Flow Measurement

1
Flow Measurement(basics)

• Ashvani Shukla
• CI
• Reliance

2
INTRODUCTION

• In the physical world, mechanical engineers are
  frequently required to monitor or control the
  flow of various fluids through pipes, ducts and
  assorted vessels. This fluid can range from thick
  oils to light gasses. While some techniques work
  better with some groups of fluids, and less well
  with others, some are not at all suitable for
  some applications. In this primer on fluid flow
  instrumentation we will look at a wide variety of
  flow transducers and their application in the
  physical world.

3
Fluid flow measurement

• Fluid flow measurement can encompass a wide
  variety of fluids and applications. To meet this
  wide variety of applications the instrumentation
  industry has, over many years, developed a wide
  variety of instruments. The earliest known uses
  for flow come as early as the first recorded
  history. The ancient Sumerian cities of UR and
  Kish, near the Tigris and Euphrates rivers
  (around 5000 B.C.) used water flow measurement to
  manage the flow of water through the aqueducts
  feeding their cities. In this age the a simple
  obstruction was placed in the water flow, and by
  measuring the height of the water flowing over
  the top of the obstruction, these early engineers
  could determine how much water was flowing. In
  1450 the Italian art architect Battista Alberti
  invented the first mechanical anemometer. It
  consisted of a disk placed perpendicular to the
  wind, and the force of the wind caused it to
  rotate. The angle of inclination of the disk
  would then indicate the wind velocity. This was
  the first recorded instrument to measure wind
  speed. An English inventor, Robert Hooke
  reinvented this device in 1709, along with the
  Mayan Indians around that same period of time.
  Today we would look down our noses at these crude
  methods of flow measurement, but as you will see,
  these crude methods are still in use today.

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TYPE OF FLOW

• There are in general three types of fluid flow in
  pipes
• laminar
• turbulent
• transient
• Laminar flow
• Laminar flow generally happens when dealing with
  small pipes and low flow velocities. Laminar flow
  can be regarded as a series of liquid cylinders
  in the pipe, where the innermost parts flow the
  fastest, and the cylinder touching the pipe isn't
  moving at all.
• Shear stress in a laminar flow depends almost
  only on viscosity - µ - and is independent
  of density - ?.
• Turbulent flow
• In turbulent flow vortices, eddies and wakes make
  the flow unpredictable. Turbulent flow happens in
  general at high flow rates and with larger pipes.
• Shear stress in a turbulent flow is a function
  of density - ?.
• Transitional flow

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CONTINUE..

• Transitional flow is a mixture of laminar and
  turbulent flow, with turbulence in the center of
  the pipe, and laminar flow near the edges. Each
  of these flows behave in different manners in
  terms of their frictional energy loss while
  flowing and have different equations that predict
  their behavior.
• Turbulent or laminar flow is determined by the
  dimensionless Reynolds Number.
• Reynolds Number
• The Reynolds number is important in analyzing any
  type of flow when there is substantial velocity
  gradient (i.e. shear.) It indicates the relative
  significance of the viscous effect compared to
  the inertia effect. The Reynolds number is
  proportional to inertial force divided by viscous
  force.
• The flow is
• laminar when Re lt 2300
• transient when 2300 lt Re lt 4000
• turbulent when 4000 lt Re

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TYPE OF FLOW

• Uniform Flow, Steady Flow
• It is possible - and useful - to classify the
  type of flow which is being examined into small
  number of groups. If we look at a fluid flowing
  under normal circumstances - a river for example
  - the conditions at one point will vary from
  those at another point (e.g. different velocity)
  we have non-uniform flow. If the conditions at
  one point vary as time passes then we have
  unsteady flow. Under some circumstances the flow
  will not be as changeable as this. He following
  terms describe the states which are used to
  classify fluid flow
• uniform flow If the flow velocity is the same
  magnitude and direction at every point in the
  fluid it is said to be uniform.
• non-uniform If at a given instant, the velocity
  is not the same at every point the flow is
  non-uniform. (In practice, by this definition,
  every fluid that flows near a solid boundary will
  be non-uniform - as the fluid at the boundary
  must take the speed of the boundary, usually
  zero. However if the size and shape of the of the
  cross-section of the stream of fluid is constant
  the flow is considered uniform.)
• steady A steady flow is one in which the
  conditions (velocity, pressure and cross-section)
  may differ from point to point but DO NOT change
  with time. unsteady If at any point in the
  fluid, the conditions change with time, the flow
  is described as unsteady. (In practice there is
  always slight variations in velocity and
  pressure, but if the average values are constant,
  the flow is considered steady.

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CONTINUOUS

• Combining the above we can classify any flow in
  to one of four type
• 1. Steady uniform flow. Conditions do not change
  with position in the stream or with time. An
  example is the flow of water in a pipe of
  constant diameter at constant velocity. Fluid
  Mechanics Fluid Dynamics The Momentum and
  Bernoulli Equations.
• 2. Steady non-uniform flow. Conditions change
  from point to point in the stream but do not
  change with time. An example is flow in a
  tapering pipe with constant velocity at the inlet
  - velocity will change as you move along the
  length of the pipe toward the exit.
• 3. Unsteady uniform flow. At a given instant in
  time the conditions at every point are the same,
  but will change with time. An example is a pipe
  of constant diameter connected to a pump pumping
  at a constant rate which is then switched off.
• 4. Unsteady non-uniform flow. Every condition of
  the flow may change from point to point and with
  time at every point. For example waves in a
  channel.

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CONTINUOUS

• Compressible or Incompressible All fluids are
  compressible - even water - their density will
  change as pressure changes. Under steady
  conditions, and provided that the changes in
  pressure are small, it is usually possible to
  simplify analysis of the flow by assuming it is
  incompressible and has constant density. As you
  will appreciate, liquids are quite difficult to
  compress - so under most steady conditions they
  are treated as incompressible. In some unsteady
  conditions very high pressure differences can
  occur and it is necessary to take these into
  account - even for liquids. Gasses, on the
  contrary, are very easily compressed, it is
  essential in most cases to treat these as
  compressible, taking changes in pressure into
  account.

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continuous

• Three-dimensional flow Although in general all
  fluids flow three-dimensionally, with pressures
  and velocities and other flow properties varying
  in all directions, in many cases the greatest
  changes only occur in two directions or even only
  in one. In these cases changes in the other
  direction can be effectively ignored making
  analysis much more simple. Flow is one
  dimensional if the flow parameters (such as
  velocity, pressure, depth etc.) at a given
  instant in time only vary in the direction of
  flow and not across the cross-section. The flow
  may be unsteady, in this case the parameter vary
  in time but still not across the cross-section.
  An example of one-dimensional flow is the flow in
  a pipe. Note that since flow must be zero at the
  pipe wall - yet non-zero in the Centre - there is
  a difference of parameters across the
  cross-section. Should this be treated as
  two-dimensional flow? Possibly - but it is only
  necessary if very high accuracy is required. A
  correction factor is then usually applied.

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continuous

• Flow is two-dimensional if it can be assumed that
  the flow parameters vary in the direction of flow
  and in one direction at right angles to this
  direction. Streamlines in two-dimensional flow
  are curved lines on a plane and are the same on
  all parallel planes. An example is flow over a
  weir foe which typical streamlines can be seen in
  the figure below. Over the majority of the length
  of the weir the flow is the same - only at the
  two ends does it change slightly. Here correction
  factors may be applied.

One dimensional flow
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Two dimensional flow
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Flow rate.

• Mass flow rate If we want to measure the rate at
  which water is flowing along a pipe. A very
  simple way of doing this is to catch all the
  water coming out of the pipe in a bucket over a
  fixed time period. Measuring the weight of the
  water in the bucket and dividing this by the time
  taken to collect this water gives a rate of
  accumulation of mass. This is know as the mass
  flow rate.
• Volume flow rate - Discharge. More commonly we
  need to know the volume flow rate - this is more
  commonly know as discharge. (It is also commonly,
  but inaccurately, simply called flow rate). The
  symbol normally used for discharge is Q. The
  discharge is the volume of fluid flowing per unit
  time. Multiplying this by the density of the
  fluid gives us the mass flow rate.

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Type of flow measurement

• The most common principals for fluid flow
  metering are
• Differential Pressure Flow meters
• Velocity Flow meters
• Positive Displacement Flow meters
• Mass Flow meters
• Open Channel Flow meters

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1.Differential Pressure Flow meters

• In a differential pressure drop device the flow
  is calculated by measuring the pressure drop over
  an obstructions inserted in the flow. The
  differential pressure flow meter is based on
  the Bernoulli's Equation, where the pressure drop
  and the further measured signal is a function of
  the square flow speed.

15

• Common types of differential pressure flow meters
  are
• Orifice Plates
• Flow Nozzles
• Venturi Tubes
• Variable Area - Rota meters
• Orifice Plate
• An orifice plate is a device used for measuring
  flow rate, for reducing pressure or for
  restricting flow (in the latter two cases it is
  often called a restriction plate). Either a
  volumetric or mass flow rate may be determined,
  depending on the calculation associated with the
  orifice plate.With an orifice plate, the fluid
  flow is measured through the difference in
  pressure from the upstream side to the downstream
  side of a partially obstructed pipe. The plate
  obstructing the flow offers a precisely measured
  obstruction that narrows the pipe and forces the
  flowing fluid to constrict.

16
continuous

• Orifice Plate is the heart of the Orifice Meter.
  It restricts the flow and develops the
  Differential Pressure which is proportional to
  the square of the flow rate. The flow measuring
  accuracy entirely depends upon the quality of
  Orifice plate, its installation and maintains.
• When measuring wet gas or saturated steam a weep
  hole is drilled in a concentrically bored orifice
  plate. This is a small hole drilled on the
  orifice plate such that its location is exactly
  on ID of the main pipe.

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• The Orifice plates are manufactured as per ISA /
  AGA/ API / ANSI standards and in various
  materials
• such as SS304 /SS316 / SS316L /Hestoly C / Monel
  / PTFE coated.
• Various bores are used for various applications.
• Orifice Plate is categories in two types -
  Paddle Type Universal Palate.
• Paddle Type Orifice Plate
• This plate is sandwiched between two Orifice
  Flanges. Tag Plate of orifice plate projects out
  from Orifice flanges and it indicates the
  existence of Orifice plate. Details such as Tag
  NO /Orifice ID / Pipe ID / Plate Material are
  stamped on one side of the tag plate which faces
  upstream side of the pipe line. Outside diameter
  of the orifice plate equals to PCD-1 Bolt Dia.
  This ensures the concentricity with the main pipe
  line. The other method to maintain the
  concentricity is by using sleeves on the bolts or
  by providing dowel pins on the Orifice Flanges.
• Universal Orifice Plate
• This is a circular plate designed to fit in the
  Orifice fittings / Plate holders / carrier rings
  / Ring Type Joints(RTJ).

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Technical Specification
1.Size for Integral Design 15, 20, 25, 40 mm
2.Size for Flanged Design 25, 40, 50, 65, 80,
100, 150 ...250 mm 3.Material- Flanges
Carrier Ring A105 / SS304/ SS316 / SS316L / CS
Other materials on request. 4.Orifice
Plate SS304, SS316, SS316L, Hast C, Monel, PP,
PVC,PTFE, Coated or Clad with PP / HDPE / PTFE.
5.Gasket CAF / SS Spiral Wound CAF / PTFE /
PVC / Rubber, Other materials as per special
request. 6.Stud / Nut ASTM A193 Gr B/ASTM
A194 CI 2H A193 B16/A194 C14 7.Standards
Applicable Design - ISA RP 3.2 / DN 1952 / BS
1042 - 1981-84 8.Bore Calculation ISO 5167 /
BS 1042 / RW Miller / L. K. SPIN / AGE - 3.7
9.Flanges ANSI B-16-36 / or Equivalent
10.Types Square edge concentric, Quadrant
edges, Conical entrance, Eccentric.
11.Pressure Toppings For 1" to 16" - Flange
Taps / Corner Taps. Above 16" - D x D/2
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Type Orifice Plate

• Paddle Type Orifice Plate
• Concentric Beveled Bore
• Application  This Most Common Bore Used In The
  Industries. This Is The Only Type Generally
  Accepted For Use In Custody Transfer Measurement,
  Since Adequate Data Is Not Available For Other
  Bores. Used Primarily For Clean Homogeneous
  Liquids, Gases, Non Viscous Fluids. The Bevel Is
  Matched At 45 Angle To The Desired Throat
  Thickness.

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2) Restriction BoreApplication This Type Is
Not Used For Flow Measurement But For Dropping
The Pressure Considerably And Reducing The Flow
Accordingly. The Bore Is Not Beveled But Kept
Straight. The Beta Ratio Has No Limit As Accuracy
Is Not The Goal
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Eccentric Bore

• Application  Used For Measurement Of Flow For
  Fluids Containing Solids And Slurries. It Is Also
  Used For Vapors And Gases Where Condensation Is
  Present. The Eccentric Bore Is Offset To Where
  The Bore Edge Is Inscribed In A Circle That Is
  98 The Line Id.

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4) Segmenta Bore

• Application
• The Segmental Bore Is Located In The Same Way
  That The Eccentric Bore Is. This Type Is Used
  Primarily For Slurries Or Extremely Dirty Gases
  Where The Flow May Contain Impurities Heavier
  Than The Fluid.

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Quadrant Bore

• Application  Used For High Viscous Fluids Such
  As Heavy Crude, Syrups  And Slurries. It Is
  Always Recommended For Flow Where Reynolds Number
  Is Less Than 10,000.The Inlet Is Quarter Of A
  Circle And The Plate Thickness Must Be At Least
  Radius Of The Inlet.

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6) Ring Type Joint Integral

• Application
• These Are Available In Oval Or Octal Shapes.
  Orifice Plate Is A Part Of RTJ Gasket.

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Ring Type Joint- Separate

• Application  These Are Available In Oval Or
  Octal Shapes. The  Orifice Plate Is Universal
  Type And Snap Fitted On The RTJ Gasket By Screws.

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Universal Orifice Plates
Application  This Is A Circular Plate Designed
To Fit In The Orifice Fittings / Plate Holders /
Carrier Rings / Ring Type Joints(RTJ).
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Various Orifice Assemblies
WNRF - Flange Taps
WNRF - Corner Taps
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Orifice working principle

• Working
• The orifice plate, being fixed at a section of
  the pipe, creates an obstruction to the flow by
  providing an opening in the form of an orifice to
  the flow passage.

When an orifice plate is placed in a pipe
carrying the fluid whose rate of flow is to be
measured, the orifice plate causes a pressure
drop which varies with the flow rate. This
pressure drop is measured using a differential
pressure sensor and when calibrated this pressure
drop becomes a measure flow rate. The flow rate
is given by.
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Where, Qa flow rateCd Discharge
coefficientA1 Cross sectional area of pipeA2
Cross sectional area of orificeP1, P2 Static
Pressures

• The main parts of an orifice flow meter are as
  followsA stainless steel orifice plate which
  is held between flanges of a pipe carrying the
  fluid whose flow rate is being measured.
• It should be noted that for a certain distance
  before and after the orifice plate fitted between
  the flanges, the pipe carrying the fliud should
  be straight in order to maintain laminar flow
  conditions.
• Openings are provided at two places 1 and 2 for
  attaching a differential pressure sensor (U-tube
  manometer, differential pressure gauge etc) as
  shown in the diagram.

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Operation of Orifice Meter

• The detail of the fluid movement inside the pipe
  and orifice plate has to be understood.
• The fluid having uniform cross section of flow
  converges into the orifice plates opening in its
  upstream. When the fluid comes out of the orifice
  plates opening, its cross section is minimum and
  uniform for a particular distance and then the
  cross section of the fluid starts diverging in
  the down stream.
• At the upstream of the orifice, before the
  converging of the fluid takes place, the pressure
  of he fluid (P1) is maximum. As the fluid starts
  converging, to enter the orifice opening its
  pressure drops. When the fluid comes out of the
  orifice opening, its pressure is minimum (p2) and
  this minimum pressure remains constant in the
  minimum cross section area of fluid flow at the
  downstream.
• This minimum cross sectional area of the fluid
  obtained at downstream from the orifice edge is
  called VENA-CONTRACTA.
• The differential pressure sensor attached between
  points 1 and 2 records the pressure difference
  (P1 P2) between these two points which becomes
  an indication of the flow rate of the fluid
  through the pipe when calibrated.

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• Applications of Orifice Meter
• The concentric orifice plate is used to measure
  flow rates of pure fluids and has a wide
  applicability as it has been standardized.
• The eccentric and segmental orifice plates are
  used to measure flow rates of fluids containing
  suspended materials such as solids, oil mixed
  with water and wet steam.
• Advantages of Orifice Meter
• It is very cheap and easy method to measure flow
  rate.
• It has predictable characteristics and occupies
  less space.
• Can be use to measure flow rates in large pipes.
• Limitations of Orifice Meter
• The vena-contracta length depends on the
  roughness of the inner wall of the pipe and
  sharpness of the orifice plate. In certain cases
  it becomes difficult to tap the minimum pressure
  (P2) due to the above factor.
• Pressure recovery at downstream is poor, that is,
  overall loss varies from 40 to 90 of the
  differential pressure.
• In the upstream straightening vanes are a must to
  obtain laminar flow conditions.
• Gets clogged when the suspended fluids flow.
• The orifice plate gets corroded and due to this
  after sometime, inaccuracy occurs. Moreover the
  orifice plate has low physical strength.
• The coefficient of discharge is low.

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Venturi Tube

• Due to simplicity and dependability, the Venturi
  tube flowmeter is often used in applications
  where it's necessary with higher TurnDown Rates,
  or lower pressure drops, than the orifice plate
  can provide.
• In the Venturi Tube the fluid flow rate is
  measured by reducing the cross sectional flow
  area in the flow path, generating a pressure
  difference. After the constricted area, the fluid
  is passes through a pressure recovery exit
  section, where up to 80 of the differential
  pressure generated at the constricted area, is
  recovered. With proper instrumentation and flow
  calibrating, the Venturi Tube flowrate can be
  reduced to about 10 of its full scale range with
  proper accuracy. This provides a TurnDown
  Rate 101.

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Venturi tube
34
Flow Nozzles

• Flow nozzles are often used as measuring elements
  for air and gas flow in industrial applications

35

• The flow nozzle is relative simple and cheap, and
  available for many applications in many
  materials.
• The Turndown Rate and accuracy can be compared
  with the orifice plate.
• The Sonic Nozzle - Critical (Choked) Flow Nozzle
• When a gas accelerates through a nozzle, the
  velocity increase and the pressure and the gas
  density decrease. The maximum velocity is
  achieved at the throat, the minimum area, where
  it breaks Mach 1 or sonic. At this point it's not
  possible to increase the flow by lowering the
  downstream pressure. The flow is choked.
• This situation is used in many control systems to
  maintain fixed, accurate, repeatable gas flow
  rates unaffected by the downstream pressure.

36
Recovery of Pressure Drop in Orifices, Nozzles
and Venturi Meters

• After the pressure difference has been generated
  in the differential pressure flow meter, the
  fluid pass through the pressure recovery exit
  section, where the differential pressure
  generated at the constricted area is partly
  recovered. As we can see, the pressure drop in
  orifice plates are significant higher than in the
  venturi tubes.

37
Variable Area Flow meter or Rota meter

• The Rota meter consists of a vertically oriented
  glass (or plastic) tube with a larger end at the
  top, and a metering float which is free to move
  within the tube. Fluid flow causes the float to
  rise in the tube as the upward pressure
  differential and buoyancy of the fluid overcome
  the effect of gravity.

38
continuous

• The float rises until the annular area between
  the float and tube increases sufficiently to
  allow a state of dynamic equilibrium between the
  upward differential pressure and buoyancy
  factors, and downward gravity factors.
• The height of the float is an indication of the
  flow rate. The tube can be calibrated and
  graduated in appropriate flow units.
• The rotameter meter typically have a TurnDown
  Ratio up to 121. The accuracy may be as good as
  1 of full scale rating.
• Magnetic floats can be used for alarm and signal
  transmission functions.

39
Velocity Flow meters

• In a velocity flow meter the flow is calculated
  by measuring the speed in one or more points in
  the flow, and integrating the flow speed over the
  flow area

40
Pitot Tubes

• The pitot tube are one the most used (and
  cheapest) ways to measure fluid flow, especially
  in air applications like ventilation and HVAC
  systems, even used in airplanes for speed
  measurent. The pitot tube measures the fluid flow
  velocity by converting the kinetic energy of the
  flow into potential energy.
• The use of the pitot tube is restricted to point
  measuring. With the "annubar", or multi-orifice
  pitot probe, the dynamic pressure can be measured
  across the velocity profile, and the annubar
  obtains an averaging effect.

41
Calorimetric Flow meter

• The calorimetric principle for fluid flow
  measurement is based on two temperature sensors
  in close contact with the fluid but thermal
  insulated from each other.

42
One of the two sensors is constantly heated and
the cooling effect of the flowing fluid is used
to monitor the flow rate. In a stationary (no
flow) fluid condition there is a constant
temperature difference between the two
temperature sensors. When the fluid flow
increases, heat energy is drawn from the heated
sensor and the temperature difference between the
sensors are reduced. The reduction is
proportional to the flow rate of the
fluid. Response times will vary due the thermal
conductivity of the fluid. In general lower
thermal conductivity require higher velocity for
proper measurement. The calorimetric flow meter
can achieve relatively high accuracy at low flow
rates
Vortex Flow Meter An obstruction in a fluid flow
creates vortices in a downstream flow. Every
obstruction has a critical fluid flow speed at
which vortex shedding occurs. Vortex shedding is
the instance where alternating low pressure zones
are generated in the downstream.
43

• Electromagnetic Flowmeter
• An electromagnetic flowmeter operate on Faraday's
  law of electromagnetic induction that states that
  a voltage will be induced when a conductor moves
  through a magnetic field. The liquid serves as
  the conductor and the magnetic field is created
  by energized coils outside the flow tube.
• The voltage produced is directly proportional to
  the flow rate. Two electrodes mounted in the pipe
  wall detect the voltage which is measured by a
  secondary element.
• Electromagnetic flowmeters can measure difficult
  and corrosive liquids and slurries, and they can
  measure flow in both directions with equal
  accuracy.
• Electromagnetic flowmeters have a relatively high
  power consumption and can only be used for
  electrical conductive fluids as water.
• The Electromagnetic Flowmeter Principle - An
  introduction to the electromagnetic flowmeter
  principle

44
Ultrasonic Doppler Flow meter

• The effect of motion of a sound source and its
  effect on the frequency of the sound was observed
  and described by Christian Johann Doppler.
• The frequency of the reflected signal is modified
  by the velocity and direction of the fluid flow
• If a fluid is moving towards a transducer, the
  frequency of the returning signal will increase.
  As fluid moves away from a transducer, the
  frequency of the returning signal decrease.
• The frequency difference is equal to the
  reflected frequency minus the originating
  frequency and can be use to calculate the fluid
  flow speed.
• The Ultrasonic Doppler and Time of Flight Flow
  meter

45
Positive Displacement Flowmeter

• The positive displacement flow meter measures
  process fluid flow by precision-fitted rotors as
  flow measuring elements. Known and fixed volumes
  are displaced between the rotors. The rotation of
  the rotors are proportional to the volume of the
  fluid being displaced.
• The number of rotations of the rotor is counted
  by an integral electronic pulse transmitter and
  converted to volume and flow rate.
• The positive displacement rotor construction can
  be done in several ways
• Reciprocating piston meters are of single and
  multiple-piston types.
• Oval-gear meters have two rotating, oval-shaped
  gears with synchronized, close fitting teeth. A
  fixed quantity of liquid passes through the meter
  for each revolution. Shaft rotation can be
  monitored to obtain specific flow rates.
• Notating disk meters have movable disks mounted
  on a concentric sphere located in spherical
  side-walled chambers. The pressure of the liquid
  passing through the measuring chamber causes the
  disk to rock in a circulating path without
  rotating about its own axis. It is the only
  moving part in the measuring chamber.
• Rotary vane meters consists of equally divided,
  rotating impellers, two or more compartments,
  inside the meter's housings. The impellers are in
  continuous contact with the casing. A fixed
  volume of liquid is swept to the meter's outlet
  from each compartment as the impeller rotates.
  The revolutions of the impeller are counted and
  registered in volumetric units.
• The positive displacement flowmeter may be used
  for all relatively nonabrasive fluids such as
  heating oils, lubrication oils, polymer
  additives, animal and vegetable fat, printing
  ink, Dichlorodifluoromethane R-12, and many more.
• Accuracy may be up to 0.1 of full rate with a
  TurnDown of 701 or more.

46
Mass Flow meters

• Mass meters measure the mass flow rate directly.
• Thermal Flow meter
• The thermal mass flowmeter operates independent
  of density, pressure, and viscosity. Thermal
  meters use a heated sensing element isolated from
  the fluid flow path where the flow stream
  conducts heat from the sensing element. The
  conducted heat is directly proportional to the
  mass flow rate and the temperature difference is
  calculated to mass flow.
• The accuracy of the thermal mass flow device
  depends on the calibrations reliability of the
  actual process and variations in the temperature,
  pressure, flow rate, heat capacity and viscosity
  of the fluid.
• Coriolis Flow meter
• Direct mass measurement sets Coriolis flowmeters
  apart from other technologies. Mass measurement
  is not sensitive to changes in pressure,
  temperature, viscosity and density. With the
  ability to measure liquids, slurries and gases,
  Coriolis flowmeters are universal meters.
• Coriolis Mass Flowmeter uses the Coriolis effect
  to measure the amount of mass moving through the
  element. The fluid to be measured runs through a
  U-shaped tube that is caused to vibrate in an
  angular harmonic oscillation. Due to the Coriolis
  forces, the tubes will deform and an additional
  vibration component will be added to the
  oscillation. This additional component causes a
  phase shift on some places of the tubes which can
  be measured with sensors.
• The Coriolis flow meters are in general very
  accurate, better than /-0,1 with an turndown
  rate more than 1001. The Coriolis meter can also
  be used to measure the fluids density.

47
Open Channel Flow meters

• A common method of measuring flow through an open
  channel is to measure the height of the liquid as
  it passes over an obstruction as a flume or weir
  in the channel.
• Common used is the Sharp-Crested Weir, the
  V-Notch Weir, the Cipolletti weir, the
  Rectangular-Notch Weir, the Parshall Flume or
  Venturi Flume.