric method:

Fluid flowing in the flow meter is diverted into a tank of

known volume. When the tank is completely filled, then

the known volume is compared with the integrated,

volumetric quantity registered by the flow meter under

test.

Flow Measurements2. Gravimetric method:

Fluid flowing in the flow meter is diverted into a vessel

which can be weighted either continuously or in the

vessel after a pre-determined time. The weight of the

liquid collected is compared with the gravimetric quantity

registered by the flow meter under test.

B. Positive Displacement Meters:

Liquid flows through a meter and moves the measuring


Liquid flows through a meter and moves the measuring

element that seals the measuring chamber into a series of

measuring compartments each holding a definite volume.

Each element is successively filled from the flow at the

inlet and emptied at the outlet of the meter. In other

words these meters chop the flow into number of pieces

of known size and then count the number of pieces.

Flow MeasurementsFeatures:

• Widely used in low flow rate metering applications where

high accuracy & repeatability are required.

• Easy to install

• Moderate cost

• Due to moving parts involved, wear of the components may

alter the accuracy.

• Suitable to clean fluids only.


• Suitable to clean fluids only.

• These devices are flow totalizes and generally and do not

give the instantaneous rate of flow.

• Cause pressure loss in the fluid system whose flow is being

metered.

Flow Measurements

1. Nutating Disc Meters:


Flow MeasurementsWorking:

• Liquid enters the left side of the meter, alternately above &

below the disc, forcing it to rock in a circular path while

rotating about its own axis.

• A small spindle attached to the sphere traces a circular path

and is used to drive the counter, which can be calibrated in

terms of liquid discharge.

Features:


Features:

• Simple & rugged construction

• Low pressure drop

• Low cost

• High accuracy of the order of ±1%.

• Widely used as a domestic water meter

Flow Measurements

2. Sliding Vane Type Meters:



• Liquid entering at inlet revolves the rotor and the vanes

around cam causing the vanes to move radially.

• Vane nearest to the inlet port begins to move outwards &

becomes fully extended at point A. While vane ahead at

point B is already fully extended, so a measuring chamber

of known volume is formed.

• A continuous series of chambers at the rate of four per


• A continuous series of chambers at the rate of four per

revolution are formed.

Features:

• Low pressure drop

• High accuracy of the order of ±0.2% of measured values.

• Can be used as a positive displacement pump which can

cause the flow and measure it simultaneously.

Flow Measurements

3. Lobed Impeller Meters:



• The incoming fluid is trapped between the two rotors and is

conveyed to outlet as a result of the rotor rotations.

• For every rotation of each rotor, the swept volume

corresponding to twice the area A is passed through meter.

• Number of revolutions of the rotor gives and indication of

volumetric flow.

• Rotation of lobed impellers can be monitored by magnetic


• Rotation of lobed impellers can be monitored by magnetic

or photo-electric pick up.

Features:

• Available for pressures up to 60 atm and flow range 10 to

10,000 m3/hr.

• High accuracy of the order of ±1% of rated capacity.

• Low pressure drop of the order of 50 mm of Hg.

• Relatively expensive.

• Applicable to clean fluids like gas metering.

Flow MeasurementsC. Secondary or Rate Meters:

• Also termed as inferential type of flow measuring devices.

• They do not measure flow directly but instead measure

another physical quantity which is related to flow.

• Two categories:

� Flow rate meters

� Velocity meters or Anemometers


Flow Measurements

1. Variable Head Meters:

Features:

• Generally termed as obstruction type flow meters.

• The main forms of obstructions are: Venturi, orifice plate

and nozzle.

• The position of minimum pressure is located slightly

downstream from the restriction at a point where the


downstream from the restriction at a point where the

stream is narrowest and is called vena- contracta.

• Beyond this point the pressure the pressure again rises but

does not return to upstream value and thus there is

permanent pressure loss.

• No moving parts & so require no maintenance practically.

• Major disadvantage is square root relationship between

flow and head.

• Not suitable for flow below 20% of rated meter capacity.

Flow Measurementsa) Venturi Meter:


Flow Measurementsb) Orifice Meter:


Flow Measurementsc) Nozzle Meter:


Flow Measurements

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Flow MeasurementsCoefficient of discharge depends on:

• Type of flow

• Obstruction type configuration

• Reynolds number

Venturimeter Features:

• Best accuracy

• Least head loss as compared to orifice & nozzle meters


• Least head loss as compared to orifice & nozzle meters

• Due to low value of losses coefficient of discharge is high

• Can be used with dirty fluids also

• Relatively expensive

• Occupies substantial space

Flow MeasurementsNozzle Flow meter Features:

• Offers all the advantages of venturimeter but to a lesser

extent

• Occupies less space

• Nozzle is difficult to install and the installation is limited to

small to moderate pipe sizes

Orifice Flow meter Features:


Orifice Flow meter Features:

• Simple geometry, so cost is low

• Easy to install

• Takes almost no space

• Susceptible to inaccuracies due to erosion, corrosion,

clogging etc.

• Suffers from a head loss of the order of 30-40%

Flow Measurements2. Variable Area Meters: Features:

• In variable area meters the

area of restriction can be

altered to maintain the steady

pressure difference.

• Common variable area meter

is rotameter.

• The float will rise to a point in

the tube where the drag force


the tube where the drag force

(Up) & Buoyant force (Up) is

balance by the weight of the

float.

• The position of the float in

the tube is taken as an

indication of flow rate.

• It is also called as variable

area flow meter.

Flow Measurements

)p-p()(

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AA

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Where

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ft

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ft

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ρ

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ρ

Flow Measurements

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11 & Eq.10 From

21

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fii

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πθ

π

θππ

Flow Measurements

Rotameter Features:

• Rotameter tube is made of high strength borosilicate glass

to allow direct observation of the float position.

• Main advantage is that they give direct visual indication on

linear scale.

• Can be used for wide ranges (from 0.1 ml/min to several

hundred lpm)


hundred lpm)

• Accuracy is generally ±1% of max flow rate, but up to ±5%

of rated capacity for very cheaper units.

• Can handle wide variety of fluids including corrosive ones.

• The disadvantage is that it can only be mounted in vertical

direction.

• Can be subjected to oscillations in pulsating flows.

Flow Measurements3. Variable Head & Variable Area flow Meters:


Flow Measurements

dyy 2gLCQ

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KK

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−=∴

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Flow Measurements

Weir type Flow meters Features:

• Variable head variable area flow meters.

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• Variable head variable area flow meters.

• Principle: If a restriction of a specified shape and form is placed

in the path of the flow, a rise in upstream liquid level occurs

which is a function of the rate of flow through the restricted

section.

• Rectangular weir suitable for large flow while triangular weir is

used for smaller flows below 50l/s.

• Simple in construction, easy to install and accurate devices.

• Main disadvantage is their non-linear characteristics.

Flow Measurements4. Linear Resistance Element flow Meters:



• Principle: Based on Hagen- Poiseuille equation for laminar flow.

Where Q is the flow rate, D is the inside diameter of the tube, L is

length of tube, µ is the coefficient of viscosity and is the

pressure drop along the length of tube.

• Used for very small flow rates of highly viscous flows.

( )21

4

128pp

L

DQ −=

µ

π

( )21 pp −


• Used for very small flow rates of highly viscous flows.

• Flow rate is directly proportional to pressure drop (Linear

characteristics)

• Main disadvantage is that metering element is subjected to

plugging if fluid is not clean.

• Relatively expensive.

• High pressure losses.

Flow Measurements5. Pitot Static Tube:



• Principle: Variable head velocity measuring device.

• Consists of two concentrically arranged tubes bent at right

angle.

• Inner tube faces the flow impingement and hence measures

static and dynamic pressures while outer tube measures the

static pressure.


( )

pressure. static is

and pressure stagnation is density, fluid is velocity,flow is V Where

2

2 :Eq sBernoulli'By

flow, fluid ssfrictionle and ibleincompress l,dimensiona one aFor

2

statstag

statstag

stagstat

pp

ppV

pVp

ρ

ρ

ρρ

−=∴

=+

Flow MeasurementsAdvantages of Pitot tube:

• Simple & low cost device

• No appreciable pressure loss

• Easy installation

• Useful in measuring flow velocities

Limitations of Pitot tube:

• Not suitable for measuring low velocities i.e. below 5 m/s

• Sensitive to misalignment of the probe with respect to free


• Sensitive to misalignment of the probe with respect to free

stream velocity.

• Not suitable for measuring highly fluctuating velocities.

• Not commonly used in industrial applications as numerous Pitot

tube traverses are required for velocity distribution data which is

quite tedious & time consuming.

Flow Measurements6. Target Flow Meter:



• Principle: The drag force onto a suitably shaped body into a flow

stream can be a measure of the flow rate after suitable

calibration.

)m(in section cross of AreaA

drag oft Coefficien Where

2

1

2

2

=

=

= ρ

d

dd

C

AgVCF


velocityFluidV

)kg/m(in density Fluid

)m(in section cross of AreaA

3

=

=

=

ρ

• Main advantage is its very good dynamic response.

• Overall accuracy is ±0.5%

• Applications: Highly viscous flows of tar, oils or slurries at high

pressures of the order of 100 bars.

Flow Measurements7. Turbine Flow Meter:



• Principle: Turbine meters are inferential measurement devices

that measure a flow rate indirectly by using the natural kinetic

energy of the flow as it passes through the angled blades of the

turbine rotor. This causes the turbine to spin and as the blades

pass by a close pre positioned magnetic (Or other

technology)“pick up” coil. The resulting interruption of the coils

magnetic field by each blade results in a pulse being produced.


magnetic field by each blade results in a pulse being produced.

The frequency of this pulse is directly proportional to the point

velocity reading it is taking. From this point velocity and cross

sectional area of the pipe a rough volumetric flow rate can be

calculated.

Q = k X n

Where k = Constant for any given meter

n = rotor angular velocity


• These meters have linear relationship between volume flow rate

and the angular velocity of the rotor.

• Accuracy of instruments is ±1%

• Commercially available for flow rates ranging from 0.5 to 150000

LPM (for liquids) and from 5 to 100000 LPM (For air).

• Bearing maintenance is a problem and accuracy drops off greatly


• Bearing maintenance is a problem and accuracy drops off greatly

at low flow rates.

Flow Measurements8. Vortex Shedding Flow Meter:



• Principle: If a bluff body or an obstruction is placed in a fluid

stream, vortices are shed alternately on each side of the bluff

body. The vortex shedding frequency of the bluff body is a

measure of the average flow velocity of the fluid flow.

• Strouhal number S governs the operation of vortex shedding

flow meter:


flow meter:

number StrouhalS

flow theof velocity AverageV

body bluff theoflength sticcharacterior Diameter D

frequency sheddingVortex Where

=

=

=

=

=

s

s

f

V

DfS

Flow Measurements• h= height of the bluff body, generally the ratio of height to the

pipe dia should not be less than 0.2

• L=Length of shedder taken approx 1.3h

• Value of Strouhal number S for triangular wedge shaped body

remains nearly constant value of 0.88±0.01 over a wide range of

Reynolds number.

1.136D)(V

88.0

s

s

for

DfV

=

=∴


)1.136D(4

Q

shedder theof downstream flow ofVelocity

shedder theof upstream flow ofVelocity Where

44Q

:as evaluated becan meter flow vortex in the rate Flow

1.136D)(V

2

22

s

d

u

du

s

fhDD

V

V

VhDDVD

for

−=∴

=

=

−==

=

π

ππ

Measurement of vortex shedding frequency is done by monitoring the alternating

strain signals from the strain gauge.

Flow Measurements

Advantages of Vortex shedding Flow meter:

• No moving parts and causes very low pressure loss

• Linear characteristics with respect to the shedding frequency

• Calibration constant is same for all fluids

• Calibration constant is not affected by the changes in the fluid

properties.

• Accuracy is of the order of ±0.5%


• Accuracy is of the order of ±0.5%

• Main disadvantage is inability to give accurate results for

Reynolds number <103 and > 107

Flow MeasurementsD. Special Methods

1. Ultrasonic Flow Meters:

Travel time difference

method (Single Transmitter-

Receiver System)

Fig.1


Travel time difference

method (Twin Transmitter-

Receiver System)

Fig.2

Flow Measurements

Oscillating Loop Method

Fig.3


fluid in the sound of

receiver &ansmitter between tr distance Where

t

:bygiven isreceiver theto

er transmitt thefrom pulse theof time transit t the velocity,flow zeroWith

0

0

VelocityV

l

V

l

s

s

=

=

=

Flow Measurements

( )

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,V gConsiderin

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>>>

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2.1or t

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==∆

==∆

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Flow Measurements

VV-tt

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Flow Measurements

time.acoustical theequals period timepath whoseeach in pulses of

train a generates This sion.retransmisfor r transduceing transmitt theback to

fedously instantane and amplified is pulse This . tafter time R transducer

receiving by the received is and Tr transduceing transmittby the emitted

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Flow Measurements

Advantages of Ultrasonic Flow meter:

• Negligible resistance to metering fluid system

• Reasonably good Accuracy of the order of ±2%

• Is suitable for both liquids and gases

• Output read out can easily be either analog or digital

• Has a linear relationship between the velocity and output


Flow Measurements2. Electromagnetic Flow Meters:


Flow Measurements

Features:

• Suitable for electrically conducting fluids

• Principle: Faraday’s law of electromagnetic induction for solid

conductors. It states that whenever a conductor cuts lines of

magnetic field, an induced emf is generated and the magnitude

of this emf is proportional to the rate at which these lines are

cut and the emf is perpendicular to the plane of conductor and

magnetic field. The direction of induced emf is given by the


magnetic field. The direction of induced emf is given by the

Fleming’s right hand rule.

cm/s)(in conductor theofVelocity v

cm)(in conduction theof

)s/cm-V(in density flux MagneticB

voltageInduced Where

Volts 10 X

2

0

8

0

=

=

=

=

= −

Lengthl

E

BlvE

Flow Measurements

.E emf induced the toalproportiondirectly is rate flow that theshows This

Volts 10 X d

4BQor

4Q

:asgiven pipeiscircular for the rate flow Volume

0

8

2

−=

=

Q

vd

π

π

• This type of flow meter causes no obstruction to the flow

line of metered fluid


line of metered fluid

• Device is quite accurate and has wider range with good

transient response

• Can be used for dirty, corrosive, cement slurries etc flows

• These meters are expensive and their use is limited to fluids

having conductivity at least of the order of 1 X 10-6

Flow Measurements3. Hot Wire Anemometer:


Flow Measurements

Features:

• Used for measuring mean and fluctuating velocities

• Flow sensing element is a Platinum- Tungsten wire

• Principle: When a heated probe is introduced in the fluid

stream, it tends to get cooled by the instantaneous velocity and

consequently there is a decrease in its resistance. If the same is

connected by a Wheatstone bridge then the instrument

response is the direct measure of the flow velocity and hence


response is the direct measure of the flow velocity and hence

the flow.

• The velocity measurement is done in two ways:

1. Constant current method: In this method, the sensing element

is feed constant current, the knowledge of this current and the

resistance of the wire defines the power being fed to the wire,

which is a function of the flow velocity. This method can be

used at higher frequencies and relatively small signals.

Flow Measurements

2. Constant resistance or constant temperature method: In this

method, the current to the hot wire is continuously adjusted

by means of a suitable servo system to maintain the resistance

of wire and hence the wire temperature at a contant value

throughout the range of hot wire operation. The current or

voltage is then the measure of the heat transfer rates and

hence the fluid velocity.

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=

=

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=

=

+=

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µ

ρEq

VD

k

hDKK

Flow Measurements

:follows as written be alsocan Eq.1

hot wire of DiaD

fluid theof viscosityoft Coefficien

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fluid theofDensity

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3.

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f

w

c

w

2

21

=

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KK

Flow Measurements( )( )

( )

( )

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6.RR

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5.RR

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fw

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−

Flow Measurements

( ) 2

1

0

2

0

2

9.4

2

12

:get weating,Differenti velocity.zeroat emf theis

8.

:as written be alsocan 7 Eq

EqEdEV

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EWhere

EqVBEE

=

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+=

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KK


2

0

2

2

0

2

2

0

2

4is level e turbulenc theHence

4

4 :9 & 8 eq From

EE

Ee

EE

EdE

V

dV

EdEV

dV

V

EE

V

−=

−=

=−

Flow Measurements

Advantages of hot wire anemometer:

• Negligible resistance/disturbance to metering fluid system

• Excellent accuracy of the order of ±0.1% in the measurement of

mean velocities and ±2% in the measurement of turbulence

levels

• Exhibits excellent dynamic characteristics




• Measuring range can be from very low velocities to supersonic

velocities

• Shown on next slide are the Typical forms of hot wire film

anemometer probes used for liquid flows

Flow Measurements


Flow Measurements4. Laser Doppler Anemometer:


Flow Measurements

Features:

• It is an optical type of velocity meter

• Principle: Operating principle of this device is based on the

Doppler shift in frequency of the light scattered by an object

moving relative to the radiating source. The technique basically

consists of focusing laser beams at the point in the fluid where

the velocity is to be measured. At this focal point, the laser light

scattered from the fluid or fluid particles entrained in the fluid is


scattered from the fluid or fluid particles entrained in the fluid is

sensed by a photo-detector. Signal processing of the photo-

detector output yields the magnitude of Doppler frequency shift

which is directly proportional to the instantaneous velocity of

the flow.

2sin

2 θ

λ

vf =∆

NOTE: Refer the detailed theory of Laser Doppler Anemometer from text book.

flow measurement pdf

Engineering

flow meter

flow measurements features

instantaneous rate of

temperature flow rates

quantity meters

detailed theory

reference purpose

measuring contents