An Introduction to Flow Measurement

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FLOWMETERED
An Introduction to Flow Measurement Training
Course
2
INTRODUCTION

• Definition of flow measurement
• Why flow is measured
• Oil gas applications
• Flow basics
• Fluid properties
• Reynolds number
• Velocity profile
• Oil and gas flows
• Multiphase flow

3
INTRODUCTION

• Metering basics
• Measurement concepts
• Accuracy and uncertainty
• Calibration concepts
• Flow meter performance measures

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Definition of flow measurement

• Flow is the movement of material from one place
  to another
• We see examples of flow around us in everyday life

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Definition of flow measurement

• Water flowing in a river to the sea

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Definition of flow measurement

• Sand flowing in an egg timer

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Definition of flow measurement

• Traffic flowing on a busy road

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Definition of flow measurement

• Normally when we describe flow our descriptions
  are in qualitative terms

9
Definition of flow measurement

• e.g. the flow in the river is very fast

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Definition of flow measurement

• e.g. The traffic on the road is very heavy

11
Definition of flow measurement

• Flow measurement is the practice of describing
  flow in quantitative terms, i.e. putting a
  numerical value on the flow
• It is easy to see how this could be done for some
  of the flow examples we have already introduced

12
Definition of flow measurement

• Traffic flowing on a busy road

10 cars in 20 seconds or 1800 cars per hour
13
Definition of flow measurement

• When we are quantifying flow in terms of both
  quantity and time then we are describing a flow
  rate
• The units of quantity and time that we use can be
  changed to suit the situation
• One person may be interested in total number of
  cars per day pass on a certain road
• Another may want to know how many cars pass per
  hour in the morning and how per hour in the
  afternoon

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Definition of flow measurement

• Sand flowing in an egg timer
• We could count the grains of sand before we put
  then in and seal the timer
• 9,000 grains of sand
• 3 minutes to flow from top to bottom
• 50 grains per second

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Definition of flow measurement

• If we were to use a similar device to add salt to
  food in production process in a factory then we
  would want to know the quantity in grams not
  grains
• We could do this by weighing a quantity of salt
  and timing how long it took to flow out
• We could then quantify the flow in mass terms
• e.g. Grams per minute

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Definition of flow measurement

• If we then know that the salt flows at our device
  flows at 5 grams per minute and that our recipe
  calls for 10 grams of salt, then we can allow the
  device to flow for two minutes to add the correct
  amount of salt to our recipe

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Definition of flow measurement

• In summary
• Flow measurement is the practice of quantifying
  the movement of material
• Flow rate is specified in units of quantity and
  time
• e.g.
• cubic meters per hour
• gallons per minute
• barrels per day
• kilograms per second

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INTRODUCTION

• Definition of flow measurement
• Why flow is measured
• Oil gas applications
• Flow basics
• Fluid properties
• Velocity profile
• Metering basics
• Measurement concepts
• Accuracy and uncertainty
• Calibration concepts
• Flow meter performance measures

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Why flow is measured

• Flow is normally measured for one of two reasons.
  Either
• The fluid has a value that is determined by
  quantity and quality, or
• The flow rate or total amount of flow has
  important consequences such that the flow must be
  controlled or the process managed accordingly

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Why flow is measured

• Examples where the fluid has a value that is
  determined by quantity and quality include
• Water
• Steam
• Crude oil
• Natural gas
• Petrol
• Lubricants
• Bottled gases
• Pharmaceuticals

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Why flow is measured

• Examples where the flow rate or total amount of
  flow has important consequences such that the
  flow must be controlled or the process managed
  accordingly include
• Intravenous drug injection in hospitals
• Chlorination of drinking water supplies
• Feedwater flow in nuclear power plants

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Why flow is measured

• Why does Fiscal Metering attract so much effort
  and expense ??
• We Meter because we need to know the MARKET VALUE
  of the hydrocarbon products
• The MARKET VALUE impacts on the Operating
  Companys PROFITS which impacts on WAGES and
  BONUSES !!!!

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Why flow is measured

• Its a MIS CONCEPTION that Production Quantity
  must be MAXIMISED at any cost
• The REALITY is that you need to MAXIMISE YOUR
  PROFIT
• So you MUST MEASURE CUSTODY ALLOCATION
  ACCURATELY and MINIMISE UNCERTAINTY

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Why flow is measured
CRUDE OIL CRUDE OIL CRUDE OIL CRUDE OIL CRUDE OIL
THE COST OF UNCERTAINTY THE COST OF UNCERTAINTY THE COST OF UNCERTAINTY THE COST OF UNCERTAINTY THE COST OF UNCERTAINTY
   
Assume an Oil Field Producing - Assume an Oil Field Producing - 50,000 bbls / day  
Assume a market value of - Assume a market value of - 100 / bbl  
   
Metering Uncertainty Daily Flow Error Daily Cost Error Annual Flow Error Annual Cost Error
bbls / day bbls / day
         
0.1 50 5,000.00 18250 1,825,000.00
0.2 100 10,000.00 36500 3,650,000.00
0.25 125 12,500.00 45625 4,562,500.00
0.3 150 15,000.00 54750 5,475,000.00
0.5 250 25,000.00 91250 9,125,000.00
1 500 50,000.00 182500 18,250,000.00
         
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Why flow is measured
GAS GAS GAS GAS GAS
THE COST OF UNCERTAINTY THE COST OF UNCERTAINTY THE COST OF UNCERTAINTY THE COST OF UNCERTAINTY THE COST OF UNCERTAINTY
   
Assume a Gas Field Producing - Assume a Gas Field Producing - 100 mmscfd  
Assume a market value of - Assume a market value of - 5000 / mmscf  
   
Metering Uncertainty Daily Flow Error Daily Cost Error Annual Flow Error Annual Cost Error
mmscfd mmscfd
         
0.6 0.6 3000 219 1095000
0.8 0.8 4000 292 1460000
1 1 5000 365 1825000
1.5 1.5 7500 547.5 2737500
2 2 10000 730 3650000
5 5 25000 1825 9125000
         
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Why flow is measured

• It is obvious that flow could be measured by
  collecting the fluid in appropriate containers
  and counting those
• However, it is also obvious that transporting
  fluids in pipes has great advantages and that it
  is inconvenient to measure all of the input or
  output by using buckets or barrels
• Filling your car at the petrol station would be
  very inconvenient if you had to transfer the
  petrol using 1 litre containers

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Why flow is measured

• In order to avoid the inconvenience of measuring
  the flow in containers, we require a device that
  can be installed in a pipe to make a continuous
  measurement of flow as illustrated below
• We call this a flow meter
• Flow measurement is often referred to as metering

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INTRODUCTION

• Definition of flow measurement
• Why flow is measured
• Oil gas applications
• Flow basics
• Metering basics

29
Oil and gas applications

• Oil and gas flow measurement applications
  normally fall into at least one of the four
  categories below
• Custody Transfer
• Fiscal
• Allocation
• Management and Control
• In oil and gas applications the flow can be a
  mixture of oil, water and gas we will discuss
  the importance of this later

30
Oil and gas applications

• Custody transfer is a term used to describe a
  change of ownership or responsibility
• A custody transfer transaction normally involves
  one party paying the other for a product on the
  basis of the total quantity delivered
• e.g. 500,000 barrels of crude oil at 100 per
  barrel

31
Oil and gas applications

• Typical examples of custody transfer include
• crude oil shuttle tanker loading and offloading
• oil and gas pipeline export from offshore
  facilities and
• road tanker loading and offloading of refined
  products
• Custody transfers normally involve processed
  fluids, i.e. stabilised crude oil with water
  and gas removed, refined oil products or dry
  natural gas

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Oil and gas applications

• Fiscal metering refers to flow measurement that
  is related to government tax revenues, i.e. the
  results are used to calculate oil and gas
  production figures that determine how much tax
  the oil companies have to pay the government
• It is normal for most countries to have specific
  regulations for fiscal metering
• In addition to the regulations, which will tend
  to be of a legalistic nature, there may also be
  technical guidelines to follow or standards to
  comply with

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Oil and gas applications

• Fiscal metering is normally associated with
  custody transfer but allocation metering can also
  have a fiscal use

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Oil and gas applications

• Allocation is a term used to describe the process
  of attributing a proportion of production to a
  particular source
• Allocation metering often has a similar purpose
  to custody transfer metering, i.e. it has a
  financial implication

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Oil and gas applications

• In this example, if field A and field B have
  different owners, allocation metering is required

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Oil and gas applications

• Allocation metering often has a fiscal
  implication
• If field A and B have different owners then the
  tax on the export will be split according to the
  allocation
• Even if A B have the same owner, if different
  tax regimes are applied then the allocation
  meters would have a fiscal use

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Oil and gas applications

• Management and control application for oil and
  gas production tend to fall into the following
  broad categories
• Reservoir management
• Well management and control
• Production management and control
• Environmental management
• There are many different specific uses for flow
  measurement
• A few are given here as examples

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Oil and gas applications

• In reservoir management it is important to
  quantify both production and injection flow rates

Oil production
Water injection
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Oil and gas applications

• In well and flow line management flow measurement
  can play an important part in operations such as
• Production monitoring to evaluate the
  effectiveness of well clean up operations
• Gas lift to optimise the gas flow rate to get
  the maximum oil flow without using excessive
  amounts of gas
• Injection of corrosion inhibitor to protect
  flow lines and risers
• Injection of scale and hydrate inhibitors to
  prevent blockage of flow lines and risers

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Oil and gas applications

• In the production train flow measurement is used
  to control the rate of flow through heaters and
  separators to ensure the fluids are processed
  properly prior to export, storage or overboard
  discharge

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Oil and gas applications

• Measurement of production into storage on a
  platform with gravity base storage and shuttle
  tanker export is an other example of an important
  measurement that is not itself an allocation or
  custody transfer measurement

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Oil and gas applications

• Gas can be flared as part of the production
  process, either if there is no gas export
  facility or more often to dispose of waste gases
  or to cope with emergency process shut downs
• Quantification of flare gas is often required to
  comply with environmental legislation
• A growing number of companies are now taxed and
  hence the flare is a fiscal measurement

43
INTRODUCTION

• Definition of flow measurement
• Why flow is measured
• Oil gas applications
• Flow basics
• Fluid properties
• Reynolds number
• Velocity profile
• Oil and gas flows
• Multiphase flow
• Metering basics

44
Flow basics

• Fluids are materials that flow easily, i.e.
  liquids and gases
• Liquids will sit in a open container and cannot
  easily be compressed

45
Flow basics

• Gases can be compressed and will expand to fill
  whatever contains them

46
Flow basics

• Density is used to describe a fluid in terms of
  mass per unit volume
• In metric units density is normally expressed in
  kilograms per cubic meter (kg/m3)
• The approximate densities of some common
  materials are given below
• Steel 7800 kg/m3
• Water 1000 kg/m3
• Oil 800 kg/m3
• Atmospheric air 1.3 kg/m3

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Flow basics

• Density is important in flow measurement for two
  main reasons
• It influences the way fluids behave and interact
  with objects and other fluids
• For certain fluids the value of the fluid can be
  related to its density
• If we mix two liquids of different densities,
  such as oil and water, and then allow them to
  settle out, then the liquid with the lowest
  density will float to the top and high density or
  heavy liquid will sink to the bottom

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Flow basics

• As far as flow measurement is concerned, the
  density of the fluid will influence how some
  types of flow meter respond to the flow
• A useful example is to consider the flow of air
  in a breeze and the flow of water in a river
• A fast flowing river has more power to displace
  objects than wind blowing at the same speed

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Flow basics

• All fluids are compressible to some extent and
  density changes occur in both gases and liquids
  with both temperature and pressure
• Because gases expand and compress much more
  easily than liquids, the density changes with
  pressure in a gas are more significant than for a
  liquid

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Flow basics

• Density changes with pressure
• Liquid change is /- 4 kg/m3, less than 0.5
• Gas change is /- 50 kg/m3, around 80

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Flow basics

• Density changes with temperature
• Liquid change is /- 20 kg/m3, around 2
• Gas change is /- 5 kg/m3, around 10

52
Flow basics

• Because the density of liquids and gases are
  sensitive to pressure and temperature, it is
  common for volumes to be quantified at a
  reference or normal temperature and pressure
• The most commonly used reference conditions are a
  temperature of 15 degrees Celsius and an
  atmospheric pressure of 1 bar

53
Flow basics

• Viscosity is a term used to describe the tendency
  of a fluid to resist flow and is sometimes
  described as internal friction
• The more viscous a fluid the more it resists flow
• A commonly used unit of absolute viscosity is the
  centipoise (1 cP 10-3 Pascal seconds)
• Water has a viscosity of approximately 1 cP
• Honey has a viscosity of around 10,000 cP
  

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Flow basics

• Gases have low absolute viscosities
• For example, air at atmospheric conditions has a
  viscosity of approximately 0.018 cP
• Another commonly used measure of viscosity is the
  centistoke (1 cSt 10-6 m2/s)
• The viscosity in cSt is called the kinematic
  viscosity is obtained by dividing the absolute
  viscosity in cP by density in kilograms per litre

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Flow basics

• The viscosity of liquids tends to decrease with
  increasing temperature whereas for gases the
  opposite is true
• The variation of viscosity with temperature is
  most significant in liquids

56
Flow basics

• Hydrocarbon liquid viscosities vs temperature
• Note that for oils of higher viscosity, the
  gradient of viscosity change with temperature is
  steeper

57
Flow basics

• The viscosity of a fluid affects its flow
  behaviour
• Viscosity can also have a direct effect on the
  performance of some types of flow meter
• The way in which viscosity influences fluid flow
  is best described with reference to Reynolds
  number

58
INTRODUCTION

• Definition of flow measurement
• Why flow is measured
• Oil gas applications
• Flow basics
• Fluid properties
• Reynolds number
• Velocity profile
• Oil and gas flows
• Multiphase flow
• Metering basics

59
Flow basics

• Reynolds number describes the balance between the
  dynamic (driving) forces and the viscous
  (frictional) forces in a fluid flow
• For circular pipes Reynolds number, Re, can be
  written as

where U is the average flow velocity in meters
per second, D is the diameter of the pipe in
meters and ? is the kinematic viscosity in cSt
60
Flow basics

• Reynolds number can be used to classify flow in
  to different flow regimes
• If Reynolds number is small (less than around
  2,000) then viscous forces dominate and the flow
  is described as laminar flow
• When the flow is laminar it can be thought of as
  moving along in thin layers with no mixing
  between the layers

61
Flow basics

• If we were to inject dye or tracer particles into
  a laminar flow, we would expect them to move in a
  straight line, parallel with the pipes axis

Flow
62
Flow basics

• If Reynolds number is large (more than around
  5,000) then dynamic forces dominate and the flow
  is described as turbulent flow
• When the flow is turbulent the general motion is
  parallel to the pipes axis but with mixing
  occurring between the different layers

63
Flow basics

• If we were to inject dye or tracer particles into
  a turbulent flow then we would expect the dye
  stream to break up or the particles to move
  forward with random movements superimposed
• This superimposed random motion can be in any
  direction and is called turbulence

Flow
64
Flow basics

• In between laminar and turbulent flow the flow is
  described as transitional
• In this flow regime, the flow switches back and
  forth between laminar and turbulent behaviour
• The transitional flow regime is less predictable
  than turbulent or laminar flow and can cause
  difficulties in terms of flow measurement

65
INTRODUCTION

• Definition of flow measurement
• Why flow is measured
• Oil gas applications
• Flow basics
• Fluid properties
• Reynolds number
• Velocity profile
• Oil and gas flows
• Multiphase flow
• Metering basics

66
Flow basics

• Velocity profile is a term used to describe how
  fast the fluid is flowing at different points in
  a circular cross-section of the pipe
• If we had a long straight pipe with a completely
  frictionless pipe wall, then the fluid would move
  all at the same velocity, as if it were a solid
  plug

67
Flow basics

• In the uniform velocity profile shown on the
  previous slide is not possible as the pipe wall
  creates friction
• This means that a tiny thin layer of fluid next
  to the pipe wall will not move, i.e. its velocity
  will be zero

68
Flow basics

• As we move away from the pipe wall, the drag
  caused by the pipe wall becomes less and the
  fluid moves at increasingly higher velocities

69
Flow basics

• In a very long straight piece of pipe the highest
  velocity occurs in the centre of the pipe, i.e.
  at the point that is furthest away from the pipe
  wall

70
Flow basics

• The way that the velocity changes from the pipe
  wall to the centre of the pipe is dependent on
  flow regime and Reynolds number
• In laminar flows the viscosity effect or
  internal friction dominates and does an
  effective job of transferring the drag of the
  pipe wall from layer to layer
• This results in a profile where the velocity
  changes fairly gradually as you move from the
  pipe wall to the centre line

71
Flow basics

• When flow enters a long straight piece of pipe
  from a tank, a pump or some pipe bends, the
  velocity profile will be changing shape
• As the flow moves down the straight piece of pipe
  the profile will gradually stabilise or develop
• Once the shape of the velocity profile is no
  longer changing, the flow is said to be fully
  developed

72
Flow basics

• The length of straight pipe that is required to
  ensure fully developed flow is dependent on the
  velocity profile entering the straight section of
  pipe

Flow profile no longer changing i.e. fully
developed flow
Flow profile changing
73
Flow basics

• The shape of a fully developed velocity profile
  can be described mathematically
• The equations are not required in this course but
  it is useful to understand some features of fully
  developed velocity profiles
• Fully developed laminar flow has a velocity
  profile that has a parabolic shape as shown in
  the following slide

74
Flow basics

• The maximum velocity in the centre of the pipe
  has a value that is twice the average velocity

75
Flow basics

• Fully developed laminar flow has the same
  velocity profile shape for the full range of
  Reynolds number in the laminar regime, i.e. from
  0 to 2,000
• Fully developed turbulent flow has a velocity
  profile that is flatter in the middle and varies
  steeply in the boundary layer next to the pipe,
  as shown in on the following slide

76
Flow basics

• The maximum velocity in the centre of the pipe
  has a value between around 1.1 and 1.3 times the
  average velocity

77
Flow basics

• In fully developed turbulent flow the profile
  shape varies slightly with Reynolds number and
  also pipe roughness
• As Reynolds number increases, the central area of
  the velocity profile gets flatter
• A smooth pipe will also produce a flatter
  turbulent velocity profile than a rough pipe

78
Flow basics

• When fluid flowing in a pipe encounters a
  pipeline component such as a partially closed
  valve, a reducer or a bend, then the flow is
  forced to change direction and this will distort
  the velocity profile
• We can consider flow round a 90-degree bend as a
  good example of how velocity profiles become
  distorted

79
Flow basics

• As the flow goes round the bend the fluid on the
  outside of the bend is forced to move more
  quickly
• This means that instead of the symmetrical flow
  profile that we get in a straight pipe, we get a
  skewed or asymmetric profile

80
Flow basics

• The previous animation illustrates that the
  greatest distortion occurs close to the fitting
  (the bend in this case) and that it gradually
  returns to a fully developed profile the further
  along a straight pipe we go
• As most flow meters rely on the flow profile
  being relatively close to a fully developed
  profile, guidance from standards or meter
  manufacturers will often advise on the minimum
  length of straight pipe that should be installed
  in front of the flow meter

81
Flow basics

• Flow round bends or through any other pipe
  components that cause a significant change in
  direction of the flow also create secondary flows
• Taking the example of a single pipe bend again,
  the flow coming round the bend is forced against
  the pipe wall on the outside of the bend and
  rebounds towards the opposite side of the pipe

82
Flow basics

• This sets up two rotating vortices

83
Flow basics

• This shows that rather than having all of the
  fluid moving forward in the pipe in a straight
  line we can have some up and down and side to
  side movement in the cross section of the pipe

84
Flow basics

• If two changes in direction in two different
  planes occur relatively close to one another, the
  result is a distortion of the axial velocity
  profile and a single vortex

85
Flow basics

• The vortex causes the distorted profile to rotate
  such that the high velocity will corkscrew down
  the pipe
• The single vortex shown in the previous slide is
  commonly called swirl
• Swirl can persist very far downstream of the
  bends and could have an impact on flow meter
  accuracy

86
Flow basics

• Flow profiles that are different from ideal fully
  developed flow tend to cause flow meter to
  indicate a flow rate that is different from what
  they would indicate in the ideal situation
• The reading could be higher or lower and could be
  large or small, depending on the meter type and
  the nature of the distorted flow profile
• These differences in indicated flow rate are
  often referred to as installation effects, as
  they are dependent on the layout of the pipe
  system into which the meter is installed

87
INTRODUCTION

• Definition of flow measurement
• Why flow is measured
• Oil gas applications
• Flow basics
• Fluid properties
• Reynolds number
• Velocity profile
• Oil and gas flows
• Multiphase flow
• Metering basics

88
Flow basics

• Oil flow pipelines and systems tend to be
  designed such that the flow velocity is around5
  m/s
• Pipe sizes from 1-inch to 24-inch then most of
  the likely conditions will be covered
• Many light crudes and refined oils have viscosity
  in the range of 2 10 cSt
• Some fuel oils and lubricating oils may have
  viscosity of around 50 cSt
• Heavier crude oils can exceed 1000 cSt

89
Flow basics

• The table below shows that most oil flow
  applications are in the turbulent flow regime
  (i.e. Re gt 5000)
• For heavy oils or small pipes it is possible that
  flow can be in laminar and transitional regimes

90
Flow basics

• If we recalculate Reynolds number for a lower
  velocity of 1 m/s then some more of the
  applications now come into the laminar and
  transitional regimes

91
Flow basics

• Flow of water and other liquids of relatively low
  viscosity such as condensate and petrol will
  generally be in the turbulent regime
• Gas pipelines and systems tend to be operated at
  higher velocities than liquid systems
• For the majority of oil and gas applications,
  pipe sizes, pressures, viscosities and flow
  rates, gas flow is in the turbulent regime

92
Flow basics

• The compressibility of gas results in the
  possibility of more dramatic changes in flow
  conditions in gas systems than in liquid systems
• For all fluids mass flow is conserved, that
  means if we look at a cross section of the pipe,
  the mass flow into and out of the cross section
  are the same
• For liquids this seems obvious. However, for a
  gas, consider what happens if you change the gas
  the pressure significantly

93
Flow basics

• Because density changes significantly with
  pressure, the volume occupied by the gas will
  change
• If we reduce pressure, the flow in volume terms
  will increase, and hence the velocity of the flow
  will increase
• This means that the same mass flow of gas could
  have significantly different behaviour depending
  on the pressure and temperature at a particular
  time or location

94
Flow basics

• Because density changes significantly with
  pressure, the volume occupied by the gas will
  change
• If we reduce pressure, the flow in volume terms
  will increase, and hence the velocity of the flow
  will increase
• This means that the same mass flow of gas could
  have significantly different behaviour depending
  on the pressure and temperature at a particular
  time or location

95
INTRODUCTION

• Definition of flow measurement
• Why flow is measured
• Oil gas applications
• Flow basics
• Fluid properties
• Reynolds number
• Velocity profile
• Oil and gas flows
• Multiphase flow
• Metering basics

96
Flow basics

• When oil and/or gas is produced from a well, it
  is often in the form of a mixture of oil, water
  and gas, sometimes also containing sand
• This means that flow measurement can be required
  for multiphase mixtures of oil and/or water
  and/or gas as well as for the individual fluids
  once separated
• Sometimes measurements are made following a
  first stage separation, where the fluid is
  composed mainly of one component but could have
  small quantities of the other components

97
Flow basics

• Multiphase flow is much more complicated than
  single component flow of oil, water or gas
• The way that individual fluids mix and flow is
  dependent on a number of factors including the
  properties of the fluids, the flow rate of each
  component and the layout of the pipe
• In horizontal pipes the heavier fluids tend to
  separate out to the bottom of the pipe so that we
  tend to get mainly water at the bottom, with a
  layer of oil above and gas at the top

98
Flow basics

• This separation effect is particularly true when
  the flow is slow moving, which is why separator
  vessels have a large diameter so that the flow is
  slowed down as it comes out of the inlet pipe
• At higher flow rates and in vertical pipes the
  fluids mix more evenly in the cross section
• Like single component flow, multiphase flow can
  be described in terms of a number of different
  flow regimes

99
Flow basics

• General multiphase flow regimes found in
  horizontal pipes are described in the following
  slides
• In these descriptions the oil and water are
  considered to behave as a combined liquid phase,
  though these can sometimes separate into layers
  also

100
Flow basics

• Stratified flow is oil, water and gas flowing in
  layers at relatively slow velocities

101
Flow basics

• Slug flow is similar to stratified flow but with
  occasional slugs of liquid that completely fill
  the pipe cross section

102
Flow basics

• Bubble flow is mainly liquid flow at relatively
  high velocity and with small bubbles dispersed
  throughout the liquid

103
Flow basics

• Annular/mist flow is mainly gas flow at quite
  high velocity with some liquid spread around the
  pipe walls in a film and also carried in the gas
  form of droplets

104
Flow basics

• In vertical pipes the flow regimes have some
  differences as the effects of gravity are
  different
• General multiphase flow regimes found in vertical
  pipes are described in the following slides

105
Flow basics

• Bubble flow in vertical pipes is similar to
  horizontal bubble flow, with relatively small gas
  bubbles dispersed throughout the liquid

106
Flow basics

• Slug flow has some similarities to slug flow in
  horizontal pipes, but in this case gas bubbles
  join together to form large bullet shaped gas
  bubbles which fill the cross section are
  separated by regions of liquid with dispersed gas

107
Flow basics

• Churn flow occurs at higher flow velocities and
  the liquid appears to oscillate back and forward
  as the multiphase mixture flows up the pipe
• This results in a very chaotic churning flow
  pattern

108
Flow basics

• Annular/mist flow in vertical pipes is very
  similar to annular/mist flow in horizontal pipes

109
Flow basics

• Multiphase flow is a complex subject and not one
  that can be covered in great detail in this
  course
• Measurements tend to be inaccurate when flow
  meters designed for measurement of a single fluid
  are subjected to multiphase flow
• In general it is true that the greater the
  quantity of the additional fluid components, the
  poorer the accuracy will be
• It is also generally true that liquid/liquid
  mixtures such as oil/water flows are less
  difficult to measure than liquid/gas flows

110
INTRODUCTION

• Definition of flow measurement
• Why flow is measured
• Oil gas applications
• Flow basics
• Metering basics
• Measurement concepts
• Accuracy and uncertainty
• Calibration concepts
• Flow meter performance measures

111
Metering basics

• Earlier in this section we introduced the concept
  of a flow meter as a device that is installed in
  a pipe to quantify the flow through that pipe
  without having to take the fluid out of the pipe
• Over the years many different types of flow meter
  have been developed and some of the most common
  and useful ones will be introduced later in this
  course
• Now we will discuss some of the concepts and
  common issues that are important for all flow
  meters

112
Metering basics

• Most flow measurement systems use a primary
  element or flow meter that gives an output in
  mass or volume flow rate
• Mass flow rate is given by

where t is the time taken for the mass M to pass
through a given cross-section
113
Metering basics

• In practice, the mass flow rate can also be
  obtained by making simultaneous measurements of
  density ? and volumetric flowrate, Q, and
  multiplying the two

114
Metering basics

• Volumetric flowrate is defined as the passage of
  a given volume of fluid, V, in a given time

115
Metering basics

• In practice, the volumetric flow rate can also be
  obtained by making a measurement of the flow
  velocity U and multiplying this by the
  cross-sectional area of the pipe, A

What the equation above shows in a very basic
form is that it is possible to obtain flow rate
information in mass or volumetric terms by
measuring some other property of the flow, in
this case the flow velocity
116
Metering basics

• Referring back to the initial examples of
  different types of flow at the start of this
  course, if we take the example of a fast flowing
  river we can now devise a method of flow
  measurement

117
Metering basics

• If we drop a stick into the river and measure the
  time it takes to flow downstream a certain
  distance we can estimate the flow velocity

d 20 m t 2 s U 10 m/s
118
Metering basics

• We can then measure the width and depth of the
  river to estimate the flow area

w 10 m h 2m A 20 m2 U 10 m/s Q 200 m3/s
119
Metering basics

• The fact that the word estimate is used in this
  example is important. The reasons for this will
  be discussed later in this lecture when we talk
  about accuracy and uncertainty

120
Metering basics

• Another useful example of how flow measurements
  can be made by measuring some other property of
  the flow is the use of a hot wire flow meter

121
Metering basics

• If electricity is passed through a thin wire
  (like a light bulb element) then it will heat up
• If there is no flow then a constant current
  flowing through the wire will produce a constant
  heat

122
Metering basics

• If we now blow air over the element this will
  have a cooling effect on the wire
• We can then use a thermometer to control the
  electric current to maintain a constant
  temperature

123
Metering basics

• The electrical current will now vary as a
  function of the flow rate, i.e. the faster the
  air flows the more current will be needed to heat
  the element
• The above example illustrates another form of
  inferential flow measurement where flow can be
  inferred by measurement of a related property in
  an physical system of some sort
• In this example we now have an electrical signal
  that represents the flow rate

124
Metering basics

• It is a common feature of many modern flow meters
  that they use sensors or transducers that convert
  flow to electronic signals
• It is also common that the reading or output from
  the flow meter can be given in electronic form,
  typically as a current in the range 4 20
  milliamps, or as a voltage pulse output where
  each pulse corresponds to a certain mass or
  volume of fluid

125
INTRODUCTION

• Definition of flow measurement
• Why flow is measured
• Oil gas applications
• Flow basics
• Metering basics
• Measurement concepts
• Accuracy and uncertainty
• Calibration concepts
• Flow meter performance measures

126
Metering basics

• If we think of the previous example of the river
  flow measurement, we would not expect our attempt
  to quantify the flow rate to be very accurate
• Perhaps we were unable to measure the width of
  the river and had to estimate it, or perhaps we
  realised that we only measured the depth in the
  middle of the river and that it is shallower at
  either side

127
Metering basics

• Maybe we even realised that the velocity of the
  stick on the surface of the water may not be the
  same as the velocity of the water below the
  surface
• All of these factors create uncertainty in our
  estimate of the flow rate

128
Metering basics

• Consider also a simple example of transferring
  liquid from one tank to another using a bottle of
  around 1 litre volume
• If we very carefully count the number of bottles
  of liquid we put into the tank then we should be
  able obtain a fairly accurate result

129
Metering basics

• However, if at the end of the transfer we have
  put 100 full bottles of liquid into the tank and
  the last bottle is not full then we would have to
  estimate the remaining quantity, e.g. between ½
  and ¾ of a litre
• It is obvious then that there is some uncertainty
  in our estimate of the volume of liquid

130
Metering basics

• If we repeat the transfer of fluid again and this
  time after 100 full bottles we have less than ½ a
  bottle left then it is obvious that there is
  further uncertainty, probably arising from
  differences in how much liquid we have in each
  full bottle

131
Metering basics

• It can be noted that the terms accuracy and
  uncertainty have both been used in the above
  examples
• Accuracy and uncertainty tend to be used
  interchangeably in industry
• The term accuracy should only really be used for
  qualitative descriptions of measurements

132
Metering basics

• e.g. it is acceptable to say that our estimate of
  the river flow rate is not very accurate but we
  should not say that our measurement of the liquid
  transfer has an accuracy of better than half a
  litre
• In the latter case, as it is a quantitative
  description of how good we think our estimate is,
  it is more appropriate to use the term
  uncertainty
• i.e. we would say the measurement has an
  uncertainty of plus or minus half a litre

133
Metering basics

• The term uncertainty is used to quantify the
  doubt about how well the result of the
  measurement represents the quantity being
  measured
• Our earlier example of the river flow can be used
  to show that when a result is calculated from a
  number of input quantities then it too will have
  an associated uncertainty

134
Metering basics
w 10 m /- 1 m h 2m A 20 m2 /- 2 m U 10
m/s Q 200 m3/s /- 20 m3/s
135
Metering basics

• Uncertainty analysis has been applied for many
  years in various fields of industry and has been
  applied in the field of flow measurement since
  the 1960s
• Standard mathematical and statistical methods are
  now well established for estimating the
  uncertainty
• Detailed exploration of these methods is beyond
  the scope of this course but a few important
  points should be recognised

136
Metering basics

• Uncertainty defines the range of values around
  the measurement result within which the true
  value is expected to lie
• A statement of uncertainty should have an
  associated statistical statement of probability,
  which is often referred to as a confidence level
• A confidence level of 95 is most commonly used
  in flow measurement

137
Metering basics

• If the result of a measurement is a value of 100
  and is stated to have an uncertainty of /-10 at
  a confidence level of 95 then it is expected
  that the result will lie between 90 and 110, 19
  out of 20 times
• This represents the central part of a probability
  distribution that looks like this

138
Metering basics

• Every flow measurement has an associated
  uncertainty
• The level of uncertainty that can or should be
  tolerated is dependent on the application
• If we are measuring produced water discharge on
  an offshore platform then the local government
  regulations may require an uncertainty of /-10
• However, it would be unwise to accept a
  measurement uncertainty of even /- 1 for an oil
  custody transfer

139
Metering basics

• Consider a custody transfer of a load of 500,000
  barrels of crude at 100 a barrel
• An uncertainty of /- 1 would mean that the
  seller could give away up to 500,000 worth of
  oil without getting paid for it
• On the other hand it would also be possible that
  the buyer could pay the full amount 50,000,000
  and only receive 495,000 barrels of crude

140
Metering basics

• It is generally true that the lower the
  uncertainty of the metering system the more it
  will cost to purchase and/or operate
• Therefore, there is a point at which it no longer
  makes economic sense to try to continue to reduce
  the uncertainty of the metering system

141
Metering basics
Cost of metering system
Financial exposure

Increasing uncertainty
142
INTRODUCTION

• Definition of flow measurement
• Why flow is measured
• Oil gas applications
• Flow basics
• Metering basics
• Measurement concepts
• Accuracy and uncertainty
• Calibration concepts
• Flow meter performance measures

143
Metering basics

• Calibration is the set of operations that
  establish, under specified conditions, the
  relationship between values of quantities
  indicated by a measuring instrument or measuring
  system, and the corresponding values realised by
  standards.
• The above is the ISO definition of calibration
• This tells us that calibration is a process of
  comparison and that the comparison is performed
  between our measurement and a standard of some
  sort

144
Metering basics

• Using time as an example it is relatively easy to
  understand what calibration means
• We would normally set the time on an old
  mechanical clock to match the time on modern
  wristwatch
• A week later we might return to see if the old
  clock is running fast or slow
• It is this comparison process is a calibration
  and in this case the wristwatch is our standard

145
Metering basics

• In this example we might also adjust the old
  clock after we have compared it with our watch
• The important thing to recognise is that it is
  the process of comparison that is the
  calibration, not the adjustment

146
Metering basics

• When calibrating the old clock we have used the
  wristwatch as our standard because we expect a
  modern digital watch to sufficiently accurate for
  our purposes
• We assume the watch to be accurate and expect
  that the manufacturer of the watch has checked it
  against an even more accurate standard
• The manufacturer in turn should have checked his
  standard against a more accurate standard, and so
  on

147
Metering basics

• Eventually the process of comparison should lead
  back to the internationally recognised definition
  of the second, which in fact is defined by as the
  time taken for caesium atoms to emit
  9,192,631,770 periods of a specific form of
  radiation
• This process of linking a measurement back to a
  definitive standard via a number of intermediate
  standards is called traceability

148
Metering basics

• Traceability back to a common standard is vital
  for many purposes including trade
• For example, the price of a barrel of oil is
  meaningless unless the seller and the buyer are
  both agreed on a standard to use for the barrel

149
Metering basics

• It has already been stated that calibration is
  the process of comparison, and not the process of
  adjustment of the measuring instrument
• However, calibration is very often carried out to
  obtain information that is used to adjust the
  measurement device or to apply conversions or
  corrections to the measured values
• We will now use our earlier example of
  transferring liquid from one tank to another
  using a bottle as another illustration of the
  importance aspects of calibration

150
Metering basics

• Transferring 100 litres of liquid 1 at a time is
  laborious
• If we decided to use a bucket instead of the 1
  litre bottle then we could probably transfer the
  liquid a lot quicker
• If we want to transfer 100 litres of liquid using
  the bucket then we need to know how many litres
  our bucket will hold

151
Metering basics

• If we fill up the bucket using the 1 litre bottle
  and count how many bottles are required then we
  have just calibrated our bucket
• If the liquid is being transferred for sale, then
  we could find that the accuracy of our 1 litre
  bottle is questioned and it will be necessary to
  calibrate it also
• i.e. we then need to establish traceability of
  our calibration

152
Metering basics

• If, halfway through transferring the liquid we
  were to drop the bucket and dent the side of it,
  we would then have to check it using our 1 litre
  bottle
• i.e. we would have to recalibrate the bucket

153
Metering basics

• In summary, we can see that calibration is a
  useful and necessary process when
• The relationship between the measurement and the
  standard is not yet known
• We know approximately what the relationship is
  but want to reduce the uncertainty in the
  measurement
• We suspect that the relationship has changed
  during use of the measurement device

154
INTRODUCTION

• Definition of flow measurement
• Why flow is measured
• Oil gas applications
• Flow basics
• Metering basics
• Measurement concepts
• Accuracy and uncertainty
• Calibration concepts
• Flow meter performance measures

155
Metering basics

• When we are making flow measurements it is
  normally important to know how well we can expect
  the flow meter we are using to perform
• For a custody transfer application we would be
  interested in the overall uncertainty of the
  measurements as illustrated earlier in this
  course
• However, there may be other characteristics of
  the flow meters that we are interested in

156
Metering basics

• Normally we are interested in particular
  characteristics either because these are more
  important to us than the overall uncertainty, or
  because we need to know these characteristics in
  order to quantify uncertainty

157
Metering basics

• K-factor is a term that is used to describe the
  output of a meter that provides an output in the
  form of a series of electronic pulses
• These pulses can be counted on an electronic
  counter and then converted to a quantity using
  the k-factor, which has normally been obtained
  previously by calibrating the flow meter

158
Metering basics

• Error is a term used to describe the difference
  between the measurement made by a flow meter and
  the actual flow rate quantified using a
  standard, and is normally quantified in
  percentage terms

159
Metering basics

• Error and uncertainty are related but different
• If a flow meter reads 101 and the uncertainty is
  /- 10 then we would expect the true value to
  lie between 91 and 111
• If the actual flow rate were 100 then the meter
  would be reading with an error of 1
• However, if our flow meter is the only
  measurement device we have in this application,
  then although we have quantified the uncertainty,
  we cannot know what the error is

160
Metering basics

• We can only quantify errors when comparing the
  flow meter with a standard by the process of
  calibration
• The purpose of such a calibration is normally to
  quantify and then correct for any apparent
  errors, to reduce the uncertainty of the
  measurement

161
Metering basics

• Presenting results in the form of an error plot
  is often more useful than plotting measured
  results against actual results, as illustrated in
  the following slides

162
Metering basics

• The table and graph below show the results of a
  flow meter calibration

163
Metering basics

• If we calculate the error in percentage terms we
  can visualise the performance of the meter much
  more clearly

164
Metering basics

• Linearity is a term used to describe how close to
  a straight line the results would be if we
  plotted the meter reading against the actual flow
  rate
• Examples of calibration results from linear and
  non-linear flow meters are given in below

165
Metering basics

• Linearity is normally expressed as a percentage
  difference between the meter reading and the
  expected result using a straight line fitted to
  the data
• Linearity is important in terms of ease of
  calculation of flow measurements
• Good linearity (i.e. a response close to a
  straight line) also makes it easier to check
  meter performance as it means we do not have to
  attempt to check detailed characteristics by
  carrying out calibration tests at many flowrates

166
Metering basics

• In order for a statement of linearity to be
  useful, it should be accompanied by a statement
  of the range of flow rate (sometimes called
  turndown) that it is applicable for

167
Metering basics

• Repeatability is a term used to describe the
  spread of results obtained from a flow meter when
  the flow is not changing
• Repeatability is determined by repeating a
  calibration test several times one after the
  other
• There are a number of different ways of
  quantifying repeatability

168
Metering basics

• The most widely recognised method is to calculate
  the standard deviation of the repeat results,
  multiply it by a factor of 2.83 and express the
  result as a percentage of the average value of
  the repeats
• A more simple and convenient way of calculating
  repeatability is to simply take the difference
  between the maximum and minimum values as a
  percentage of the average

169
Metering basics

• For example, consider the following data obtained
  during a series of five repeat calibrations
• 50 52 49 48 51
• The difference between the maximum and minimum is
  4 and the average value is 50 so the
  repeatability according to the simple method is
  8
• Calculating the repeatability using 2.83 times
  the standard deviation, the repeatability is 8.9

170
Metering basics

• Repeatability and uncertainty are related but
  different
• Repeatability is essentially a measure of the
  randomness of the result
• The different concepts of repeatability, error
  and uncertainty are illustrated in the following
  slides

171
Metering basics

• If the repeatability of a device is good and the
  possibility of error small, then the uncertainty
  is low

172
Metering basics

• If the repeatability is poor then the uncertainty
  must be high

173
Metering basics

• However, if the repeatability is good but the
  possibility of error is large, then the
  uncertainty is high

174
Metering basics

• Calibration is used to reduce errors or offsets
• Good repeatability is important in order to make
  it easy to perform a calibration of the flow
  meter
• Because repeatability is a description of the
  randomness of the measurement, it holds that even
  with poor repeatability, taking many repeat
  measurements will eventually give an average
  result that we can use for calibration
• This is best illustrated by example

175
Metering basics

• The figure below shows 100 randomly generated
  numbers between 0 and 1 representing individual
  measurements from a flow meter

176
Metering basics

• With a large enough set of measurements the
  average result would be very close to 0.5

177
Metering basics

• The bold line shows the running average
  calculated after each measurement is taken

178
Metering basics

• We can see from the graph that the more
  measurements we average together, the closer to
  the true average we get

179
Metering basics

• Sometime flow meter performance claims can be
  confusing or misleading
• For example, sometime uncertainty or accuracy
  is stated as a percentage of full scale
• In this case, the uncertainty at the flow rate
  that the meter is used at could be larger than
  you might initially think
• This is best illustrated by a couple of examples

180
Metering basics

• A flow meter is designed to operate up to a
  maximum of 1000 m3/hr and the statement
  accuracy, /- 1 of full scale is made
• This actually means that the uncertainty of the
  flow measurement is /- 10 m3/hr irrespective of
  the actual flow rate
• That means that if the meter was being used at 50
  m3/hr then the uncertainty in percentage terms at
  the flow rate is /- 20

181
Metering basics

• Similarly a statement of linearity in these terms
  can be deceptive
• Such methods are often used when manufacturers
  claim to cover very wide flow rate ranges with
  one meter
• Here is another example

182
Metering basics

• One manufacturer claims an uncertainty of
• 2 of flow rate over a range of 101
• Another manufacture claims an uncertainty of
• 1 of max flow rate over a range of 201
• Both meters cover the same flow rate range
• Which is best?

183
Metering basics

• 2 of flow rate over a range of 101
• 1 of max flow rate over a range of 201

184
Metering basics

• Linearity or repeatability are sometimes used
  alone to describe the performance of a flow meter
• It should be clear from earlier discussions that
  a flow meter could have a significant uncertainty
  even if it has good linearity and repeatability
• For example, a meter could be linear and
  repeatable, but the slope of the relationship
  between actual and indicated flow could be very
  sensitive to installation effects or to the
  effects of changes in fluid viscosity

185
Metering basics

• An example of how installation conditions may
  affect a flow meter

X
X
X
X
X
186
Metering basics

• The presentations have introduced some basic
  concepts that are important in flow measurement
• It has illustrated that it is important to
  consider fluid properties and flow conditions
  when measuring flow
• Basic concepts of calibration and uncertainty
  have been introduced
• In most circumstances meters should have a
  traceable c