FLOW Measurement

1. ELET 241
PROCESS INSTRUMENTATION
Chapter#5
FLOW MEASURMENTS

2. DISCLAIMER
•These powerpoint slides act only as a tool in
delivering lectures to the students.
•The materials presented in these slides are not
comprehensive as most of the materials are
explained to the students verbally with the guide of
these PowerPoint slides, smartboard and ELET241
reference book.
•Hence, the students are reminded that the main
reference for ELET241 is Instrumentation and
Process Control by Franklyn W. Kirk text book
2

3. Flow
•Fluid flow is the movement of liquids in pipes or channels, and gases or
vapors in pipes or ducts.
•The most important characteristic of a fluid that affects flow is whether the
fluid is a liquid, gas, or vapor.
•At certain temperatures and pressures, most fluids can change phase
between vapor, liquid, or solid.
•For example, water can be heated to make steam or cooled to become ice.
Gases can be condensed to liquids like liquid nitrogen or liquid oxygen, or to
a solid like dry ice.

4. Flow Measurement Basics
The most important factors affecting fluid flow are:
• density,
• specific gravity,
• viscosity,
• The compressibility of the fluid.

5. Flow Measurement Basics
Compressibility
•An incompressible fluid is a fluid where there is very
little change in volume when subjected to a change in
pressure. Liquids are essentially incompressible.
•A compressible fluid is a fluid where the volume and
density change when subjected to a change in pressure.
Gases and vapors are compressible fluids.

6. TYPES OF FLOW MEASUREMENT
•Flow rate (Velocity)

7. •
TYPES OF FLOW MEASUREMENT

8. TYPES OF FLOW MEASUREMENT

9. TYPES OF FLOW MEASUREMENT
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10. TYPES OF FLOW MEASUREMENT

12. Types of Flow
•There are three types of fluid flow types:
• Laminar Flow
• Turbulent flow
• Transitional flow

13. Laminar flow
•It is a smooth layered flow with no mixing between the streamlines.
•It is observed as several streams of liquid flowing smoothly alongside each
other.

14. Turbulent flow
•It is agitated and disturbed flow.
•Turbulent flow appears to have small, high frequency fluctuations that travel
in all direction forming currents.

15. Flow Measurement Basics

16. Transitional flow
•Transitional flow is mix of both laminar and turbulent flow.
•There is a sudden transition between laminar flow and turbulent flow as the
flow rate increases.
•Like turbulent flows, transitional flows are difficult, if not impossible, to
accurately measure

17. Flow Measurement Basics
Reynolds Number
•A Reynolds number is the ratio between the inertial forces moving a fluid
and viscous forces resisting that movement. It describes the nature of the
fluid flow.
•It is used to differentiate between the turbulent and laminar flow of a
pattern and predict how it will flow in different fluid situations.
•The Reynolds number has no units of measure and is calculated from
velocity or flow rate, density, viscosity, and the inside diameter of a pipe.

18. Reynolds number
•The Reynolds number is an experimental number used in fluid flow to
predict the flow velocity at which type of flow will occur.
1. At low Reynold numbers flow is laminar. Any flow having Reynolds number up to 2000 is considered laminar.
2. When the Reynolds number is greater than 4000 flow is considered to be in
turbulent region.
3. When Reynolds number is in the range of 2000 to 4000 flow is considered to be in the transitional region.

19. Primary Flow Elements
Primary Flow Element
•It is a pipeline restriction that causes a pressure drop used to measure
flow.
•Primary flow elements are designed to provide accuracy, low cost, ease
of use, and pressure recovery, but not necessarily all in the same
element.
•The different types of the primary element used are:
• Orifice Plate
• Flow Nozzles
• Venturi Tubes

20. Primary Flow Elements
Orifice Plates
•An orifice plate is a primary flow element consisting of a
thin circular metal plate with a sharp-edged round hole
in it and a tab that protrudes from the flanges.

21. ORIFICE PLATE AND ITS TYPES
•The Orifice system has two elements primary and secondary
•The orifice plate, the primary elements, when inserted in the line,
causes an increase in flow velocity and a corresponding decrease in
pressure.
•secondary element which is a device used to measure the differential
pressure created

22. ORIFICE PLATE AND ITS TYPES
Working Principle:
•As the fluid approaches the orifice plate, the pressure P1 increases and the
velocity is decreases because of the orifice hole edges.
•After the fluid passes the orifice hole, the pressure p2 start to decreases and
the velocity start to increases unit it reach the vena contracta.
•The minimum pressure and maximum velocity are occurred on vena contraca.

23. Differential Pressure Flow meters

24. ORIFICE PLATE AND ITS TYPES
Advantages of Orifice Plates
• Simple construction
• Inexpensive
• Suitable for gas and liquids
• No moving parts
• ease of installation and replacement.
Disadvantages of Orifice Plate
• Not accurate
• Physical damage affect its measurement
• Needs another instrument to measure the differential pressure

25. ORIFICE PLATE AND ITS TYPES
Types of ORIFICE PLATE:
•Concentric
•Eccentric
• Segmental
Eccentric and segmental orifices are preferable to concentric orifices for
measurement of slurries or dirty liquids as well as for measurement of gas or
vapor where liquids may be present, especially large slugs of liquid.

26. THE VENTURI TUBE
•The Venturi tube consists of a converging conical inlet section in
which the cross section of the stream decreases and the velocity
increases consequently.
•a cylindrical throat that provides small diameter inlet where the
fluid pressure decreases
•a diverging recovery cone in which velocity and pressure are
recovered.
•Pressure taps are taken at one half diameter upstream of the inlet
cone and at the middle of the throat.

27. Primary Flow Elements
Venturi Tubes
• A Venturi tube is a primary flow element consisting of a fabricated pipe section
with a converging inlet section, a straight throat, and a diverging outlet section.
• Venturi tubes are much more expensive than orifice plates but are more accurate
and recover 90% or more of the differential pressure.
• This reduces the burden on pumps and the cost of power to run them.
• Venturi tubes are frequently used to measure large flows of water.

28. THE VENTURI TUBE
Advantages of Venturi meter:
• They can handle large flow volumes at low pressure drops.
• Venturi tube flowmeters involve no moving parts.
• They can be used with nearly all liquids, as well as those containing extreme solids
content.
Disadvantages of Venturi meter:
• Highly expensive
• Occupies considerable space

29. Differential-Pressure Flowmeters
• One of the most common methods for measuring the flow of liquids in process
pipes is to introduce a restriction in the pipe and then measure the resulting
differential pressure (ΔP) drop across the restriction. This restriction causes an
increase in flow velocity at the restriction and a corresponding pressure drop
across the restriction. The relationship between the pressure drop and the rate of
flow is given by the following equation:

30. ULTRASONIC FLOWMETERS
ULTRASONIC FLOWMETERS
•An ultrasonic flowmeter is a flowmeter that uses the principles of sound
transmission in liquids to measure flow.
•They use either the change in frequency of sound reflected from moving
elements or measure the change in the speed of sound in a moving
liquid.

31. ULTRASONIC FLOWMETERS
Transit Time Ultrasonic Meters
• This devices contains of two ultrasonic transducers . Used to beam a high frequency sound wave
• (ultrasonic wave), alternatively upstream and downstream at an angle θ, across the flow, as shown in
Figure
• The difference in times required for the sound waves to travel upstream (Tu) and downstream (Td), can be
used to calculate both the sound speed and the mean fluid velocity along the path followed by the sound.
• This measurement method has been applied successfully to very large pipes carrying clean, noncorrosive,
bubble-free liquids.

32. ULTRASONIC FLOWMETERS

33. Differential Pressure Flow meters
•