7/30/2019 flow measurement.pdf
1/58
Measurement Techniques for
Engineers
Flow Measurement
7/30/2019 flow measurement.pdf
2/58
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
7/30/2019 flow measurement.pdf
3/58
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
7/30/2019 flow measurement.pdf
4/58
Volume Flow Rate
Turbine flowmeters Electromagnetic flowmeters
Vortex-shedding flowmeters
Gate-type meters
Ultrasonic flowmeters
Optical flowmeters
7/30/2019 flow measurement.pdf
5/58
Volume Flow Rate
Need to think about following before choosingmeter
Temperature
Pressure
Density
Viscosity
Chemical properties
Abrasiveness (if contains particles)
Phase (liquid or gas)
7/30/2019 flow measurement.pdf
6/58
Figure 7.1
7/30/2019 flow measurement.pdf
7/58
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
7/30/2019 flow measurement.pdf
8/58
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
7/30/2019 flow measurement.pdf
9/58
Venturi Meter
22
222
211 vpvp +=+
2211 vavaQ ==&
)(2
)(
212
2
2
1
21 pp
aa
aaQ
=
&
7/30/2019 flow measurement.pdf
10/58
Venturi Meter
Simplifies to
where is the ratio: throat diameter
pipe diameter
)1(
)(24
212
=
ppaQ
&
7/30/2019 flow measurement.pdf
11/58
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
7/30/2019 flow measurement.pdf
12/58
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.
7/30/2019 flow measurement.pdf
13/58
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
7/30/2019 flow measurement.pdf
14/58
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)
7/30/2019 flow measurement.pdf
15/58
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
7/30/2019 flow measurement.pdf
16/58
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)
7/30/2019 flow measurement.pdf
17/58
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
7/30/2019 flow measurement.pdf
18/58
Mechanical Flow Meters
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
7/30/2019 flow measurement.pdf
19/58
Mechanical Flow Meters
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
7/30/2019 flow measurement.pdf
20/58
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
7/30/2019 flow measurement.pdf
21/58
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
7/30/2019 flow measurement.pdf
22/58
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)
7/30/2019 flow measurement.pdf
23/58
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
7/30/2019 flow measurement.pdf
24/58
Electromagnetic Flowmeter
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
7/30/2019 flow measurement.pdf
25/58
Electromagnetic Flowmeter
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
7/30/2019 flow measurement.pdf
26/58
Electromagnetic Flowmeter
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
7/30/2019 flow measurement.pdf
27/58
Electromagnetic Flowmeter
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
7/30/2019 flow measurement.pdf
28/58
Electromagnetic Flowmeter
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
7/30/2019 flow measurement.pdf
29/58
Electromagnetic Flowmeter
Measure reverse flow corrosive liquids and liquids carrying abrasive
solids in suspension can be measured
can measure viscous slurries and non-Newtonian liquids
7/30/2019 flow measurement.pdf
30/58
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
7/30/2019 flow measurement.pdf
31/58
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
7/30/2019 flow measurement.pdf
32/58
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
7/30/2019 flow measurement.pdf
33/58
Hot Wire Anemometer
To overcome wires fragility a thin film ofplatinum deposited onto glass may be used,
called hot film
7/30/2019 flow measurement.pdf
34/58
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
7/30/2019 flow measurement.pdf
35/58
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
7/30/2019 flow measurement.pdf
36/58
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)
7/30/2019 flow measurement.pdf
37/58
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
7/30/2019 flow measurement.pdf
38/58
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
7/30/2019 flow measurement.pdf
39/58
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
7/30/2019 flow measurement.pdf
40/58
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
7/30/2019 flow measurement.pdf
41/58
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
7/30/2019 flow measurement.pdf
42/58
Doppler Flowmeters
Received echoes very weak and areamplified
frequency decreased by doppler effect
calibration gives relationship between
frequency shift and velocity
7/30/2019 flow measurement.pdf
43/58
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
7/30/2019 flow measurement.pdf
44/58
Laser Doppler Flowmeters
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
7/30/2019 flow measurement.pdf
45/58
Laser Doppler Flowmeters
Frequency of signal detected (differencebetween laser light frequency and doppler
shifted frequency of reflections) allows
speed to be computed
Main advantage - non-intrusive
7/30/2019 flow measurement.pdf
46/58
Laser Doppler Flowmeters
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)
7/30/2019 flow measurement.pdf
47/58
Laser Doppler Flowmeters
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
7/30/2019 flow measurement.pdf
48/58
Laser Doppler Flowmeters
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
7/30/2019 flow measurement.pdf
49/58
Laser Doppler Flowmeters
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)
7/30/2019 flow measurement.pdf
50/58
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
7/30/2019 flow measurement.pdf
51/58
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
7/30/2019 flow measurement.pdf
52/58
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
7/30/2019 flow measurement.pdf
53/58
Particle Image Velocimetry (PIV)
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
7/30/2019 flow measurement.pdf
54/58
Particle Image Velocimetry (PIV)
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
7/30/2019 flow measurement.pdf
55/58
Particle Image Velocimetry (PIV)
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
7/30/2019 flow measurement.pdf
56/58
Particle Image Velocimetry (PIV)
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)
7/30/2019 flow measurement.pdf
57/58
Particle Image Velocimetry (PIV)
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
7/30/2019 flow measurement.pdf
58/58
Particle Image Velocimetry (PIV)
Image shifting - super-imposing shiftvelocity to flowfield.
Particle tagging - laser pulses are of
different durations or colours