Flowmeters

1
Flowmeters

• Andre Pennington
• Kat Witherspoon
• Pam Buzzetta

2
Introduction

• Flowmeters are process instruments that measure a
  fluids linear or non-linear flow at volumetric
  or mass flow rates
• A fluid can either be a liquid or a gas

3
Introduction

• Key features to consider in flow meter selection
• Fluid properties
• Liquid or gas
• Temperature and pressure
• Density
• Viscosity
• Chemical properties
• Presence of other phases

4
Introduction

• Key features to consider in flow meter selection
• Installation considerations
• Orientation
• Flow direction
• Upstream/downstream pipe work
• Location for servicing
• Location of valves
• Effects of local vibration
• Electrical connections
• Effects of unsteady flow

5
Introduction

• Key features to consider in flow meter selection
• Performance considerations
• Accuracy
• Repeatability
• Linearity
• Rangeability (turndown)
• Pressure drop
• Output signal characteristics
• Response time

6
Introduction

• Key features to consider in flow meter selection
• Economic considerations
• Cost of ownership (i.e. purchase, installation,
  operation, maintenance, calibration, meter life,
  spares)
• Pressure loss
• Environmental considerations
• Ambient temperature effects
• Humidity effects
• Safety factors
• Electrical interference

7
Positive Displacement Flowmeters

• Fluid goes through a chamber with a unit that
  repeatedly fills and discharges a fixed volume
• The total volumetric flow rate can then be
  calculated from the rate of filling and
  discharging the discrete volumes

8
Positive Displacement Flowmeter

• Accuracy 0.25 to 1
• Line sizes ¼ to 3
• Rangeability 21 to 101
• Common Applications
• Fluids generally need a degree of lubricity
• Clean, non-abrasive, medium to high viscosity
  liquids
• Good for batch operation, low-tech plants
• Often used in oil and gas refining, chemical,
  pulp and paper

9
Positive Displacement Flowmeter
Nutating Disc
Rotating Valve
10
Positive Displacement Flowmeter
Oscillating Piston
Oval Gear
11
Positive Displacement Flowmeter

• Advantages
• Moderately inexpensive
• No Reynolds number constraints
• No upstream/downstream requirements
• High accuracy 0.25 to 1 of rate
• Can measure very low and very viscous flows

12
Positive Displacement Flowmeter

• Disadvantages
• Moving parts
• Can create large pressure drops
• Maintenance is necessary must disassemble to
  unplug if using a dirty fluid and subject to
  deterioration
• Measures discrete fluid flows instead of actual
  flow rate
• May take up a lot of space

13
Differential Pressure Flowmeter

• Flow goes through a section with different cross
  section areas that cause pressure and velocity
  variations
• Employ Bernoulli equation by observing
  relationship between pressure drop and velocity
  to get volumetric flow

14
Differential Pressure Flowmeter

• Most common method to measure flow
• Smart transmitters simplify use
• Accuracy 2 of full scale
• Line size greater than ½
• Rangeability 41
• Common Applications
• Most gases and low viscosity fluids
• Used for chemical, oil and gas refining, power,
  and transfer of natural gas

15
Differential Pressure Flowmeter

• Orifice Plates

Calculate mass flow mactual KAt(2?(p1-p2))0.5
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Differential Pressure Flowmeter

• Venturi Tube

Flow Nozzle
17
Differential Pressure Flowmeter

• Advantages
• Well known system of measurement
• Versatile
• Line size flexibility
• Inexpensive initially
• Highly repeatable
• East to maintain
• Economical to correct sizing mistake

18
Differential Pressure Flowmeter

• Disadvantages
• High installation costs
• Moderate system accuracy
• An abrasive or sticky process will erode accuracy
  and increase maintenance cost
• Moderate rangeability
• High relative pressure loss

19
Turbine Flow MetersOverview

• Uses flow to turn a turbine rotor
• Magnetic sensor transmits a voltage pulse to a
  processor
• Axial-vane rotor is free turning
• Rotor continuously moving under pressure of the
  liquid
• Accuracy in the range of /- 0.25 with 101
  turndowns.

20
Turbine Flow Meters Common Applications

• Turbine flow meters are widely used for both
  liquid and gas applications
• Typical applications include
• Oil and gas, refining, chemical, semiconductor,
  agricultural, pharmaceutical, food beverage
  dispensing, photo development, process control,
  and more

21
Turbine Flow MetersBenefits/Advantages

• High degree of accuracy at low cost, especially
  when combined with a flow computer
• Flexibility in connecting to associated
  electronic readout devices for flow control and
  computer interface
• Wide flow rangeability
• Construction materials that permit use with many
  process fluids
• Simple, durable, field-repairable construction
• Operation over a wide range of temperatures and
  pressures

22
Turbine Flow MetersLimitations/Disadvantages

• Poor interchangeability from unit to unit
• Bearings depend on lubricity and cleanliness of
  process fluid
• Turbine blades are susceptible to wear and must
  be frequently calibrated
• Liquid applications may be suspect to problems
  involving cavitation, specific gravity, and
  viscosity
• Intended for clean fluid applications

23
Magnetic Flow MetersOverview

• Nonmagnetic tube surrounded by coils
• Must pump conductive liquids
• Flow rate inferred by sensing linear velocity
• Principle of operation based on Faradays Law,
  EkBDV
• 301 rangeability
• Accuracy 0.5 of volumetric rate
• Line size of 0.15 to 60

24
Magnetic Flow MetersCommon Applications

• Turbine flow meters are widely used for
  corrosive, dirty, or slurry-like liquids
• Typical applications include
• Wastewater applications or any dirty liquid which
  is conductive or water based (large water flows)
• Pulp paper industry, acid flows or other highly
  corrosive liquids, abrasive fluids such as mining
  ore slurries and pulp stock
• Also ideal for applications where low pressure
  drop and low maintenance are required

25
Magnetic Flow MetersBenefits/Advantages

• Relatively unaffected by changes in liquid
  density or viscosity (compatible with wide range
  of process fluids)
• Liquid turbulence has a very limited affect
• Suitable for high viscosity and slurries
• Low maintenance, high accuracy and rangeability
• No pressure loss
• Obstructionless flow
• Flow profile has minimum effect on measurement
  accuracy (Re constraints and little flow
  conditioning needed)

26
Magnetic Flow MetersLimitations/Disadvantages

• Measures conductive liquids only
• High initial cost
• 4-wire device (requires external power source)
• Must be lined with non-conductive material (lower
  temperature and pressure limits)
• Grounding problems
• Unstable zero with empty meter

27
Ultrasonic Flow MeterOverview

• Use transmitted sound waves to determine flow
  rate
• Measures liquids and gases with different designs
• Accuracy 1-5 for microprocessor-based units
• Rangeability 20 to 501
• Can be divided into 2 types
• Transit Time (pulsed type)
• Doppler (frequency shift type)

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Ultrasonic Flow MeterTransit Time (Pulsed Type)

• Sonic transducers are mounted diagonally on
  opposite sides of a pipe
• Requires clean liquid and uniform flow profile
• Rangeability 101
• Accuracies/- 1 of rate
• Advantages
• Bi-directional and non-intrusive

29
Ultrasonic Flow MeterDoppler (Frequency Shift
Type)

• Established 1843 by Christian Doppler
• Measures the shift in frequency due to motion of
  particles or bubbles in the process pipe
• Turndowns 101
• Accuracy /- 1 of rate
• Not suitable for clean liquids
• Requires straight pipe runs for installation
• Pipe must have good acoustical properties

30
Open Channel (Weirs and Flumes)

• Oldest method to measure flow, used by Romans to
  measure flow in their aqueducts
• Any time the fluid flows with a free surface
• Examples aqueducts, log flumes, channels, etc.
• Flow measured by inserting a calibrated
  restriction to the channel
• Two types of restrictions
• Weirs
• Flumes

31
Ultrasonic Flow MetersCommon Applications

• Liquids and some gas applications
• Doppler flowmeters require entrained gas or
  particles to reflect ultrasonic energy
• Where non-wetted sensors are applicable
• Existing installations where pipe modifications
  are difficult or uneconomical
• Where exotic materials make other flowmeter
  uneconomical
• Large pipes where in-line meters are uneconomical
• Temporary installations
• Typical applications include
• Water and wastewater, chemical, refining, oil and
  gas

32
Ultrasonic Flow MetersBenefits/Advantages

• Some designs allow measurement to be made
  external to the pipe (utilize no wetted parts)
• Low maintenance

33
Ultrasonic Flow MetersLimitations/Disadvantages

• Fluid changes ( solids, bubbles, etc) affect
  measurement
• Proper installation is critical
• Longer upstream/downstream straight piping
  requirements
• Minimum Reynolds number constraint
• 4-wire operation (external power source)
• Low user confidence
• Only mixed success in industrial flow applications

34
Oscillatory Flowmeters

• Two types
• Vortex Shedding
• Fluidic

Fluidic Flowmeter
Vortex Shedding Flowmeter
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Vortex Shedding

• Vortex shedding is caused by fluid flowing around
  an object
• Blunt object placed in the flowing stream
• The frequency of the vortices is measured
• The relationship between flow and frequency is
  V kdf
• The frequency is directly proportional to the
  flow rate.

36
Measuring the Vortices

• Different objects manufactured to produce stable
  vortices
• Vortices are measured by
• High frequency pressure transducers
• Measuring variations in heat transfer from a
  heated resistor
• Ultrasonics

37
Pros Cons

• Pros
• Good accuracy (/- 0.5) and rangeability (401)
• No moving parts, less to break
• Moderate costs
• Can handle liquid, gas, and steam
• Low pressure drop
• Not affected by fluid density changes

• Cons
• Intrusive, obstruct flow
• If using ultrasonics to measure the vortices,
  straight runs of pipe are needed
• Re lt 20,000 (high) for linear performance
• Sensitive to increasing Viscosity
• Expensive in larger sizes

38
Common Applications

• Low viscosity fluids
• Pressurized gases
• Steam and other utility fluids
• Pressurized gases with high densities
• Single-phase fluids (no particulate matter)

39
Fluidic

• As fluid enters device, flows along one interior
  wall
• Some fluid diverted back to inlet (feedback flow)
  causing the fluid to be pushed against other wall
• The flow shifts from side to side creating
  oscillations
• Oscillations sensed by an electronically heated
  thermistor on one side
• Alternating flow causes the thermistor to be
  cooled, this signal is directly proportional to
  velocity

40
Pros Cons

• Pros
• Accuracy between 0.5 and 1.0 of rate
• Minimum maintenance
• Inexpensive

• Cons
• Can only be used on clean low-viscosity fluids
• Re 3,000 (requires turbulent flow)
• Only used in pipes 4 or less diameter

41
Target Flowmeters
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Target Flowmeter

• Use an object that is placed in the fluid flow
• Object mounted at right angle
• Force exerted on the target is measured by strain
  gauges
• Gauges produce electronic output that is
  proportional to the square of flow rate
• Optimum size of target depends on liquid being
  studied

43
Drawbacks

• While accuracy is good at low scale, at full
  scale the accuracy can vary as much as 5
• Straight pipe length requirements
• 20 x diameter upstream
• 10 x diameter downstream

44
Mass Flowmeters

• Thermal
• Angular Momentum
• Coriolis
• Two general categories
• Inferred mass (uses density to convert volumetric
  to mass flow)
• Direct Mass (actually measure mass)

45
Thermal Mass

• Measures heat loss from a heat source
• Measures temperature rise as flow passes a hot
  tube
• Mass flow is inferred from known physical
  properties of fluid
• Usually used for gas applications

46
Coriolis

• Operates on gyroscopic principle
• Based on coriolis forces (angular velocity of
  earth imparts force on a moving object)
• Fluid flows through U- or S-shaped tube which
  vibrates at its natural frequency

47
Coriolis

• Motion of fluid in the tubes resist this
  vibration (the tubes twist)
• Velocity of the tube deflection is proportional
  to mass flow

48
Pros Cons

• Pros
• Extremely accurate (0.15)
• Directly measures mass
• No Re constraints
• Low maintenance
• Can measure density, temperature, mass and
  volumetric flow

• Cons
• High initial capital costs
• Small pipe diameters needed cause large pressure
  drop
• Not recommended for measurements involving gases

49
Angular Momentum

• Measures the force required to resist the angular
  momentum of flowing fluid
• This force proportional to mass
• Device consists of
• Motor
• Impeller (imparts the momentum)
• Turbine to resist the angular momentum torque is
  applied
• The torque needed to resist rotation of the
  turbine is transmitted to a display

50
Drawbacks

• Only clean liquids can be used
• Lots of moving parts that often require
  maintenance
• Expensive

51
Sources

• http//www.manufacturing.net/ctl/article/CA185726
• http//www.efunda.com/designstandards/sensors/flow
  meters/flowmeter_pd.cfm
• http//www.efunda.com/designstandards/sensors/flow
  meters/flowmeter_dp.cfm
• www.manufacturing.net/ctl/article/CA325984
• http//www.jlcinternational.com/gas_liquid_turbine
  _flowmeters.htm
• http//www.ddc-online.org/inout/inout_chapt02_ana_
  06flow.aspx
• http//www.omega.com/prodinfo/magmeter.html
• http//www.envitech.co.uk/Product_Images/FlowMeter
  4210.jpg
• http//www.seilenterprise.co.kr/English/Technology
  /flowmetertypes.htm
• http//www.efunda.com/designstandards/sensors/flow
  meters/flowmeter_tar.cfm
• http//www.omega.com/literature/transactions/volum
  e4/images/10_Fig_01_l.GIF