flow measurement.pdf

7/30/2019 flow measurement.pdf

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Measurement Techniques for

Engineers

Flow Measurement


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Introduction

Very important for process and foodindustry

Mass flow rate measurement of solids,small particles created by crushing or

grinding. Transported by conveyer,

calculation of mass of material on givenlength of conveyor times speed of conveyer


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Volume Flow Rate

Used for quantifying flow of gaseous, liquid,semi-liquid slurry (solid particles suspendedin liquid)

Classes of instruments used to measurevolume flow

Differential pressure meters Variable Area meters

Positive Displacement meters


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Volume Flow Rate

Turbine flowmeters Electromagnetic flowmeters

Vortex-shedding flowmeters

Gate-type meters

Ultrasonic flowmeters

Optical flowmeters


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Volume Flow Rate

Need to think about following before choosingmeter

Temperature

Pressure

Density

Viscosity

Chemical properties

Abrasiveness (if contains particles)

Phase (liquid or gas)


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Figure 7.1


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Venturi Meter

Restriction in pipe in form of taperingsection leading to narrow throat

Beyond throat more gradually returns to

original pipe diameter

Pressure taping at entry to venturi and at

throat led to differential pressure transducer Pressure difference governed by Bernoullis

law


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Venturi Meter

Bernoullis law in any continuous body ofliquid the sum of potential energy, pressureenergy and kinetic energy is constant at all

points In horizontal pipe potential energy is

constant and cancels So sum pressure energy and kinetic energyat entry and throat are equal


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Venturi Meter

22

222

211 vpvp +=+

2211 vavaQ ==&

)(2

)(

212

2

2

1

21 pp

aa

aaQ

=

&


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Venturi Meter

Simplifies to

where is the ratio: throat diameter

pipe diameter

)1(

)(24

212

=

ppaQ

&


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Venturi Meter

To obtain actual flow rate need to multiplyby a coefficient of discharge, Cd which is

found by calibration and is normally in

range 0.97 to 0.99

Typo in notes page 121, second whole

paragraph, second line, depends only onthe square root


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Venturi Meter

Limitations differential pressure transducers tend to become

less accurate at pressure differences less than

1/10th upper limit of their range, so venturi and

transducer can only be relied on down to about

one-third of maximum allowable flow rate.

Another source of error at low flow rates is the

decrease in Cd as Re number decreases.


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Orifice Plate

Obstruction method like Venturi relies onpressure drop induced

Flat plate inserted into pipework with sharp-edged hole in it which fluid passes through.

Constricts fluid as though passing through a

venturi

Figure 7.2 shows the streamlines of the

fluid as it passes through


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Orifice Plate

Shaded areas denote stationary flow Equations derived for Venturi hold for

orifice plate Coefficient of Discharge is much lower

(0.63 typically for fully turbulent flow) than

venturi (0.97-0.99)


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Orifice Plate

Two main advantages it gives larger pressure difference for given

flow rate than venturi

cheap and compact, can be inserted into

pipework between flange

Main Disadvantage permanent pressure drop, higher power loss in

pumping through orifice as opposed to Venturi,

therefore higher running cost


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Orifice Plate

Slurries tend to silt up the stationary regionson either side of the plate

Abrasive particles round off plate hole and

alter conditions (therefore re-calibration

required)


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Mechanical Flow Meters

Fluid made to do work on some kind ofmachine

quantity of fluid passing through the

machine is proportional to number of

oscillations or rotations of mechanism

Example - domestic gas meter

gas inflates alternately each chamber of a pair

of bellows being diverted to the other chamberonce one becomes full


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Turbine Flow Meter (figure 7.3) fluid flows past rotor with skewed blades

spins rotor at speed proportional to flow rate

flow upstream is straightened by radial vanes

vanes act as spacers to centralise rotor bearings

rotor may drive a mechanical counter (throughreduction gearing) or generate a digital signal

by means of a magnetic transducer


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Due to friction in bearings the rotor speed isalways slightly less than theoretical speed -

therefore classed as inferential

Error becomes more serious at lower flow

rates

Error usually less than 2% provided flow

rate is >7% of rate maximum

Fluid being measured must be clean - nosolids


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The Rotameter

Variable area flow meter - figure 7.4 Tube (high-strength glass) arranged

vertically, fluid enters narrow end and rises

to wide end

Float achieves equilibrium position where

weight (acting downwards) is balanced by

drag and buoyancy forces


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The Rotameter

To take a reading top of float sightedagainst scale engraved on glass tube

Alternatively to withstand higher pressures

tube may be made of metal and position

detected magnetically

Used for either gas or liquid, limited to fairly

small rates of flow


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The Rotameter

Float usually cylindrical with pointed bottomend, sharp edges to create turbulence and a

helical groove around the rim to spin the float

and stabilise its axis gyroscopically

Usually inaccuracy 2% of full scale

repeatability 0.25% of reading Calibration is for a particular density of fluid

Accurate range of 10:1 (max to min range)


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Electromagnetic Flowmeter

Figure 7.5 If a conductor is moving through a magnetic

field a current is induced

conductor is liquid in this case

Flemings right-hand rule for generators

applies


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Electrodes pick off voltage generated atright angles to flow and magnetic field

Voltage obtained is directly proportional to

rate of flow of liquid

In practice magnetic field produced by coils

immediately above and below pipe

Short piece of pipe is made of non-magnetic

material


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If pipe isnt insulating material is lined withone, so that it does not short-circuit the

output voltage

Cannot be used for petroleum products

(electricity)

Any liquid which separates into ions hassufficient conductivity


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Solutions of acids, alkalis and water can bemeasured provided water is not completely

pure

To stop build up of an insulating layer of

neutral molecules on the electrodes AC or

interrupted


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Solutions of acids, alkalis and water can bemeasured provided water is not completely

pure

To stop build up of an insulating layer of

neutral molecules on the electrodes AC or

interrupted DC is used


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Advantages no obstruction to pipe therefore no pumping

losses

calibration unaffected by changes in viscosity

or by disturbances in density or flow of liquid

provided that velocity is symmetrical about

vertical centre line of pipe

wide linear range of measurement


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Measure reverse flow corrosive liquids and liquids carrying abrasive

solids in suspension can be measured

can measure viscous slurries and non-Newtonian liquids


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Hot Wire Anemometer

Figure 7.6 fine tungsten wire stretched between tips of

a streamlined forked support

diameter 0.008mm, length 1mm, resistance

1

Flow cools wire and alters electrical

resistance


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Hot Wire Anemometer

Mainly used for gases but can be used forliquids (not common)

Particularly useful at measuring rapid

fluctuations of velocity

Turbulence measurements

Bridge circuit used and power supply

voltage varied to keep resistance of probe

constant


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Hot Wire Anemometer

Two practical difficulties hot wire may vibrate in high flow velocities

causing it to fatigue and break

fluid needs to be very clean, otherwise wire is

coated and calibration will be out, or wire is

broken by large particles


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Hot Wire Anemometer

To overcome wires fragility a thin film ofplatinum deposited onto glass may be used,

called hot film


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Vortex Flowmeter

Von Karman vortices are caused by flowpassing an object such as a cylinder

Vortices are shed of alternate sides of the

object creating a street of vortices

When Re>10,000 the distance between

vortices are constant for a given cross-section of an obstacle


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Vortex Flowmeter

Therefore number of vortices passing apoint in time is a measure of the velocity of

the fluid

Liquids, gases and steam may be measured

by this method


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Vortex Flowmeter

Vortices may be detected by strain gauging the obstacle (alternating side

forces)

obstacle fitted with piezoelectric transducers to

count vortices

local velocity change on surface of obstacle byhot-film anemometry

downstream of obstacle may be detected by

ultrasonic vibrations (figure 7.7)


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Vortex Flowmeter

Because of this direct proportionality, themaximum to minimum flow rate ratio is

quite large. 20:1 200 per second to about 10

per second


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Doppler Flowmeters

Doppler effect is the change in frequency ofa wave reaching a receiver when there is a

relative velocity between receiver and

transmitter

Can be used to measure flow velocity in

pipes either by beam of ultrasonic vibrationor by a beam of laser light projected

through a transparent section of pipe


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Doppler Flowmeters

Ultrasonic mainly for liquids, laser for bothliquids and gases

Cannot measure pure fluid, needs to be

carrying particles or bubbles in suspension

Doppler works by reflections from bubbles

or particles

For ultrasonic pipe diameter 200mm

particle sizes 100m to 40m minimumrequirement


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Doppler Flowmeters

Laser doppler flowmeter can operate withsub-micron particles

Ultrasonic doppler flowmeter - figure 7.8 -

clamp-on device

piezoelectric transmitter and receiver

(similar to vortex flowmeter)

transmitter applies narrow beam of

vibrations to a wedge block


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Doppler Flowmeters

Vibrations pass through wall pipe andechoes are reflected from particles in fluid

receiver mounted on another wedge picks

up echoes only from particles which are in

narrow cylindrical volume corresponding to

that through which transmitters vibrationsare propagated


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Doppler Flowmeters

Received echoes very weak and areamplified

frequency decreased by doppler effect

calibration gives relationship between

frequency shift and velocity


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Laser Doppler Flowmeters

Laser emits stable monochromatic light ofknown frequency

Beam of light is split into two beams of

equal intensity

two beams are focused on a point in the

fluid where they cross

at crossing point (measurement control

volume) a fringe or interference pattern isformed


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Particles in the fluid reflect the light in thefringe pattern as they cross it in all

directions

Small proportion of the reflected light will

be in the direction of a photodetector which

is placed on the transmitting (back-scatter)or receiving (forward-scatter) end of the

flowmeter


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Frequency of signal detected (differencebetween laser light frequency and doppler

shifted frequency of reflections) allows

speed to be computed

Main advantage - non-intrusive


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Two main sections transmitting optics (laser and optical

components used to project measurement

control volume in the fluid medium) receiving optics (components used to collect

scattered light and produce an analogue or

digital signal)


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Different arrangements reference beam mode

dual beam mode

dual scatter mode

based on method used to obtain optical

frequency shift of laser light dual beam (figure 7.9) most common


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To measure 2 components of velocity need2 fringe patterns of different planes of

polarisation or different colours

superimposed at right angle to each other

Polarisation or colour sensitive detectors

must be used


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Argon-Ion laser can be split to produce onegreen, one blue and one green-blue beam, 3

intersect and produce 2 fringe patterns (1

blue & 1 green)


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Directional Ambiguity in LDA

Signal yielded by photodetector is samewhatever direction of particle

Commonest method is to use frequency

shifting of one of the illuminating beams

Dual beam mode then sees movement of

fringe pattern in a prescribed direction


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Directional Ambiguity in LDA

particle velocity information extracted bythe signal processor contains the imposed

frequency shift

if shift is positive it is subtracted from final

velocity information

The frequency shifting can be made eitherby acousti-optic systems, by rotating

gratings, or by electro-optic methods


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Particle Image Velocimetry (PIV)

Allows measurement of instantaneouswhole fields

non-intrusive

Based on measurement of velocity of tracer

particle carried by fluid

Plane of field is illuminated by narrow sheet

of light spread of region of interest


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Tracer particles are visible and images ofilluminated particles are recorded

Recordings show successive images of the

whole flow field

displacement of particles determined by

analysis of records record image process image to

determine tracer displacement


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Laser usually used well-collimated source that can easily be

turned into sheet using cylindrical lenses or

scanning mirrors

continuous or pulsed lasers used

Basic optical arrangement shown in figure7.10


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Light sheet produced by pulsed laser andcylindrical lens

camera placed perpendicular to light sheet

to obtain a well-focused image of the

illuminated tracer particles

shutter opened for time long enough topulse laser twice


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Tracer particles therefore show up twice onone exposure

light has to be intense and short to avoid

blurring image

film has to be sufficiently sensitive to the

wavelength of the laser slow flow can use a shutter and continuous

laser (2 m/s)


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Tracer particles must be small enough tofollow the flow

reflected light very low intensity so have to

use larger aperture of camera, therefore

small depth of field and focusing is difficult

Similarly to LDA there is difficulty inestablishing direction of flow


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Image shifting - super-imposing shiftvelocity to flowfield.

Particle tagging - laser pulses are of

different durations or colours

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flow measurement.pdf