Chapter 2 Introduction to flow measurement

Flow

1. Process
Measurement

2. “
▪•Flow meters
▪•Pressure transmitters
▪•Temperature sensors
▪•Level transmitters
2

3. Flow measurement

4. Topics
▪Introduction to Flow Measurement
▪Flow Measuring Device:
1. Differential Pressure Meter
-Orifice, Venturi,Nozzle
2. Electromagnetic Flowmeter
3. Vortex Flowmeter
4. Turbine Flowmeter
5. Ultrasonic Flowmeter
-Transit Time Model, Doppler Ultrasonic Flowmeters
4

5. Topic outcomes
▪Describe the flow measurement.
▪Explain the basic operation, list the
advantages and disadvantages of each flow
measurement device.
5

6. What is “flowrate”?
▪Amount of material passing one point for
certain time.
▪Give one simple method to measure…
…water flowrate from tap water.
6

7. Why flow rate is important variable?
▪Most process involves moving material from
one part of the plant to another .
▪Flow of
1.Material -Gas, liquid, solid particle
2.Energy
7

8. Common terms
▪Velocity = measure the speed and direction the object, the rate of
flow of fluid particles in a pipe. The units; feet per second (fps),
meters per second (mps), and etc.
▪Laminar flow = when its average velocity is comparatively low and
the fluid particles tend to move smoothly in layers.
▪Turbulent flow = when the flow velocity is high and the particles no
longer flow smoothly in layers and turbulence or a rolling effect
occurs.
▪Viscosity = property of a gas or liquid that is a measure of its
resistance to motion or flow. 8

9. Differential Pressure Meter
•Sensors that rely on the pressure drop or head occurring
as a fluid flows by a resistance or obstruction.
•Velocity is increased after passing the obstruction while
pressure is decreased.
q= Flow rate K = constant ∆P= Pressure difference
•Types of differential pressure meter; orifice meter,
venturimeter, and flow nozzle.pKq =
9

10. Orifice, nozzle, venturi
Orifice
Venturi
Nozzle
10

11. ▪An orifice plate is a restriction with an opening smaller than
the pipe diameter which is inserted in the pipe perpendicular
to the flow stream.
▪Because of the smaller area the fluid velocity increases,
causing a corresponding decrease in pressure.
▪Typical orifice meter has a concentric, sharp edged opening
Orifice plate
Orifice Plate
11

12. Orifice Plate
12

13. ▪Advantages
Most commonly used flow
sensor-inexpensive, easy to
install, no moving part, simple
configurations, required
virtually no maintenance
▪Disadvantages
Creates low recoverable
pressure (may recover only
50% of pressure drop) due to
turbulence around the plate –
high energy consumption.
.
Orifice plate
13

14. Example Orifice calculation
▪Q is the volumetric flowrate, Aois the area of the orifice, ρ
(kg/m
3
), is the liquid density, ΔP(Pa) is the pressure drop, P
1-P
2
▪From the Bernoulli equation,
V (m/s) is the velocity
�=??????
????????????
??????
2∆�
??????
∆�=�
1−�
2=
1
2
????????????
2
2
−
1
2
????????????
1
2
14

15. Example Orifice calculation
▪The flow coefficientC
fis found from experiments and is tabulated in
reference books; it ranges from 0.6 to 0.9 for most orifices.
▪The mass flowrate can be found by multiplyingQwith the fluid
density,
??????=�??????
15

16. Derivation of orifice equation
▪Bernoulli equation
�
1+
1
2
????????????
1
2
=�
2+
1
2
????????????
2
2
▪Continuity equation
??????=??????
1??????
1=??????
2??????
2
▪Combine both equations
�
1−�
2=
1
2
????????????
2
2
−
1
2
????????????
1
2
▪Rearrange the equations
∆�=
1
2
??????
??????
??????
2
2
−
1
2
??????
??????
??????
1
2
∆�=
1
2
????????????
2
1
??????
2
2
−
1
??????
1
2
∆�=
1
2
????????????
2
1
??????
2
2
1−
??????
2
??????
1
2
16

17. Derivation of orifice equation
▪Determine q
??????=
2∆�
??????
??????
2
1−
??????
2
??????
1
2
17

18. •It consists of a convergent cone section, cylindrical
bore, and a divergent cone section.
•The cylindrical bore restricts the fluid flow resulting in a
pressure drop.
•The change in cross sectional area causes a pressure
change.
•Can be used on slurries and dirty fluids, high installation
cost.
Venturimeter
18

19. Venturimeter
▪More expensive than orifice meter but produce substantially higher
recoverable pressure drops recovers up to 85% of pressure drop),due to
the continuous geometry of the contraction, throat, and divergent
sections.
19

20. Example Venturimetercalculation
20
Q is the volumetric flowrate, A and A1 are the areas in the venturi, ρ(kg/m
3
), is
the liquid density, P and P1 are the pressures in the venture, v (m/s) is the
velocity
Therefore;
A A1
�−�
1=
1
2
????????????
1
2
−
1
2
????????????
2
�=????????????=??????
1??????
1
�=??????
2
??????
�
1−�
1−
??????
??????
1
2

21. ▪Advantages
Widely used for high flow
rates. They can handle 25-
50% more flow rate than an
orifice.
▪Disadvantages
The pipe design need not be
straight, insensitive to the
changes in the velocity of flow
.
Venturimeter
21

22. ▪Can handle flowrates that are larger than the capabilities of orifice plate
with acceptable pressure loss
▪Consist of an elliptical converging section and cylindrical throat section.
▪Flow Nozzles have a smooth elliptical inlet leading to a throat section
with a sharp outlet. This restriction in the fluid flow causes a pressure
drop.
Flow nozzle
22

23. Advantages
▪Suitable for high-velocity, non-viscous, erosive flows.
▪produce less differential pressure (low pressure loss) than
venturi
Disadvantages
▪offers lower than venturi installation costs but higher than
orifice
Flow nozzle
23

24. 24

25. ▪This instrument operates based on Faraday’s Law.
▪It is ideal for liquids that conduct electricity.
▪A charged particle moving through the magnetic field produces a voltage
proportional to the velocityof the particle.
▪The magnetic field is developed by electric coils.
Electromagnetic flowmeter
25

26. Electromagnetic flowmeter
▪This conductor in the magnetic field will generate an electric
voltage that is proportional to its average velocity.
▪The pipe section in which measurement is made must be
insulated or nonconductor itself, so that the generated voltage
not dissipated through the pipeline
▪A pair of electrodes is installed across the pipe wall to detect
the induced voltage.
26

27. Advantages:
▪Minimum pressure drop
▪can be used in hazardous environments or measure corrosive or
slurry fluid flow
▪low maintenance cost because of no moving parts
▪high linearity between the output and flow rate
▪more accurate than differential pressure meter (orifice, venturi
and flow nozzle) since this flow meter does not introduce any
pressure drop
Electromagnetic flowmeter
27

28. Disadvantages/Limitations
▪material must be liquid that conduct electricity
▪requires electrical conductivity of fluid higher than 3 µS/cm in
most cases, (particle caused bias in reading).
▪expensive
Electromagnetic flowmeter
28

29. ▪Vortex meter operate on the principle that when a non-
streamlined object (barrier/bluff body) is placed in the middle
of flow stream
▪A series of vortices are shed alternately downstream of the
object
▪The frequency of vortex shedding down each side of the bluff
body is directly proportional to meanflow velocity and
therefore to volumeflow.
▪A device that counts the vortices passing per second will also
measure the flowrate.
Vortex flowmeter
29

30. Advantages
▪Low cost installation-do not required impulse tubing and valve
manifold.
▪Universally suitable for measuring liquids, gases and steam
▪Largely unaffected by changes in pressure, temperature and
viscosity
▪No moving parts
▪Marginal pressure loss
▪Easy to install and commission
Vortex flowmeter
30

31. Vortex flowmeter
Disadvantages
▪Vortexes are inhibited in viscous fluid at low flow rate.
▪At high fluid velocity the obstructions may introduce excessive
pressure drop-limited to higher flow rate.
▪Vaporization of liquid may damage the vortex detector-care
must be taken.
31

32. ▪Turbine meters have a spinning rotor with blades that is mounted on
bearings in a housing on the central longitunidalaxis of the pipeline.
▪The rotor spins as water or other fluid passes over it.
▪Magnets are embedded in the rotor housing and a pickup coil, isolated
from the fluid is placed outside the rotor blades.
▪The rotating magnets induce a voltage pulse in the coil each time they
pass the coils. Blade movement is often detected magnetically.
▪The pulse frequency is proportional to the velocity of the fluid. When the
fluid moves faster, more pulses are generated
Turbine
32

33. Turbine
▪Turbinmeter excel at measuring steady, low flows of liquid and gas.
Widely used in utility applications to measure the amount of water used in
commercial and industrial buildings.
▪Have excellent accuracy.
▪The size (diameter) same as the pipe in which they are fitted, and pressure
loss is quite low.
▪Disadvantages: limited to clean fluid, require maintenance at their bearing
from time to time, expensive.
▪Acceleration of fluid beyond the normal at upstream cause excessive
rotational speed and have serious consequences on the life of the meter.33

34. ▪Advantages
Suitable for high pressure
measurement, high accuracy
and repeatability.
Measurement of non-
conductive liquids
▪Disadvantages
Can only measure clean fluids,
and filters are normally
needed when installing the
sensor. Not effective with
swirling fluids. Not applicable
to higher viscosity fluids.
.
Turbine flowmeter
34

35. ▪Ultrasonic flowmeters can be categorized into two types based on the
installation method: clamped-on and inline.
▪The clamped-on type is located outside of the pipe and there are no
wetted parts. It can easily be installed on existing piping systems without
worrying about corrosion problems.
▪Clamped-on designs also increase the portability of the flowmeter.
▪The inline type, on the other hand, requires fitting flanges or wafers for
installation. However, it usually offers better accuracy and its calibration
procedures are more straightforward.
Ultrasonic flowmeter
35

36. •A pair of transducers is placed on the pipe wall, one on the upstream
(transmitter) and the other on the downstream (receiver).
•It sends pulses of ultrasonic energy diagonally across the pipe.
•Flowrateis measured based on the time for the sound to travel between a
transmitter and a receiver
•The time for acoustic waves to travel from the upstream transducer to the
downstream transducer is shorter than the time it requires for the same
waves to travel from the downstream to the upstream.
Time Transit Model -for clean fluids
transmitter
receiver
36

37. ▪Use only one transducer for transmitting and receiving ultrasonic frequency
▪Apply principle of frequency shift between the ultrasonic frequency source,
receiver and fluid carrier due to the fluid velocity.
▪Usually requires someparticles or undissolved gasses (bubble) in the flow to
reflect the signals.
▪As the objects are moving, the reflected ultrasonic energy will have a
different frequency. The amount of difference between the original and
returned signals is proportional to the flow velocity.
Doppler ultrasonic flowmeter -for dirty, slurry-type flow 37

38. ▪Advantages
No moving parts, low
maintenance and high
accuracy.
No obstruction in the flow
path, no pressure drop.
Can be used to measure
corrosive or slurry fluid flow.
▪Disadvantages
Higher upfront cost, cannot be
used for heavily contaminated
liquids or slurry.
Turbulence or even the
swirling of the process fluid
can affect the ultrasonic
signals.
.
Ultrasonic flowmeter
38

39. “
There are key questions you can ask
yourself when trying to determine which
flowmeter is the best choice for you. The
purpose of the measurementand the
physical characteristics of the fluid
being measured are the two main
considerations.
How to choose the right
flowmeter?
39

40. Some factors to consider are
▪Type of the fluid being measured (air, water, gas, oil, slurry, etc.)
▪Fluid properties (clean, dirty, viscous, slurry, corrosive, conductive
etc.)
▪Measurement device accuracy, turndown and repeatability
▪Total cost involve (equipment cost, total cost installation,
maintained cost, and operating cost)-cheap or expensive?
▪Type of construction materials (stainless steel, fiberglass, etc.)
▪Pressure loss (high, low, intermediate?)
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41. 41

42. Vortex Flowmeter
Orifice plate
Venturi meter
Turbine flowmeter
Electromagnetic
flowmeter
Flow Nozzle
Time Transit Model
Doppler Electromagnetic flowmeter
42

43. Thanks!
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