1. Types of Questions Asked in the Interview
 What is the difference between volumetric flow meter and mass flow meter?
Why Flow measure in Square-root?
What type of flow measurement devices is best for slurries?
What are the different types of primary elements used for flow measurement?
What are the different types of orifice plates and state their uses.
What is Bernoulli’s theorem and where is it applicable?
ect.

2. Flow Measurement Prepared by: Anuradha Tandon Assistant Professor,
IC Section, EE Dept.,
IT, NU

3. WHAT IS FLOW ?
Measure of the velocity of a fluid per unit area in a closed conduit; i.e., pipe or duct
FLOW = VELOCITY (fluid) X Area of Pipe or Duct or Stack

4. Why Flow Measurement? Mainly for the purpose of
cost accounting for the services like water, gas, steam etc.
plant material balancing
quality control

5. Types of Fluid Fluids whose flow is required to be measured may be
Clear or opaque,
Clean or dirty,
Wet or dry, or
Erosive or corrosive

6. How Flow Measurement is Done?
Commonly used Flow Measurement Devices are:
Differential Pressure (Head) Type
Orifice Plate - Concentric, Eccentric, Segmental, Quadrant Edge, Integral, Conditioning
Venturi Tube
Flow Nozzles
Pitot Tube, Averaging Pitot Tube (Annubar)
Elbow
Wedge Meter
V-Cone
Variable Area Meters
Rotameter
Cylinder and piston type flow meter

7. How Flow Measurement is Done? (cont….)
Mass Type – measures the mass flow rate directly.
Thermal
Velocity Type
Magnetic
Ultrasonic - Transit Time, Doppler
Turbine
Vortex
Open Channel Type
Weir
Parshall Flume
Other Types
Positive Displacement
Target

8. Flow Measurement - Terms
DENSITY (r)
A Measure Of Mass Per Unit Of Volume (lb/ft3 or kg/M3).
SPECIFIC GRAVITY
The Ratio Of The Density Of A Material To The Density Of Water Or Air Depending On Whether It Is A Liquid Or A Gas.
COMPRESSIBLE FLUID
Fluids (Such As Gasses) Where The Volume Changes With Respect To Changes In The Pressure. These Fluids Experience Large Changes In Density Due To Changes In Pressure.
NON-COMPRESSIBLE FLUID
Fluids (Generally Liquids) Which Resist Changes In Volume As The Pressure Changes. These Fluids Experience Little Change In Density Due To Pressure Changes.

9. Flow Measurement – Terms (cont..)
Rangeability
The ratio of maximum flow to minimum flow, but not zero flow.
Repeatability
The ability of a flow meter to indicate the same readings each time the same flow conditions exist. These readings may or may not be accurate, but will repeat.
Linear
Transmitter output is directly proportional to the flow input.
Square Root
Flow is proportional to the square root of the measured value.

10. Flow Measurement – Terms (cont..)
Beta Ratio (d/D)
Ratio of a differential pressure flow device bore (d) divided by internal diameter of pipe (D).
A higher Beta ratio means a larger orifice size. A larger orifice plate bore size means greater flow capacity and a lower permanent pressure loss.
Pressure Head
The Pressure At A Given Point In A Liquid Measured In Terms Of The Vertical Height Of A Column Of The Liquid Needed To Produce The Same Pressure.

11. Flow Measurement – Units
Flow is measured as a quantity (either volume or mass) per unit time
Volumetric units
Liquid
gpm, bbl/day, m3/hr, liters/min, etc.
Gas or Vapor
ft3/hr, m3/hr, etc.
Mass units (either liquid, gas or vapor)
lb/hr, kg/hr, etc.
Flow can be measured in accumulated (totalized) total amounts for a time period
gallons, liters, meters passed in a day, etc

12. Reynolds Number
The Reynolds number is the ratio of inertial forces (velocity and density that keep the fluid in motion) to viscous forces (frictional forces that slow the fluid down) and is used for determining the dynamic properties of the fluid to allow an equal comparison between different fluids and flows.
It is used to determine the point at which the flow goes from viscous to the turbulent stage.

13. Reynolds Number (cont….)
The Reynolds number is the most important value used in fluid dynamics as it provides a criterion for determining similarity between different fluids, flow rates and piping configurations.

14. Types of Fluid Flow in Pipes
Based on Reynolds Number fluid motion in a pipe can be characterized as one of three types:
1). Stream line or Viscous or Laminar - occurs at low Reynolds number
2). Turbulent - occurs at high Reynolds Number
3). Combined viscous or Transitional

15. 1). Stream line or Viscous or Laminar
Laminar flow , is characterized by concentric layers of fluid moving in parallel down the length of A pipe. The highest velocity (Vmax) is found in the center of the pipe. The lowest velocity (V=0) is found along the pipe wall.
Vavg Vmax = 2*Vavg

16. 2). Turbulent Flow
Turbulent flow is characterized by A fluid motion that has local velocities and pressures that fluctuate randomly. This causes the velocity of the fluid in the pipe to be more uniform across A cross section.

17. 3). Combined Viscous or Transitional
If the flow is transitional , then both types may be present at different points along the pipeline or the flow may switch between the two.

18. Differential Pressure (Head) Type
Also called the inferential type of flow meters as the quantity under measurement is inferred from the measurement of some other quantity.
They include:
Orifice Plate - Concentric, Eccentric, Segmental, Quadrant Edge, Integral, Conditioning
Venturi Tube
Flow Nozzles
Pitot Tube, Averaging Pitot Tube (Annubar)
Elbow
Wedge Meter
V-Cone

19. Differential Pressure (Head) Type (cont….)
Produce a pressure difference when fluid flow is maintained through them
Diff. Pressure proportional to square of flow rate
Basic operating principle - Bernoulli’s theorem

20. Bernoulli’s Theorem
The Bernoulli equation defines the relationship between fluid velocity (v), fluid pressure (p), and height (h) above some fixed point for a fluid flowing through a pipe of varying cross-section, and is the starting point for understanding the principle of the differential pressure flow meter.

21. Bernoulli’s Theorem (cont….)
Bernoulli’s equation states that:
Pressure head + velocity head + elevation head (at a point) =
pressure head + velocity head + elevation head ( at some other point) + loss due to friction

22. Bernoulli’s Theorem (cont….)
Bernoulli’s law explains the movement of fluid through an orifice or nozzle. It shows that the total energy upstream of the nozzle is equal to the energy at the nozzle throat.
It explains how the potential energy is converted to kinetic energy as the fluid speeds up and the pressure drops at the nozzle throat.

23. Bernoulli’s Theorem (cont….)
The basic equations are:
V = Velocity of the flowing fluid
Q = Volume flow rate
W = Mass flow rate
A = Cross sectional area of pipe through which fluid is flowing
h = differential head (pressure) across the restriction element
ρ = density of the flowing liquid g = acceleration due to gravity
K =

24. Bernoulli’s Theorem (cont….)
K = = a constant
where, C= discharge coefficient
β = Diameter ratio
d (diameter of the restriction element) =
D (diameter of the pipe)

25. 1. Orifice Meter

26. 1. Orifice Meter (cont….)
The Orifice Plate is a simple device, considered to be a precision instrument.
It is simply a piece of flat metal with a flow-restricting bore that is inserted into the pipe between flanges.
Orifice Plate
Orifice Flanges

27. 1. Orifice Meter (Cont…)
It is inserted into the pipe to create a partial restriction to flow
Pressure before orifice plate rises and pressure after it reduces but velocity increases

28. 1. Orifice Meter (Cont…)
Position where velocity is maximum & static pressure is minimum is known as vena contracta

29. 1. Orifice Meter (Cont...) Differential Pressure Calculation:
Calculate pressure using the basic formula or use the flow processor/computer

30. 1. Orifice Meter (Cont….)
Usually pressure tapping is at a distance D & D/2 for up stream & down stream
D- diameter of pipe
For pipe size 0.05m or more
Orifice to pipe diameter is 0.6

31. 1. Orifice Meter (Cont….)
Flow measurement using orifice meter:

32. 1. Orifice Meter (Cont…)
Normal thickness m for pipe with diameter 0.15 m & for larger
Materials used bronze, stainless steel, phosphor bronze, gun metal etc.
It is rugged and inexpensive.
It’s accuracy under ideal conditions is in the range of 0.75 to 1.5 percent.

33. Orifice plate & flanges

34. Types of orifice plates
If fluid contains suspended materials , particles tend to settle down in orifice,
To avoid this segmented or eccentric type is used

35. Quadrant Edge Type Orifice Plate
The quadrant, quadrant edge or quarter-circle orifice is recommended for measurement of fluids with high viscosity which have pipe Reynolds Numbers below 10,000.
The orifice incorporates a rounded edge of definite radius which is a particular function of the orifice diameter.

36. 1. Orifice Meter Advantages: Simple to use
Low cost, especially on large sizes
No need for recalibration
Precision instrument
Disadvantages:
Maintenance problem
Long installations (20D to 30D)
Poor accuracy

37. Orifice Selection Consideration
Quadrant Edge Orifice Plate can be considered if Reynolds number is too low.
Orifice plate must be specified with proper flange rating to account for proper bolt circle.
Typical acceptable beta ratio is .25 to .7 for non commerce meter, .3 to .6 for accounting meter but also check specifications.

38. 2. Venturi Meter

39. 2. Venturi Meter (cont….)
In a Venturi tube, the fluid is accelerated through a converging cone, inducing a local pressure drop. An expanding section of the meter then returns the flow to near its original pressure. These instruments are often selected where it is important not to create a significant pressure drop and where good accuracy is required.
Used when higher velocity and pressure recovery is required.
May be used when a small, constant percentage of solids is present.

40. 2. Venturi Meter (cont….)

41. 2. Venturi Meter (Cont….) Throat to diameter ratio 0.25 to 0.75
Discharge co-efficient – 0.9 to 1.0
Made of cast iron, gun metal, stainless steel
May be circular, square or rectangular

42. 2. Venturi Meter (Cont…) Advantages: Disadvantages:
Due to smooth design of the Venturi tube means that it is less sensitive to erosion than the orifice plate.
Thus more suitable for use with dirty gases or liquids.
The Venturi tube is also less sensitive to upstream disturbances, and therefore needs shorter lengths of straight pipe work upstream of the meter than the equivalent orifice plate or nozzle.
Disadvantages:
Not useful below 76.2 mm pipe size
Costly

43. 3. Flow Nozzles

44. 3. Flow Nozzles (Cont….)
Used when higher velocity & pressure recovery are required
Better suited for gas service than for liquid
Made from materials such as stainless steel or chrome - moly steel
Can be used at Reynolds
number of 50,000 or above
Discharge co-efficient is 0.99
or greater
Beta ratio of 0.2 to 0.8

45. 3. Flow Nozzles (cont…) Advantages: Disadvantages:
Permanent pressure loss is less than that in orifice meter
Widely accepted for high-pressure and temperature steam flow.
Disadvantages:
Costly than orifice meter
Limited to moderate pipe sizes
Requires more maintenance

46. 4. Pitot Tube

47. 4. Pitot Tube (Cont…) Operating principle is:
“When a solid body is kept centrally and stationary in a pipe line with a fluid streaming down, the velocity of the fluid starts diminishing due to the presence of the body, till it reduces to zero directly in front of the body. This point is known as the stagnation point. “
As the kinetic head (pressure) is lost by the liquid, it gains a static head and by measuring the difference pressure, the fluid velocity can be measured.

48. 4. Pitot Tube (Cont….)
Not used widely due to less accuracy, i. e., in the range of ± 0.5 % to ±5 %.

49. 4. Pitot Tube (Cont…) Advantages Disadvantages No process loss
Economical to install
Disadvantages
Accuracy is very poor
Cannot be used with dirty or sticky fluids

50. Other Type of DP Meters
There are many other types of differential pressure flowmeter, but they are not very common
the segmental wedge, V-cone, and elbow.
Each of these has advantages over the orifice plate, Venturi tube, and nozzle for specific applications.
For example, the segmental wedge can be used with flows having a low Reynolds number,
Dall tube has a lower permanent pressure loss than a Venturi tube.
However, none of these instruments are yet covered by international standards and, thus, calibration is needed to determine their accuracy.

51. Variable Area Meters
The size of the restriction is adjusted by an amount necessary to keep the pressure differential constant when the flow rate changes as opposed to that in the DP meters where the size of the restriction was fixed and the pressure differential across it changes as the flow rate changes.
The amount by which the area is changed to keep the pressure constant is proportional to flow
Includes
Rotameter, and
Cylinder and piston type flow meters

52. 1. Rotameter Rotameter is a variable area device.
The float moves up and down in proportion to the fluid flow rate and the annular area between the float and the tube wall.
As the float rises, the size of the annular opening increases. As this area increases, the differential pressure across the float decreases.
The float reaches a stable position when the upward force exerted by the flowing fluid equals the weight of the float.
Every float position corresponds to a particular flow rate for a particular fluid's density and viscosity.

53. 1. Rotameter (Cont….) Advantages: Disadvantages: Relatively low cost
Good rangeability
Suitable for liquid as well as gas flow measurements
Handles wide variety of corrosive fluids
Can be easily equipped with alarm switches or transmitting devices
Disadvantages:
Glass tube subject to breakage
Must be mounted vetically

54. 2. Cylinder and Piston Type Flow Meter
Most commonly used for measuring flow of fuel oils, tar, chemical liquors and other such highly viscous fluids

55. 2. Cylinder and Piston Type Flow Meter (Cont…)
Advantages:
Can handle slurries and greasy materials
Can handle corrosive materials
Is totally obstructionless and therefore very low pressure drop
Available in large pipe sizes and capabilities
Capable of handling extremely low flows and very high volume flow rate
Can be used as bi-directional meter
Measurement is unaffected by viscosity, density, temperature and pressure

56. 2. Cylinder and Piston Type Flow Meter
Disadvantages:
Relatively expensive
Heavy
Must be full at all times
Must be explosion proof when istalled in hazardous electrical areas

57. Mass Flow Meters Popular for the measurement of unsteady flow of gases
They are of two types:
Heat Transfer flow meter
Hot-wire flow meter

58. 1. Heat Transfer Flow Meter
Measures the rise in the temperature of the fluid after a known amount of heat has been added to it.
Mass flow rate is calculated as
Where, W= mass flow rate
Cp = Specific heat of fluid
T1 = initial temperature of the fluid after heat has been transferred
T2 = final temperature after heating the fluid

59. 1. Heat Transfer Flow Meter (Cont…)
Uses an electric immersion heater
Two thermocouples are used measure the temperature T1 and T2 placed on each side of the heater
The problem is that the heater is directly placed in the fluid and therefore can be easily damaged by corrosion and erosion.
Large input power is required if flow rate is very high.

60. 2. Hot Wire Flow Meter
The difference in temperature is measured by measuring the resistance of the resistors as all resistors have some known temperature coefficient
A Wheatstone bridge arrangement can be used to detect changes in resistance

61. Velocity Type Flow Meters
Magnetic
Ultrasonic - Transit Time, Doppler
Turbine
Vortex

62. Magnetic Type Flow Meter

63. 1. Magnetic Type Flow Meter (Cont…)
A magnetic flow meter is a volumetric flow meter which does not have any moving parts and is ideal for wastewater applications or any dirty liquid which is conductive or water based.
Magnetic flowmeters will generally not work with hydrocarbons, distilled water and many non-aqueous solutions).
Magnetic flowmeters are also ideal for applications where low pressure drop and low maintenance are required.

64. 1. Magnetic Type Flow Meter (Cont…)
The operation of a magnetic flow meter is based upon Faraday's Law, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field is proportional to the velocity of that conductor.

65. 1. Magnetic Type Flow Meter (Cont…)
The characteristics are:
Recommended Service: Clean, Dirty & Viscous Conductive Liquids & Slurries
Rangeability: 40 to 1
Pressure Loss: None
Accuracy: 0.5%
Straight Run Required: 5D Upstream, 2D Downstream
Viscosity Effect: None
Relative Cost: High
Sizes: 1” to 120”
Connections: Flanged
Type of Output: Linear

66. 2. Ultrasonic - Transit Time, Doppler

67. 2. Ultrasonic - Transit Time, Doppler (cont…)
Employ two transducers located upstream and downstream of each other.
Each transmits a sound wave to the other, and the time difference between the receipt of the two signals indicates the fluid velocity.
Transit time meters usually require clean fluids and are used where high rangeability is required.
Accuracy is within 1% for ideal applications.

68. 2. Ultrasonic - Transit Time, Doppler (cont…)
Flow is measured by measuring the difference in transit time for two ultrasonic beams transmitted in a fluid both upstream and downstream.
Ultrasonic Meters are mainly used on large size lines where high rangeability is required.
Transmitter/
Receiver (T/R)
t dn
FLOW
t up
Frequency pulse A
Transit length L
Transit time difference is proportional to mean velocity Vm, therefore Vm can be calculated as follows:

69. 2. Ultrasonic - Transit Time, Doppler (cont…)
Ultrasonic flow meters are ideal for wastewater applications or any dirty liquid which is conductive or water based.
The basic principle of operation employs the frequency shift (Doppler Effect) of an ultrasonic signal when it is reflected by suspended particles or gas bubbles (discontinuities) in motion.
Current technology requires that the liquid contain at least 100 parts per million (PPM) of 100 micron or larger suspended particles or bubbles.

70. 2. Ultrasonic - Transit Time, Doppler (cont…)
Characteristics are:
Recommended Service: Clean & Viscous Liquids, Natural/Flare Gas
Rangeability: 20 to 1
Pressure Loss: None
Accuracy: 0.25% to 5%
Straight Run Required: 5 to 30D Upstream
Viscosity Effect: None
Relative Cost: High
Sizes: > ½”
Connections: Flanged & Clamp-on Design
Type of Output: Linear

71. 3. Turbine Type Flow Meter

72. 3. Turbine Type Flow Meter (Cont….)
Turbine meter is kept in rotation by the linear velocity of the stream in which it is immersed.
The number of revolutions the device makes is proportional to the rate of flow.

73. 3. Turbine Type Flow Meter (Cont…)
Consist of a small vaned wheel (like a propeller) in the flow path
The propeller spin according to how fast the fluid flows past it
The rate of rotation will give us an indication of the flow rate

74. 3. Turbine Type Flow Meter (Cont….)
Characteristics are:
Recommended Service: Clean & Viscous Liquids, Clean Gases
Rangeability: 20 to 1
Pressure Loss: High
Accuracy: 0.25%
Straight Run Required: 5 to 10D Upstream
Viscosity Effect: High
Relative Cost: High
Sizes: > ¼”
Connections: Flanged
Type of Output: Linear

75. 4. Vortex Type of Flow Meter

76. 4. Vortex Type of Flow Meter (Cont..)
Works according to the principle that an obstruction in a pipe will produce turbulence in a known fashion
A centrally placed object, known as a bluff body, will shed vortices downstream in a very predictable and linear way
An ultrasonic beam can be used to detect the vortices
A pressure system can also be used to detect them

77. 4. Vortex Type of Flow Meter (Cont..)
Characteristics are:
Recommended Service: Clean & Dirty Liquids, Gases
Rangeability: 10 to 1
Pressure Loss: Medium
Accuracy: 1%
Straight Run Required: 10 to 20D Upstream, 5D Downstream
Viscosity Effect: Medium
Relative Cost: Medium
Size: ½” to 12”
Connection: Flanged
Type of Output: Linear

78. Positive Displacement Type Flow Meters
A positive displacement flow meter, commonly called a PD meter, measures the volume flow rate of a continuous flow stream by momentarily entrapping a segment of the fluid into a chamber of known volume and releasing that fluid back into the flow stream on the discharge side of the meter.
By monitoring the number of entrapments for a known period of time or number of entrapments per unit time, the total volume of flow or the flow rate of the stream can be ascertained.
The total volume and the flow rate can then be displayed locally or transmitted to a remote monitoring station.

79. Positive Displacement Type Flow Meters
Sliding-vane type PD meter.
Piston Type PD Meter
Tri-Rotor Type PD Meter
Oval Gear PD Meter
Birotor PD Meter

80. Positive Displacement Type Flow Meters
Advantages are:
Good accuracy at low flow rates
Good repeatability
Easy to install and maintain
Moderate cost

81. Positive Displacement Type Flow Meters
Disadvantages are:
Limited power to drive the mechanical accessories
Can be used with clean fluids only