Flowmeter course

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Flowmeter course

  1. 1. Flow Measurements1
  2. 2. 2
  3. 3. Definitions and Units Flow rate corrections Differential Pressure Flow Transmitters Differential Pressure Methods Orifice Plates Venturi Tubes Flow Nozzles Pitot Tubes Vortex Type Flow Elements Target Flowmeter Turbine Flowmeter Positive Displacement Flowmeter Ultrasonic Flowmeter Coriolis Flowmeter3
  4. 4. 4
  5. 5. It is the art and science of: 1. applying instruments 2. to sense a chemical or physical process condition.5
  6. 6. Measurement of a given quantity is an act orthe result of comparison between thequantity and a predefined standard.6
  7. 7. 7
  8. 8. In order that the results are meaningful,there are two basic requirements:1. The standard used for comparison purposes must be accurately defined and should be commonly accepted.2. The apparatus used and the method adopted must be proved.8
  9. 9. The advancement of science and technologyis dependent upon a parallel progress inmeasurement techniques.9
  10. 10. There are two major functions in allbranches of engineering:1. Design of equipment and processes.2. Proper operation and maintenance of equipment and processes.Both functions require measurements.10
  11. 11. •Direct Method: The unknown quantity is directly compared against a standard. •Indirect Method: Measurement by direct methods are not always possible, feasible and practicable. Indirect methods in most of the cases are inaccurate because of human factors. They are also less sensitive.11
  12. 12. InstrumentsIn simple cases, an instrument consists of asingle unit which gives an output reading orsignal according to the unknown variableapplied to it.In more complex situations, a measuringinstrument consists of several separateelements.12
  13. 13. These elements may consist of: •Transducer elements which convert the measurand to an analogous form. •The analogous signal is then processed by some intermediate means and then fed to •The end devices to present the results for the purposes of display and or control.13
  14. 14. These elements are: •A detector. •An intermediate transfer device. •An indicator.14
  15. 15. The history of development of instruments encompasses three phases: •Mechanical. •Electrical. •Electronic.15
  16. 16. Purpose of Process Measurement• Reaching corporate economic goals• Controlling a process• Maintaining safety• Providing product quality16
  17. 17. •No matter how advanced or sophisticated thedistributed control system,•the control system is only as effective as theprocess measurement instruments it isconnected to;•therefore, successful process control isdependent on successful instrumentapplication.17
  18. 18. To correctly apply instrumentation, an engineer must clearly understand the operations and limitations of the instrument, as well as understanding the chemical and physical properties of the process.18
  19. 19. •Fundamental to applying process instrumentation is interpreting the instrument’s performance envelope. •Every field measurement device has its own distinct envelope that constitutes the process and environmental conditions it can perform to. •Likewise, every application has a characteristic envelope that represents the applications process and environmental conditions.19
  20. 20. 20
  21. 21. MetrologyIt is the “science of measurement.”As a science, metrology uses terminology anddefinitions that the process measurementengineer must be familiar with. He must and have a clear understanding of, because vendors may vary in the use of a term.21
  22. 22. The instrument engineer must consider the following dynamic conditions that affect process measurement: • Temperature Effects • Static Pressure Effects • Interference • Instrumentation Response • Noise • Damping and Digital Filtering22
  23. 23. These dynamic conditions cause the presence of uncertainty in measuring systems. No measurement, however precise or repeated, can ever completely eliminate this uncertainty. The uncertainty of measuring systems is exemplified in the effects temperature variations can have on measurements.23
  24. 24. Temperature Effects•Temperature influences can exhibit some of themost severe effects on a process measurement,both in the process media itself and themeasurement instrument.24
  25. 25. Some obvious examples of severe temperatureinfluences include temperature-induced phasetransitions.It would be hard to determine what would happen to anorifice plate, differential pressure measurement if theprocess suddenly changed from a liquid to a solid or gas.25
  26. 26. Other temperature induce dynamic changesinclude:•Change in the dimensions of the measuringelement,•Modification of a resistance of a circuit, or•Temperature-induced change in the flux densityof a magnetic element.26
  27. 27. Similar to temperature effects, pressure changes canalso trigger phase transitions, especially in gasapplications.Pressure effects seen in differential pressure (DP)devices are an example.Because the differential pressure devices are used inflow and level applications, the importance of pressureeffects should not be underestimated.27
  28. 28. •The goal is to minimize the total error that pressure effects can cause. •To illustrate this, consider a differential pressure instrument that is calibrated in a lab at zero static pressure. •The transmitter is re-zeroed after installation by opening an equalizing valve in the process under pressure to eliminate zero shifts; •however, variations inline pressure are not accounted for during normal operations.28
  29. 29. Interference, in process measurement terms, refers toeither external power or electrical potential that caninterfere with the reception of a desired signal or thedisturbance of a process measurement signal.29
  30. 30. Instrumentation Response•The dynamic characteristic of instrumentationresponse refers to how quickly a measuring instrumentreacts or responds to a measured variable.•An ideal, perfect instrument would have aninstantaneous response, which in effect, is called zerolag.•In general, with modern electronic instrumentation,the response time is adequate for most applications.30
  31. 31. •Engineers should concern themselves with responsetime performance.•Although fast speed of response is an attribute of highquality instrumentation,•some applications with rapidly changing processeswould not benefit from fast responding devices andcould even result in instrument damage.•Depending on the application, some measurement lagis placed on the measuring device.31
  32. 32. •Noise is often described as a signal that doesnot represent actual process measurementinformation.•Noise can originate internally within the processmeasuring system or externally from the processcondition.•It makes up part of the total signal from whichthe desired signal must be read.32
  33. 33. Damping and Digital Filtering•Damping is defined as the progressive reductionor suppression of oscillation in a device or system.•In more practical terms, damping describes theinstrument’s performance in the way a pointer orindicator settles into a steady indication after achange in the value of the measured quantity.33
  34. 34. • A response is not damped at all, oscillationcontinues.• A response is underdamped or periodic, as is thecase when overshoot occurs.• A response is overdamped or aperiodic, when theresponse is slower than an ideal or desiredcondition.• A response is critically damped, when theresponse represents an ideal or desired condition.34
  35. 35. Measurement TerminologyRangeIt is defined as the region between the limitswithin which a quantity is measured, received,or transmitted, expressed by stating the lowerand upper range values.35
  36. 36. Upper Range Value (URV) is defined as the highestquantity that an instrument is adjusted to measure.Lower Range Value (LRV) is defined as the lowestquantity that an instrument is adjusted to measure.Upper Range Limit (URL) is defined as the maximumacceptable value that a device can be adjusted tomeasure.Lower Range Limit (LRL) is defined as the minimumacceptable value that a device can be adjusted tomeasure.36
  37. 37. 37
  38. 38. Rangeability is the ratio of the maximummeasurable value to the minimum measurablevalue.Turndown is defined as the ratio of the normalmaximum measured variable through themeasuring device to the minimum controllablemeasured variable.In a conventional differential pressure transmitter,if the maximum pressure is 7.45 kPa and theminimum pressure is 1.24 kPa, the span turndownis 6 to 1 (6:1).38
  39. 39. These terms are often interchanged, confused and misunderstood.39
  40. 40. 40
  41. 41. 41
  42. 42. Zero Elevation Range is defined as a range wherethe zero value of the measured variable is greaterthan the lower range value.The zero value can be between the lower rangevalue and the upper range value, at the upper rangevalue, or above the upper range value.42
  43. 43. Zero Suppression Range is defined as a rangewhere the zero value of the measured variable isless than the lower range value.In that case, the zero value does not appear on therange scale.43
  44. 44. 44
  45. 45. Response Time is defined as the time taken for thesystem output to rise from 0% to the first crossoverpoint of 100% of the final steady state value.45
  46. 46. Accuracy is sometimes referred to as the maximumuncertainty or limit of uncertainty.In practical terms, accuracy qualitatively represents thefreedom from mistake or error.In metrological terms, accuracy represents the degreeof conformity of an indicated value to an acceptedstandard value, or ideal value.46
  47. 47. Precision is confused with accuracy.Precision, by definition, is the reproducibility withwhich repeated measurements of the same measuredvariable can be made under identical conditions.47
  48. 48. 48
  49. 49. 49
  50. 50. Reproducibility is the same as precision.The close agreement among repeatedmeasurements of the output for the same valueinput that are made under the same operatingconditions over a period of time, approaching fromboth directions.If the measuring instrument is given the sameinputs on a number of occasions and the results lieclosely together, the instrument is said to be of highprecision.50
  51. 51. RepeatabilityIt is same as reproducibility except thatrepeatability represents the closeness of agreementamong a number of consecutive measurements ofthe output for the same value of input under thesame operating conditions over a period of time(approaching from the same direction).51
  52. 52. Linearity is the closeness to which a curveapproximates a straight line.Independent Linearity Terminal Linearity Zero-based Linearity52
  53. 53. Hysteresis53
  54. 54. Deadband54
  55. 55. DriftIt represents an undesired slow change oramount of variation in the output signal over aperiod of time (days, months, or years), with afixed reference input.55
  56. 56. Zero Drift represents drift with zero input signal.In practical terms, the zero of the measuringinstrument shifts.In a mechanical instrument, it is usually caused by aslipping linkage. The correction is to re-zero theinstrument.In an electronic instrument, zero shift is usually causedby environmental changes. The correction is to re-zerothe electronic instrument.56
  57. 57. Span Drift represents drift or gradual change incalibration as the measurement moves up the scalefrom zero.In a mechanical instrument, it is usually caused bychanges in the spring constant of the instrument, or bythe linkage.In a electronic instrument, span shift is usually causedby changes in the characteristics of a component.The correction can be to adjust the span of the displayelement.57
  58. 58. Partial Drift represents drift on only a portion of theinstrument’s span.In a mechanical instrument, it is usually caused by anoverstressed part of the measuring instrument.In an electronic instrument, partial shift is usuallycaused by drift in an electronic component.The correction is periodic inspection and calibration.58
  59. 59. Reliability•It represents a measuring device’s ability to perform ameasurement function without failure over a specifiedperiod of time or amount of use.•Usually reliability data is extrapolated.•Reliability is often expressed as (MTTF) specification.•After failure, repair must take place. MTTF + MTTR = MTBF59
  60. 60. Overview of Typical Design Criteria Process measurement suppliers tend to follow several rules when designing equipment to achieve reliability. • Keep the design simple, • Avoid using glass as a structural material, • Keep electronics cool as possible, • Provide easy serviceability.60
  61. 61. • Housing • Metals • Gasket Considerations • Seal Considerations • Associated Hardware Options • Process Connections Options • Installation Orientation • Effects of Vibration • Environment and Hardware Materials61
  62. 62. Environment and Hardware Materials • Reliability • Quality • Accuracy • Cost • Repeatability • Previous acceptance • Availability of spares • Compatibility with existing equipments • Flexibility of use • Compatibility with the environments • Ease of maintenance • Ease of operation • Application suitability62
  63. 63. Electrical design and instrument loop wiring considerations • Power Requirements • Power Consumption • Wiring Terminations • Output Signal • RFI Effects • Grounding of Instruments • Shielding Considerations • Lightning Protection63
  64. 64. SAFETY CONSIDERATIONS • limiting the energy level • keeping sparks away from flammable mixtures • containing an explosion • diluting the gas level • protecting against excessive temperature • Probability that a hazardous gas is present • Quantity of a hazardous gas • Nature of the gas (is it heavier or lighter than air) • The amount of ventilation • The consequences of an explosion64
  65. 65. 65
  66. 66. 66
  67. 67. • Pressure values themselves are essential datafor monitoring.• Often, the values of process variables otherthan pressure are derived from (inferred from)the values that are measured for pressure.67
  68. 68. Properties of Matter in Relation to PressureMeasurement68
  69. 69. Pressure Equation •Pressure is defined as the amount of force per unit area. P =F/A where: P = pressure F = force =ma A = area69
  70. 70. Gauge, Absolute, Differential, and Vacuum70
  71. 71. Pressure Measuring Devices Categories of pressure measuring devices : • Gravitational gauges • Deformation sensors and switches • Transducers and transmitters71
  72. 72. Gravitational Gauges72
  73. 73. Deformation (Elastic) Sensors and Switches Bourdon Tube73
  74. 74. 74
  75. 75. Advantages• They are available in a wide variety of pressure ranges.• They are proven and suitable for many pressureapplications.• They have good accuracy.Disadvantages• Vibration and shock could be harmful to mechanicallinkage.• They are susceptible to hysteresis as they age.75
  76. 76. Diaphragm Bellows76
  77. 77. Other Types of Deformation Sensors77
  78. 78. Pressure Transducer•It is a device that provides an electrical output signal that isproportional to the applied process pressure.•The output signal is specified as either a volt, current, orfrequency output.A pressure transducer always consists of two elements:  A force summing element, such as a diaphragm, converts the unknown pressure into a measurable displacement or force.  A sensor, such as a strain gauge, converts the displacement or force into a usable, proportional output signal. 78
  79. 79. Strain Gauge•The sensor changes its electrical resistance when itstretches or compresses.79
  80. 80. Potentiometer Element80
  81. 81. Capacitive Sensor81
  82. 82. Performance Advantages• They have good rangeability and response time.• They have very good accuracy.Typical accuracies are about 0.1% of reading or 0.01 % offull scale.• Typical transducers support a very wide pressure range.• High vacuum and low differential pressure ranges aresupported.82
  83. 83. Inductance-Type Transducer•Changing the spacing between two magnetic devices causes achange in the reluctance.•The change in reluctance then represents the change in pressure.•One type of reluctance pressure transducer is the linear variabledifferential transformer (LVDT).83
  84. 84. Piezoelectric Gauge•Materials that create an electrical voltage when a force isapplied.•They measure rapidly changing pressures.84
  85. 85. Performance Advantages• They provide a self generated output signal.• They have high speed of response.• They have good accuracy, about 1% of fullscale is typical.85
  86. 86. Design of Pressure Transmitters86
  87. 87. Meter Body Designs87
  88. 88. Transmitter Process Locations88
  89. 89. 89
  90. 90. 90
  91. 91. 91
  92. 92. Purpose of Flow Measurement •Monitor and control the flow rates. •Develop material and energy balances. •Sustain the efficiency and to minimize waste.92
  93. 93. Importance of Accurate Measurement• Material balances in separation processes.• Pumps and compressor operations.• Custody transfer operations.93
  94. 94. Flowmeter DefinitionA flowmeter is defined as “A device thatmeasures the rate of flow or quantity of amoving fluid in an open or closed conduit”. It usually consists of a primary device and a secondary device.”94
  95. 95. Primary Device It is defined as “The device mounted internallyor externally to the fluid conduit that produces asignal with a defined relationship to the fluidflow in accordance with known physical lawsrelating the interaction of the fluid to thepresence of the primary device.”95
  96. 96. Secondary DeviceIt is defined as “The device that responds to thesignal from the primary device and converts it toa display or to an output signal that can betranslated relative to flow rate or quantity. ”96
  97. 97. 97
  98. 98. Some Drawing Symbols98
  99. 99. General Categories of Flow Instruments Flow instrument categorization often varies. 1. Rate or quantity type. 2. Energy usage type.99
  100. 100. 1a. Rate meters •They are the most common classification of flowmeters. •Rate meters measure the process fluid’s velocity. •Because a pipe’s cross sectional area is known, the velocity is then used to calculate the flow rate.100
  101. 101. A rate meter can either infer the flow rate ormeasure the velocity of the flowing fluid todetermine the flow rate. •In differential pressure flowmeter, the flow rate is inferred from the measured differential pressure. •In turbine meter, the velocity of the fluid times the area is used to determine the flow rate.101
  102. 102. 1b. Quantity meters•They divide the flowing material intopredetermined volume segments.•Quantity meters count and keep track of thenumber of these volume segments.•An example of a quantity meter is a positivedisplacement meter.102
  103. 103. Meters that directly measure mass can alsobe considered either as a quantity meter or as a mass flow rate meter.103
  104. 104. 104
  105. 105. 2. Energy ApproachA. Extractive Energy•Flowmeters take energy from the fluidflow.•An orifice plate is an example of anextractive-type.105
  106. 106. B. Additive Energy•Flowmeters introduce some energy intothe fluid flow.•A magnetic flowmeter is an example of anadditive type.106
  107. 107. Volumetric Flow Rate•It represents the volume of fluid that passes ameasurement point over a period of time.•The calculation is based on the formula: Q=AxvwhereQ = volumetric flow rateA = cross-sectional area of the pipev = average flow velocity (flow rate) 107
  108. 108. Mass Flow Rate•It represents the amount of mass that passes a specificpoint over a period of time.•The calculation is based on the formula: W=QxwhereW = mass flow rateQ = volumetric flow rate = density108
  109. 109. Units of Measure109
  110. 110. Meter Run•It is defined as “The upstream and downstream lengthof pipe containing the orifice flanges and orifice plate ororifice plate with or without quick change fittings.”•No other pipe connections should be made within thenormal meter tube dimensions except for pressure tapsand thermowells.•The meter tube must create an acceptable flow pattern(velocity profile) for the fluid when it reaches the orificeplate.110
  111. 111. 111
  112. 112. Flow Straighteners (conditioners)•They help to provide accurate measurement when adistorted flow pattern is expected.•They are installed in the upstream section of meter tube.•They reduce the upstream meter tube lengthrequirement.112
  113. 113. 113
  114. 114. Compressible versus Incompressible Flow•Temperature and pressure changes cause the volumeof a fluid to change.•The change in volume is much more extreme in gasesthan in liquids.•For accurate gas flow measurements, thecompressibility factor is included in the measurement. z =PV/nRT114
  115. 115. 115
  116. 116. Viscosity116
  117. 117. •Viscosity is frequently described as a fluid’s resistanceto flow.•It have a dramatic effect on the accuracy of flowmeasurement.•Resistance to flow occurs because of internal frictionbetween layers in the fluid.•Water, for example, having low viscosity has lessresistance to flow.117
  118. 118. •When a fluid is in motion, layers of fluid aresubject to tangential shearing forces, causing thefluid to deform.•Fluid’s low viscosity does not become aninfluential property of the fluid upon flowmeasurement.•However, when measuring the flow rate of afluid with high viscosity, the viscosity doesbecome an influential property in flowmeasurement.118
  119. 119. Viscosity is often expressed in terms of thefollowing:• Dynamic viscosity• Kinematic viscosity• Viscosity index• Viscosity scales119
  120. 120. Dynamic Viscosity (Absolute Viscosity)•It represents a fundamental viscosity measurement of afluid.•Density of fluid does not play a part in the viscositymeasurement.•Absolute viscosity is a ratio of applied shear stress toresulting shear velocity.•The measurement units for dynamic (absolute) viscosityare centipoise, Pascal-seconds, or lb/ft-second.120
  121. 121. •One method to measure viscosity is to rotate adisk in the fluid at a particular rotational speed.•The rotational torque required to keep the diskrotating divided by the speed of rotation and bythe disk contacting surface area is a measure ofabsolute viscosity.•Another viscosity measurement that can be usedfor liquids and gases is the falling sphereviscometer.121
  122. 122. Rotational and Falling Sphere Viscometers122
  123. 123. Kinematic Viscosity (n)•It represents a ratio of dynamic (absolute) viscosity to thedensity of the fluid and is expressed in stokes (n = m / r).•In liquids, an increasing temperature usually results inlowering the kinematic viscosity.•In gases, an increasing temperature increases thekinematic viscosity.•The measurement units for kinematic viscosity are eithercentistokes, meter2/second, or ft2/second.123
  124. 124. 124
  125. 125. •The method for determining kinematic viscosityinvolves measuring the time to drain a certainvolume of liquid by gravity out of a containerthrough a capillary tube or some type of restriction.•The time it takes to drain a liquid is directly relatedto viscosity.•The flow rate of fluids by gravity, which is the forcecausing the flow, depends upon the density of thefluids.125
  126. 126. Ostwald Capillary Viscometer126
  127. 127. Viscosity Index•It represents the change in viscosity with respectto temperature.•It is used with reference to petroleum products.•A high viscosity index number means that thefluid’s viscosity does not change very much for agiven temperature, and vice versa.127
  128. 128. Viscosity Scales•It represents viscosity measurements in time units.•Commonly used viscosity scales include the following: oSaybolt Furol scales oRedwood scales oEngler scales•The three scales express kinematic viscosity in timeunits rather than centistokes.128
  129. 129. •For example, if the kinematic viscosity of a fluid at 122°F is 900 centistokes, on the Saybolt Furol scale theequivalent viscosity is expressed as 424.5 seconds(centistokes x 0.4717).•Flow engineering reference manuals often provideconversion formulas between centistokes and therespective viscosity scale.129
  130. 130. 130
  131. 131. Basic Hydraulic Equations131
  132. 132. 132
  133. 133. Bernoulli Equation P = Static Pressure (pounds force per sq. ft) r = Density (rho) (pounds mass per cubic ft) v = Velocity (feet per second) g = Acceleration of Gravity (feet per second2) Z = Elevation Head Above a Reference Datum (feet)133
  134. 134. 134
  135. 135. Continuity Equation The Equation of Continuity states that the volumetric flow rate can be calculated by multiplying the cross sectional area of the pipe at a given point by the average velocity at that point. Q=Axv where Q = volume flow rate (cubic feet per minute) A = pipe cross-sectional area (square feet) v = average fluid velocity (feet per minute)135
  136. 136. Reynolds NumberIt is a major distinctive quality of fluid flowas The ratio of Inertial Forces to Viscous Forces.136
  137. 137. 137
  138. 138. 138
  139. 139. •Laminar flow is defined by low Reynoldsnumbers with the largest flowing fluidmoving coherently without intermixing.•Turbulent flow is defined by high Reynoldsnumbers with much mixing.139
  140. 140. •Turbulent flow is best when high heat transfer iswanted,•while laminar flow is best when flowing fluid is to bedelivered through a pipe with low friction losses.•Flow is considered laminar when the Reynolds numberis below 2,000.•Turbulent flow occurs when the Reynolds number isabove 4,000.•Between these numbers, the flow characteristics havenot been defined.140
  141. 141. 141
  142. 142. 142
  143. 143. Newtonian versus non-Newtonian FluidsIn Newtonian fluids, the resistance to deformationwhen subjected to shear (consistency of fluid) isconstant if temperature and pressure are fixed.Whereas in a non-Newtonian fluid, resistance todeformation is dependent on shear stress eventhough the pressure and temperature are fixed.143
  144. 144. Hagen-Poiseuille Law•It defines viscosity in more practical terms.•Newton’s definition of viscosity is the ratio ofshear stress divided by shear rate.•Hagen-Poiseuille defines it as the ratio of shearstress divided by shear rate at the wall of acapillary tube.144
  145. 145. Rheograms•It can be used to determine the characteristics of any fluid.•Rheograms evolved from the science of rheology, whichstudies flow.•(“Rheo,” derived from the Greek language, means “aflowing.”)•Rheograms are useful as an aid to interpret viscositymeasurements.145
  146. 146. Newtonian Fluids•It exhibits the constant ratio of shear stress to shear rate (flowvelocity) when subjected to shear and continuous deformation.•When a fluid’s temperature is fixed, the fluid exhibits thesame viscosity through changing shear rates. Viscosity is notaffected by shear rate (flow velocity).•The relationship is linear between the shear stress (force) andvelocity (resulting flow).•Newtonian fluids are generally homogeneous fluids. Gasoline,kerosene, mineral oil, water and salt solutions in water areexamples of Newtonian fluids.146
  147. 147. Non-Newtonian Fluids•Fluids that do not show a constant ratio of shear stress toshear rate are defined as non-Newtonian fluids.•Fluids exhibit different viscosity at different shear rates.•In non-Newtonian fluids, there is a nonlinear relation betweenthe magnitude of applied shear stress and the rate of angulardeformation.•Non-Newtonian fluids, which have different classifications,tend to be liquid mixtures of suspended particles.•Thick hydrocarbon fluids are considered non-Newtonianfluids.147
  148. 148. 148
  149. 149. 149
  150. 150. FLOW MEASURING DEVICE SELECTION CRITERIA • Application fundamentals • Specifications • Safety considerations • Metallurgy • Installation considerations • Maintenance and calibration • Compatibility with existing process instrumentation • Custody transfer concerns • Economic considerations • Technical direction150
  151. 151. Application Fundamentals Flowchart151
  152. 152. Application Fundamentals Checklist of Selection Criteria• Flow stream conditions: – volume – temperature – pressure – density – viscosity – flow velocity• Flow measurement goals.• Accuracy requirements.• Range requirements.• Acceptable pressure drops.• Display and system requirements.• Potential problems (i.e., vibration).• Flow stream erosive/corrosive materials, entrained gases and solids (if any).• Available installation space and pipe geometry.• Economic factors (cost of ownership). 152
  153. 153. Flowmeter Applications153
  154. 154. Flowmeter Applications (Continued)154
  155. 155. Providing Protection to the Flowmeter Isolation ValvesStrainers De-aerators155
  156. 156. •Strainers are used to protect meters from debris in aliquid stream.•Strainers are not intended for filtering a liquid.•Strainers should be carefully selected to ensure that theyhave a low pressure drop when used with high velocityflowmeters. 156
  157. 157. •Deaerators are air elimination devices that protect themeter from receiving a large slug of air.•The air elimination device separates that air from theliquid through the use of special baffles.•In the case of some positive displacement meters, a largeslug of air can completely damage the meter.•In the case of a turbine meter, air may not causedamage, but will cause errors in readings (registrations).157
  158. 158. •Isolation Valves are typically provided at a meter inlet topermit meter repairability without shutting down theprocess.•Block and Bleed Valves are used in meter runs toprovide a means for calibration. These valves divert theflow to the meter prover loop.•Control Valves provide a means of controlling flowrateand/or back pressure.•For example, flowrate control is necessary to prevent apositive displacement meter from over-speeding.158
  159. 159. Typical Maintenance Concerns by Flowmeter Type159
  160. 160. Accuracy Reference•Accuracy is measured in terms of maximum positiveand negative deviation observed in testing a deviceunder a specified condition and specified procedure.•The accuracy rating includes the total effect ofconformity, repeatability, dead-band, and hysteresiserrors.•An accuracy reference of simply “2%” is incomplete.160
  161. 161. Percent of Rate Accuracy: It applies to meters such as turbine meters, DCmagnetic meters, vortex meters, and Coriolis meters.Percent of Full Scale Accuracy: It refers to the accuracy of primary meters suchas rotameters and AC magnetic meters.Percent of Maximum Differential Pressure: It applies to differential pressureflow transmitters.161
  162. 162. Totalization•It represents the process of counting the amount offluid that has passed through a flowmeter.•Its purpose is to have periodic (daily or monthly)readings of the material usage or production.•The totalization data is used for billings for materialusage or production.162
  163. 163. Multivariable Transmitters163
  164. 164. •In measuring flow, temperature is required tocompensate for changes in density.•A multivariable transmitter is essentially fourtransmitters in one package.•A multivariable transmitter measures differentialpressure, absolute pressure, and process temperature.•The multivariable transmitter also calculates thecompensated flow.•Traditionally, three separate transmitters and flowcalculation were required for this measurement.164
  165. 165. •The multivariable transmitter incorporatesmicroprocessor based technology which provides theadvantages of better readability and tighterintegration.•Additionally, the multivariable transmitter reducesinstallation costs, spares inventories, andcommissioning times.•The transmitter has the flexibility to be used inapplications such as custody transfer, energy andmaterial balances, and advanced control andoptimization.165
  166. 166. 166
  167. 167. Custody Transfer167
  168. 168. 168
  169. 169. •Flow measurement for custody transfer, whereownership of a product transfers, is on occasionregarded as a separate flow measurement topic.•There are two types of custody transfer in flowmeasurement: 1. Legal, which falls under weight and measure requirements. 2. Contract, which is a mutual agreement between seller and buyer.169
  170. 170. In process control applications, the accuracy requirement may be several percent, but for custody transfer operations the accuracy requirement may be in tenths of a percent.170
  171. 171. Custody Transfer Concerns• Reasons for metering hydrocarbons.• Classifications of custody transfermeasurements.• Meter provers required.171
  172. 172. Reasons for Metering HydrocarbonsIn typical oil processing plants, liquid hydrocarbons aremetered at each custody transfer point and often at pointswhere custody does not change.Several reasons for the metering are:• Corporate accounting requires data.• Billing is dependent upon accurate measurements.• Losses are detectable.• Business decisions are based on the measurement data.• Assist negotiations, if necessary• Provide auditable, historical records.172
  173. 173. Classification of Custody Transfer Measurements•For a custody transfer measurement of a liquidhydrocarbon, a contract requires a volumetric measurementat standard conditions of temperature and pressure.•The techniques to do this are broadly categorized as “static”and “dynamic.”•Static measurements are accomplished through automatictank gauging.•Dynamic measurements are accomplished through liquidmetering methods.173
  174. 174. 174
  175. 175. 175
  176. 176. 176
  177. 177. Meter Provers Required•Any flowmeter’s indication of a volumerepresents an unknown volume unless thevolume can be compared to a knownvolume. The known volumes are called “meter provers”177
  178. 178. For a meter to be considered accurate, themeter must be proved at the sameconditions of flowrate, temperaturepressure, and product viscosity.178
  179. 179. 179
  180. 180. FLOW METER CALIBRATION: IMPORTANCE AND TECHNIQUES•Calibration is typically performed in a laboratorysetting at several different flow rates, and usesconditions such as changing densities, pressure,and temperatures.•Proving differs from calibration in that it is donein the field, typically under a single set ofconditions.180
  181. 181. The calibration can be defined as thecomparison of a measuring instrument with specified tolerance but an undetermined accuracy, to a measurement standard with known accuracy181
  182. 182. •The use of non-calibrated instruments createspotentially incorrect measurement and erroneousconclusions and decisions.•It is calibration that: provides assurance and confidence in measurement. maintains product in specified ranges.182
  183. 183. •Calibration can be a simple dimensional check todetect measurement variables.•Before starting calibration, a decision must bemade for the following: Which variables should be measured. What accuracy must be maintained.183
  184. 184. Some element of error exists in all measurementsno matter how carefully they are conducted.The magnitude of the error can never be easilydetermined by experiments;the possible value of the error can be calculated.184
  185. 185. Method of CalibrationsIn general the flow measurement devices are calibratedby three methods:• Wet calibration uses the actual fluid flow.• Dry calibration uses flow simulation by means of anelectronic or mechanical signal.• A measurement check of the physical dimensions anduse of empirical tables relating flow rate to thesedimensions is another form of calibration.185
  186. 186. Wet Calibration•It uses actual fluid flow.•Generally it provides high accuracy for a flowmeterand is used when accuracy is a prime concern.•Precision flowmeters are usually wet calibrated atthe time of manufacture.•Wet calibration for flowmeters is usually performedwith water, air, or hydrocarbon fuels.186
  187. 187. Dry Calibration•It is performed on a flowmeter without thepresence of a fluid medium.•The input signal is Hz, mV, or P.•It is much more uncertain than wet calibration.•The overall accuracy of the flow device isinferred because the flow transducer is bypassed.187
  188. 188. •The input signal for a dry calibration mustbe provided by a measurement standard.•The value of the output signal requires useof other measurement standard.•Follow the manufacturer’s guideline andprocedures for dry calibration.188
  189. 189. Provers•The proving operation verifies the meter’sperformance and assurance.•The necessity for proving depends on how accuratethe measurement must be for the product beinghandled.•Prover is considered part of the metering station’scost and is permanently installed at the facilities.•For low value products, portable provers are used.189
  190. 190. Methods of Meter Proving•Pipe provers are one of the most commontypes of provers in industry today.•The process does not have to be shut downwhen proving a meter.•Two types of pipe provers: Unidirectional prover, Bidirectional prover.190
  191. 191. Unidirectional Provers•It displaces a known volume by means of adisplacer traveling in only one direction insidethe prover.•The displacer’s travel is detected by detectorswitches within the prover.191
  192. 192. 192
  193. 193. Bidirectional Provers•It requires a displacer to travel in both directionsto complete one prover run.•After stabilizing pressure and temperature, thedisplacer is put into the system.•It will slow down flow in the system for a timeuntil the displacer picks up speed.193
  194. 194. 194
  195. 195. Small Volume Provers•They can accommodate a wide range of flowrates.•They are compact in size and have less volumethan conventional unidirectional andbidirectional pipe provers.•The time to obtain a meter factor is significantlydecreased.195
  196. 196. Master Meter Method•It is used when a pipe prover is unavailable.•The master meter method uses a known reliablemeter configured in series with the meter to beproved.•The meter measurements are then compared.196
  197. 197. Weight and Volume Methods•Static calibration•Dynamic calibration197
  198. 198. Static Calibration•The flow is quickly started to begin the test, heldconstant during the test, and then shut off at theend of the test.•The totalized flow reading from the flowmeters iscompared with the weight or volume collected andthe performance of the meter is calculated.•The static calibration system operates best withflowmeters that have low sensitivity to low flowrates.198
  199. 199. Dynamic Calibration•The flow is kept at a constant rate before thebeginning of the test.•The flow reading from the flow meter and initialweight or volume are read together to start the testand after the desired collection period to end thetest.•The dynamic calibration systems are limited by themeter’s speed of the response.199
  200. 200. 200
  201. 201. 201
  202. 202. 202
  203. 203. Basic Equations203
  204. 204. As long as the fluid speed is sufficiently subsonic(V < mach 0.3),the incompressible Bernoullis equationdescribes the flow reasonably well.204
  205. 205. 205
  206. 206. •It is recommended that location 1 be positionedone pipe diameter upstream of the orifice, andlocation 2 be positioned one-half pipe diameterdownstream of the orifice.206
  207. 207. •For flow moving from 1 to 2, the pressure at 1will be higher than the pressure at 2;•the pressure difference as defined will be apositive quantity.207
  208. 208. •From continuity, the velocities can be replacedby cross-sectional areas of the flow and thevolumetric flowrate Q,208
  209. 209. •Solving for the volumetric flowrate Q gives,209
  210. 210. •For real flows (such as water or air), viscosityand turbulence are present and act to convertkinetic flow energy into heat.•To account for this effect, a discharge coefficientCd is introduced into the above equation tomarginally reduce the flowrate Q,210
  211. 211. •Since the actual flow profile at location 2downstream of the orifice is quite complex,•thereby making the effective value of A2uncertain, the following substitution introducinga flow coefficient Cf is made,•where Ao is the area of the orifice.211
  212. 212. •As a result, the volumetric flowrate Q for realflows is given by the equation,212
  213. 213. •The flow coefficient Cf is found from experimentsand is tabulated in reference books;•It ranges from 0.6 to 0.9 for most orifices.•Since it depends on the orifice and pipe diameters(as well as the Reynolds Number), one will oftenfind Cf tabulated versus the ratio of orificediameter to inlet diameter, sometimes defined as,213
  214. 214. Most Common P flowmeters• Orifice plates• Venturi• Flow nozzles• pitot tube / annubar• Elbow or wedge meter214
  215. 215. Meter Tube Assembly Example215
  216. 216. Orifice Plate•It is the main element within an orifice metertube.•It is the simplest and most economical type ofall differential pressure flowmeters.•It is constructed as a thin, concentric, flat metalplate.•The plate has an opening or “orifice.”216
  217. 217. •An orifice plate is installed perpendicular to thefluid flow between the two flanges of a pipe.•As the fluid passes through the orifice, therestriction causes an increase in fluid velocity anda decrease in pressure.217
  218. 218. •The potential energy (static pressure) isconverted into kinetic energy (velocity).•As the fluid leaves the orifice, fluid velocitydecreases and pressure increases as kineticenergy is converted back into potential energy(static pressure).218
  219. 219. Orifice plates always experience some energyloss – that is, a permanent pressure loss causedby the friction in the plate.219
  220. 220. The Beta ratio is defined as the ratio of thediameter of orifice bore to internal pipediameter. <1220
  221. 221. •The most common holding system for an orificeplate is a pair of flanges, upstream anddownstream piping, and a pressure tap.•The pressure taps are located either on orificeflanges or upstream and downstream of the pipefrom the orifice plate.221
  222. 222. •For precise measurement, various types offittings are used:  junior fittings,  senior fittings, and  simplex fittings.222
  223. 223. The fittings provide:•easy installation of an orifice plate,•removal of the plate for changes in flow rateservices, and•convenient removal for inspection andmaintenance.223
  224. 224. Senior Orifice Fitting•It is a dual-chamber device that reigns as themost widely used means of measurement fornatural gas.224
  225. 225. Simplex Orifice Plate HolderIt is a single-chamber fittings that house andaccurately position an orifice plate for differentialpressure measurement.225
  226. 226. Junior Orifice Fitting•It is a single-chamber fitting, engineered andmanufactured to make orifice plate changingquick and easy at installations where linemovement from flange spreading is undesirable.226
  227. 227. 227
  228. 228. 228
  229. 229. Limitations of orifice plates include a highirrecoverable pressure and a deterioration inaccuracy and long term repeatability because ofedge wear.229
  230. 230. •Two types of orifice plates designs areavailable: •Paddle type and •Universal type.230
  231. 231. The paddle type orifice plate•It is used with an orifice flange, has a handle foreasy installation between flanges.•On the paddle type plate, the orifice bore,pressure rating (flange rating), bore diameter,Beta ratio, and nominal line size are stamped onthe upstream face of the plate.•The outside diameter of a paddle plate varieswith the ANSI pressure rating of the flanges.231
  232. 232. The universal orifice plate•It is designed for use in quick change fittings.•The universal plate is placed in a plate holder,the outside diameter is the same for all pressureratings for any given size.•When using orifice fittings, the internaldiameter of the meter tube must be specifiedbecause the orifice plate is held in an orificeplate sealing unit.232
  233. 233. 233
  234. 234. Weep Hole•Some orifice plates have a small hole in theorifice plate besides an orifice bore either  above the center of the plate, or  below the center of the plate.234
  235. 235. The purpose of the weep hole is to allow the:passage of any condensate in a gas applicationorpassage of gas in liquid service applications.235
  236. 236. •The area of the weep hole must be consideredwhen sizing an orifice plate.•An orifice plate with a weep hole should not beused when accurate measurement is required in aflow measurement application, such as in gassales service.236
  237. 237. Concentric Orifice Plate237
  238. 238. The orifice plate, although a relatively simpleelement, is a precision measuring instrument andshould be treated accordingly.238
  239. 239. •Critical items considered when evaluating orificeplates are the following: • Flatness, smoothness, and cleanliness of the orifice plate. • The sharpness of the upstream orifice edge. • The bore diameter and thickness of the orifice plate.239
  240. 240. Orifice Plate Dimensions• d represents the bore of the orifice plate.• D represents the pipe inside diameter.• Dam height represents the difference of pipe inner diameter and diameter of boredivided by 2.• T represents the thickness of the plate.• e represents the orifice plate bore thickness which is 1/2 T•  is called orifice plate bevel angle. It is 45 °, +20 ° 0°.240
  241. 241. •Several types of orifice bore designs areavailable for orifice plates:  Concentric,  Segmental, and  Eccentric orifice plates.•The plates are used for a wide range ofapplications.241
  242. 242. Types of Orifice Plates242
  243. 243. Concentric Plates•The concentric orifice bore plates are used ingeneral flow measurement applications.•The concentric orifice plate has an orifice borein the center of the plate.•The concentric bore plate is used for clean fluidservices, as well as for applications requiringaccurate flow measurement.243
  244. 244. •The center of bore is either beveled or straight.•The beta ratio for the concentric plate isbetween 0.1 to 0.75.244
  245. 245. Eccentric Plates•It is similar to a concentric plate, but theeccentric plate has the bore in an offset position.•The eccentric orifice plate is used when dirtyfluids are measured, to avoid the tendency ofhole plugging if a concentric plate were used.•Flow coefficient data is limited for eccentricorifices; therefore, it provides less accuratemeasurement.245
  246. 246. 246
  247. 247. •In an eccentric orifice plate, the hole is boredtangent to the inside wall of the pipe or, morecommonly, tangent to a more concentric circlewith a diameter not smaller than 98% of thepipe’s internal diameter.•When lacking specific process data for theeccentric orifice plate, the concentric orifice platedata may be applied as long as accuracy is not amajor issue.247
  248. 248. • Make sure that flanges or gaskets do notinterfere with the plate hole.•The line size ranges from 4” to 14”.•It can be made smaller than a 4” as long as theorifice bore does not require a beveling edge.•Beta ratio is limited between 0.3 to 0.8.•Flange taps are recommended for eccentricorifice plate installations.248
  249. 249. Segmental Plates•It looks like a segment of a circle withsegmented circle hole in offset from the plate’scenter.•The orifice hole is bored tangent to the insidewall of the pipe or tangent to a more concentriccircle with a diameter not smaller than 98% ofthe pipe internal diameter.•Installation is similar to eccentric type.249
  250. 250. Quadrant Edge Plate•It is used for lower pipe Reynolds numberswhere flow coefficients for sharp-edge orificeplates are highly variable.•It is used for viscous clean liquid applications.•Nominal pipe size ranges between 1” to 6”.250
  251. 251. Orifice Plate Parameters(1) Orifice flow rate.(2) Pipe line size and pressure rating.(3) Thickness of orifice plate.(4) Orifice Bore (d).(5)Orifice plate holders: The orifice plate holder includesorifice flanges, orifice fittings.(6) Beta Ratio.(7) Differential Pressure (P).(8) Temperature.(9) Reynolds Number (Re).(10) Pressure taps.251
  252. 252. Pressure Taps252
  253. 253. 253
  254. 254. Flange Taps•Holes drilled into a pair of flanges.•Flange tap holes are not recommended whenthe pipe size is below 2 inches.254
  255. 255. Pipe Taps•Pipe taps are located at 2.5 D upstream and 8 Ddownstream from the orifice plate.• Exact location of the taps is not critical.•However, the effect of pipe roughness anddimensional inconsistencies can be severe.255
  256. 256. The uncertainty of measurement is 50 % greaterwith full flow taps than with taps close to theorifice.Pipe taps are not normally used unless it isrequired to install the orifice meter on a existingpipe, or other taps cannot be used.256
  257. 257. Corner Taps•Corner taps are a style of flange taps.•The only difference between corner and flangetaps is that the pressure is measured at thecorner between the orifice plate and the pipewall.•Corner taps are used when the pipe size is 2“ orless.257
  258. 258. Vena Contracta Taps•When an orifice plate is inserted into theflowline, it creates an increase in flow velocityand a decrease in pressure.• The location of the vena contracta point isbetween 0.35 to 0.85 of pipe diametersdownstream of the plate, depending on the betaratio and Reynolds number.258
  259. 259. Pressure and Flow Profile259
  260. 260. •Vena contracta taps are located 1D upstream and at theVena contracta location downstream.•Vena contracta Taps are the optimum location formeasurement accuracy.•They are not used for pipes less than 6” in diameter.260
  261. 261. Liquid ServiceTap Locations – The pressure tap location in liquidservice orifice meters should be located to preventaccumulation of gas or vapor in the connectionbetween the pipe and the differential pressureinstrument.The differential pressure instrument should be closeto the pressure taps or connected through downwardsloping connecting pipe of sufficient diameter toallow gas bubbles to flow back into the line.261
  262. 262. Transmitter Installation – The installation ofdifferential pressure transmitters should belocated below the pipe and sloping upwardstoward the pipe to prevent the collection of gasbubbles in the impulse tubing.Vent Holes are required for venting of any gas ina liquid service.Location of the vent hole in a liquid service is atthe top of a pipe, above the center line.262
  263. 263. Gas ServicesTap Locations – Pressure tap locations in a gasservice must be installed in the top of the linewith upward sloping connections towards a pipe.The differential pressure measuring instrumentmay be close-coupled to the pressure taps in theside of the lines or connected through upwardsloping connecting pipe of sufficient diameter toprevent liquid from accumulating in the line.263
  264. 264. Transmitter Installation – The installation ofdifferential pressure transmitters should belocated above the pipe with the impulse tubingsloping downward towards the pipe so that anycondensate drains into the pipe.Drain Holes – A drain hole is required for drainingof any liquid in a gas service.Location of the drain hole is below the center lineof the pipe.264
  265. 265. Steam ServicesTap Locations require the use of condensingchambers in steam or vapor applications becausecondensate can occur at ambient temperatures.Generally, the pressure tap connection has adownward sloping connection from the side ofthe pipe to the measuring device.265
  266. 266. Transmitter Installation – The installation ofdifferential pressure transmitters should belocated above the pipe with the impulse tubingsloping downward towards the pipe so that anycondensate drains into the pipe.Drain Holes – A drain hole is required for drainingof any condensate liquid in a steam service.The location of a drain hole is below the centerline of the pipe.266
  267. 267. Standard Flow•Flow measurement of a fluid stated in volume units at base(standard) conditions of P and T is called standard flow.•For crude petroleum and its liquid products, the vapor pressure is<= than atmospheric pressure at base temperature of 14.696 psia(101.325 kPa) at a temperature of 60°F (15.56°C).•For a hydrocarbon liquid, when vapor pressure > atmosphericpressure at base temperature, the base pressure is calledequilibrium vapor pressure.•The base condition for natural gases is defined as a pressure of14.73 psia (101.56 kPa) at a temperature of 60 °F (15.56°C).267
  268. 268. Compensated Flow•Compensated flow represents a flow under fluid conditions thatmay vary.•The conditions are measured and used along with flowmetersignal to compute the true flow rate from the flowmeter.•The output signal from a flowmeter represents the true flow ratevalue under specified fluid conditions.•For a liquid service, variations in density or viscosity can changethe meter’s accuracy.•For gas services, a change in temperature, pressure, andmolecular weight can ruin the accuracy of the meter.268
  269. 269. 269
  270. 270. Computer Programs for Sizing Orifice Plates•ORICALC-2,•EA-25,•ORSPEC,•FLOWEL,•INSTRUCALC,•ORIFICE2, and•FLOW CONSTANT270
  271. 271. http://www.pipeflowcalculations.com/orifice/271
  272. 272. Common primary element errors:• Beta ratio is too large for the meter run• Orifice plate is not flat, it is concave or convex• Orifice does not have sharp edges• Orifice plate is installed backwards• Orifice plate is damaged through poor handling• An incorrect size is used for the orifice meter tube or plate• Orifice plate is not centered in the line• Orifice meter tube is corroded• Tap locations are incorrect• Contaminants build up on orifice plate• Contaminants build up on meter run• Hydrates build up on meter run and orifice plate• Flow conditioners are dislodged and move closer to plate• Leaks occur around orifice plate• Pressure tap or thermowell installed upstream of meter• Welding meter supports distorts meter run272
  273. 273. Common secondary element errors:• Gauge lines are too small• Gauge lines are too long• Gauge lines leak• Gauge lines have sags or loops that collect condensates• Gauge line slopes are not correct• Incorrect ranges are used on secondary instruments• Differential pressure transmitter was not zeroed properly• Excessive dampening is used in secondary instrument273
  274. 274. Other Differential Pressure Flowmeters274
  275. 275. Flow Nozzles•The flow nozzle is another type of differential-producing device that follows Bernoulli’stheorem• The permanent pressure loss produced by theflow-nozzle device is approximately the same asthe permanent pressure loss produced by theorifice plates.275
  276. 276. •The flow nozzle can handle dirty and abrasivefluids better than can an orifice plate.•In a flow nozzle with the same line size, flowrate, and beta ratio as an orifice meter, thedifferential pressure is lower, and the permanentpressure loss is less.276
  277. 277. Performance and Applications•Changing a flow nozzle is more difficult thanchanging an orifice plate when there is a changein flow rate requirements.•Flow nozzles are used for steam, high velocity,nonviscous, erosive fluids, fluids with somesolids, wet gases, and similar materials.277
  278. 278. •The flow nozzles pass 60% more flow than theorifice plate of the same diameter anddifferential pressure.•A flow nozzle’s inaccuracy of ± 1% of rate isstandard with ± 0.25% of rate flow calibrated.278
  279. 279. Typical Nozzle Installations279
  280. 280. Venturi Meter•A venturi design can be described as a restrictionwith a long passage with smooth entry and exit.•Venturi tubes produce less permanent pressureloss and more pressure recovery than the othermeters.•It is one of the more expensive head meters.•Low pressure drops for non-viscous fluids.280
  281. 281. 281
  282. 282. Venturi Designs282
  283. 283. Performance Advantages:• The long form venturi develops up to 89%pressure recovery for a 0.75 beta ratio anddecreases to 86% recovery for a 0.25 beta ratio.• The short form venturi develops up to 85%recovery at 0.75 beta ratio and decreases to 7 %at 0.25 beta ratio.283
  284. 284. • A venturi meter has a low permanent pressureloss and high recovery at higher beta ratios.• A venturi meter can be used for dirty fluids andslurries.• Higher accuracy (better than orifice).284
  285. 285. Performance Disadvantages:• A venturi meter is a very expensive measuringdevice to use.• A venturi meter has limited rangeability and isonly installed when flow rate’s rangeability is lessthan 3 to 1.285
  286. 286. Pitot Tubes•The previously discussed primary differentialpressure flow metering devices utilized thedifference in static pressure perpendicular to thedirection of flow as a basis for inferring velocity.•The actual velocity was not measured, but wascalculated after many experimental laboratorymeasurements and correlations.286
  287. 287. •The Pitot tube measures a fluid velocity byconverting the kinetic energy of the flow intopotential energy.•The conversion takes place at the stagnationpoint, located at the Pitot tube entrance.287
  288. 288. 288
  289. 289. •A pressure higher than the free-stream (i.e.dynamic) pressure results from the kinematic topotential conversion.•This "static" pressure is measured by comparingit to the flows dynamic pressure with adifferential manometer289
  290. 290. 290
  291. 291. 291
  292. 292. Performance Advantages:• It creates very little permanent pressure drop and, as aresult, is less expensive to operate.• A pitot tube can be installed on 4” and more.• Performance of the pitot tube is historically proven.• A pitot tube’s installation and operation costs are low.• A pitot tube can be a standard differential producingdevice for all pipe sizes.292
  293. 293. Performance Disadvantages:• Point-type pitot tubes require traversing theflow stream for average velocity.• Poor rangeability.• Nonlinear square root characteristic.• Difficulty of use in dirty flow streams.293
  294. 294. Annubars•The sensing points are arrayed along perpendiculardiameters with the number of points in each traversebased upon the duct size.294
  295. 295. 295 Annubar Design
  296. 296. Performance• The diamond shape annubar has long termaccuracy.• The annubar has an accuracy of ±1% of actualflow and ±0.1 repeatability of the actual value.• The annubar has low installation costs; a systemshutdown is not required to install the device.296
  297. 297. • The annubar produces a repeatable signal evenwhen the run requirements are not met.• The annubar flow sensor can handle a widerange of flow conditions with two measuringinstruments.• The annubar should not be used if the viscosityapproaches 50 centipoise.• The annubar can be used on two phase flowmeasurements.297
  298. 298. Applications•The annubar can be used for liquid and gas flowmeasurement services.•Generally, the annubar is used in clean liquidservices to avoid plugging.•The annubar can be installed for low andmedium pressure applications without shuttingdown the system.298
  299. 299. Wedge Type Flowmeter•The basic system consists of a cylindrical pressurevessel into which a constriction "wedge" isfabricated thereby leaving a open segment of aknown height.299
  300. 300. •Pressure taps which receive the sensors on either sideof the "wedge" provide the differential signal to theFlow Transmitter which is then related, by formula, tothe rate of flow occurring through the open segment.300
  301. 301. Elbow Type Flowmeter•A differential pressureexists when a flowing fluidchanges direction due to apipe turn.•The pressure differenceresults from the centrifugalforce.301
  302. 302. •Since pipe elbows exist inplants, the cost for thesemeters is very low.•However, the accuracy isvery poor.•They are only appliedwhen reproducibility issufficient and other flowmeasurements would bevery costly.302
  303. 303. 303
  304. 304. 304
  305. 305. 305
  306. 306. 306
  307. 307. 307
  308. 308. 308
  309. 309. Turbine MetersFlowing fluid forces the turbine wheels to rotateat a speed proportional to the velocity of thefluid.309
  310. 310. •For each revolution of the turbine wheel, apulse is generated.•The rotational speed of shaft and frequency ofthe pulse corresponds to the volumetric flowrate through the meter.310
  311. 311. K-factorIt is the number of pulses per unit of measurementgenerated by the rotor as it turns inside the turbine.It is usually indicted as Pulses per Gallon311
  312. 312. 312
  313. 313. 313 Turbine Meter
  314. 314. Insertion Type Turbine Meter314
  315. 315. Performance Advantages• Excellent accuracy and good rangeability over thefull linear range of a meter.• Low flow rate designs are available.• Some versions do not require electrical power.• Overall meter cost is not high.• Output signal from the meter is at a highresolution rate, which helps reduce meter proving.315
  316. 316. Performance Disadvantages• Sensitive to a fluids increasing viscosity.• Two phase fluids can create usage problems.• Straight upstream piping or straighteningvanes are required in a turbine meter installationto eliminate the flow turbulence into the meter.316
  317. 317. Electromagnetic Flowmeters317
  318. 318. Faraday’s Law states that emf is created when aconductive fluid moves through a magneticfield.318
  319. 319. The axis of the conductive fluid flows at a right angle tothe magnetic field. Fluid flowing in this manner causesa voltage that is proportional to the flow rate.319 Magnetic Flowmeter Principles
  320. 320. •The voltage developed at the electrodes has anextremely low level signal.•A signal conditioner must amplify the signal.•There are two types of magnetic flowmeters: AC excitation, and DC pulse excitation.320
  321. 321. AC Excitation•In an AC type magnetic flowmeter, line voltage (120 or 240V AC) is applied directly to the magnetic coils.•This generates a magnetic field in the outer body that varieswith the frequency of the applied voltage.•An AC meter’s signal has a sine wave pattern.•The magnitude of the sine wave is directly proportional tothe flow velocity.•The system produces an accurate, reliable, fast respondingmeter.321
  322. 322. DC Pulse Excitation•In a DC type magnetic flowmeter, line voltage is themain source of power, but instead of applying it directlyto the coils, it is first applied to a magnet driver circuit.•The magnet driver circuit sends low frequency pulses tothe coils to generate a magnetic field.•The DC pulse system eliminates the zero shift problemthat occurs in an AC system.322
  323. 323. 323
  324. 324. Performance Advantages• It is non-obstructive and has no moving parts.• Pressure drop is very little.• DC pulse-type power can be as low as 15 to 20 watts.• Suitable for acid, bases, water, and aqueous solutions.• Lining materials provide good electric insulation andcorrosion resistance.• The magnetic meter can handle extremely low flow.• It can be used for bidirectional flow measurements..324
  325. 325. Performance Disadvantages• The meters only measure conductive fluid flows. (Hydrocarbons, gases, and pure substances cannot be measured)• Proper electrical installation care is required.• Conventional meters are heavy and larger in size.• Meters are expensive.325
  326. 326. InstallationProper magnetic flow meter operation is verydependent upon the installation.Installation considerations for a magnetic flowmeterprimarily involve the following:• Meter orientation• Minimum piping requirement• Grounding326
  327. 327. Magnetic Flowmeter Installation Practices327
  328. 328. Applications•It is suited for measurement of slurries and dirty fluidsbecause magnetic flowmeters do not have sensors thatenter the flowing stream of fluids.•Magnetic flowmeters are not affected by viscosity or theconsistency of Newtonian or non-Newtonian fluids.•The resulting change in flow profile caused by a changein Reynolds number or upstream configuration pipingdoes not change the meter’s performance or accuracy.328
  329. 329. Mass Flowmeters (Coriolis Flowmeters)•The mass of the fluid is measured as opposed tothe fluid volume or flow rate.•A changing density or viscosity can affect theperformance of a volumetric flowmeter,•While a mass flowmeter would not be affectedby these changes.329
  330. 330. 330
  331. 331. 331
  332. 332. 332
  333. 333. •Coriolis meters can be used on liquid and somegas applications.•The direct measurement of mass is necessaryfor applications where chemicals are balanced,combustion efficiencies are calculated, orproduction quantities must be consistent.•If a measurement volume is desired, densitycorrections are required to measure the fluid atbase conditions.333
  334. 334. •A Coriolis force is caused by flowing fluidthrough a tube. The Coriolis force equation isequivalent to Newton’s Second Law of Motion,where334
  335. 335. •In Coriolis flowmeters, fluid typically flowsthrough an U-shaped tube that vibrates at itsnatural frequency.•As the fluid flows into the U-shaped tube, thefluid is forced to conform to the verticalmomentum of the vibrating tube.•If the U-shaped tube is moving upward duringits vibration, the fluid flowing into the U-shapedtube resists and pushes downward.335
  336. 336. 336
  337. 337. •The fluid has an upward momentum as itapproaches the part of the tube where it exit.•If that portion of the tube has a downwardmotion, the fluid resists the downward motion bypushing up on the tube.•The U-shape tube then twists. The twisting iscalled the Coriolis effect.•The amount of U-shaped tube twisting becomesdirectly proportional to the mass flow rate.337
  338. 338. 338
  339. 339. •The detector senses the amount of tubetwisting.•The U-shaped tube can be vibrated by anoscillating driver at its natural frequency.•Electromagnetic devices, such as velocitydetectors, can be located on each side of thetube and be used to measure the velocity of thevibrating tube.339
  340. 340. •When no fluid flows through the tube, all pointsmove in sequence with the oscillating driver,forming a sine wave.•When fluid flows in the tube, twisting occurs.•The twisting causes a time difference to occurbetween the velocity detectors signals.•The time difference is directly related to themass flow rate.340
  341. 341. 341
  342. 342. •The mass flow of a u-shaped Coriolis flow meteris given as:Where:Ku is the temperature dependent stiffness of the tube,K a shape-dependent factor,d the width, the time lag,the vibration frequency andIu the inertia of the tube.As the inertia of the tube depend on its contents, knowledge ofthe fluid density is needed for the calculation of an accurate massflow rate. 342
  343. 343. Performance Advantage• They can handle difficult applications.• They are suitable for a large number of fluids.• They have Less susceptibility to damage, wear, andmaintenance.• They can measure bidirectional flow.• Accuracy is very good, typically ± 0.2% of rate.• The rangeability is typically 20:1 or better.• Their operation is independent of a fluid’s propertycharacteristics.343
  344. 344. Performance Disadvantages• Earlier versions were susceptible to external vibrations.• A Coriolis meter is available only up to a small size.• Special installation requirements are followed toisolate the Coriolis meter from mechanical vibration.• Avoid using Coriolis meters in piping or meter runswhich are prone to substantial vibration, shock, orextreme temperature gradients.• External meter piping must be well supported.344
  345. 345. 345
  346. 346. Ultrasonic FlowmetersFlowmeters that use sound waves to measureflow rate are called ultrasonic flowmeters.346
  347. 347. Principles• Doppler shift (frequency shift) method• Deflecting beam method• Transit time method Time difference Frequency difference347
  348. 348. Doppler Shift Method•It transmits a sound wave through the flowingfluid.•The sound waves are reflected from the fluid toa receiver on the ultrasonic flowmeter.•The frequency of the sound waves sensed at thereceiver shift are affected by the Doppler effect.348
  349. 349. •The frequency shift is used to determine flowrate.•Several types of meters are available: one type requires installation of a transducer into the flowing stream, the other is a strap-on model where installation of a transducer on the pipe is noninvasive.349
  350. 350. 350
  351. 351. Deflecting Beam Method•The transmitter sends a sound wave that is at aright angle to the flow.•The liquid carries the sound wave and the soundwave is “pushed” or deflected downstream.•The deflection is directly related to the flow rateand is used to determine the flow rate.351
  352. 352. Transit Time Method•A diagonal beam is sent across the flow path.•The beam is sent with and against the flow.•Sound travels slower against flow.•Most commonly used.•Homogeneous fluids (No entrained bubbles).•Not for heavy slurry-type applications, becauseof the high acoustic impedance.352
  353. 353. 353
  354. 354. Transmit Time Frequency Domain Meters•A pulse is sent in a given direction.•The time of pulse at the other end of sonic pathis recorded.•The same signal transmits in the oppositedirection and records the time at the arrival.•The difference between two timemeasurements provides information on motionof the fluid in a pipe.354
  355. 355. Frequency Domain Meters•The frequency domain meter uses the same type oftransducers as the transit time domain meter.•The only difference is in the processing of the signal.•The time pulse signal is converted to a frequencysignal.•The path in each direction of flow is used,•the sonic path generates two frequencies.•The difference is directly proportional to flow.355
  356. 356. Performance Advantages• Clamp-on versions are convenient for retrofits.• Usually nonintrusive.• No pressure drop.• Accuracy is comparable to orifice plates.• High rangeability; rangeability 40:1.356
  357. 357. Performance Disadvantages• Limited to clean, single-phase liquids.• Straight piping for uniform flow profile.• Attenuation may limit transmission path length.• Averaging methods for large pipes aremarginally cost-effective.357
  358. 358. Vortex Shedding Meters•Suitable for gas, steam, or liquid services.•Wide flow range capability,•Minimal maintenance,•good accuracy, and•Long term repeatability.358
  359. 359. 359
  360. 360. •Vortex shedding phenomenon is known as theVon Karman effect of flow across a bluff body.•Flow alternately sheds vortices from one side tothe other side of a bluff body.•The frequency of the shedding is directlyproportional to fluid velocity across the body.360
  361. 361. •The output depends on the K-factor.•The K-factor relates the frequency of generatedvortices to the fluid velocity.•The K-factor varies with the Reynolds number,but is virtually constant over a broad flow range.•The formula for fluid velocity is Fluid velocity =Vortex frequency/K – factor361
  362. 362. 362
  363. 363. 363
  364. 364. Positive Displacement Meters•Positive displacement (PD) meters are used formeasurement of gas and liquid.Rotating paddle meters,Oscillating piston meters,Oval gear meters,Sliding vane meters, andBi-rotor meters.•The term “displacement” refers to a discrete volumethat is flowing through the meter.364
  365. 365. PD meters are mechanically driven meters andhave one or more moving parts.365
  366. 366. The energy required to drive the meter’smechanical components is generated from theflow.366
  367. 367. The energy to drive the meter creates a pressureloss between inlet and outlet of the meter.367
  368. 368. A PD meter’s hardware can convert each unit ofvolume displacement into an electrical pulse.368
  369. 369. Positive Displacement Meters369
  370. 370. 370
  371. 371. •Accuracy is in terms of percentage registration: % Registration =(actual quantity/metered quantity) x 100•At high flow rates, the increase in pressure drop(differential pressure) increases the flow slippage rate,reducing the meter’s accuracy.•At low flow rates, the meter has low energy because ofthe lower pressure drop, so the flow is under-counted,again reducing the accuracy.•Accuracy of the meter is in the range of ± 0.1 to ± 2% ofthe actual flow.371
  372. 372. •Rangeability of PD meters typically is 5:1,although 10:1 and greater flow ranges arepossible.•Repeatability are typically ± 0.05% or better.•Output signals are available either inmechanical or electrical form.372
  373. 373. Performance Advantages• Ideal for viscous liquids• Upstream piping requirements are minimal• Some versions do not require electrical power• High rangeability in liquid and gas meter.373
  374. 374. Performance Disadvantages• Not ideal for liquids with suspended particles.• Mechanical wear.• Larger meters require extra installation care.• Meters can be damaged by over speeding.374
  375. 375. Typical PD Meter Installation375
  376. 376. Variable Area Flow Meters (Rotameter)•The rotameter’s operation is basedupon variable area principles.•The flow raises a float in a tapered tube,increasing the area for passage of theflow.•The greater the flow, the higher thefloat is raised.376
  377. 377. 377
  378. 378. Sight Flow Indicators•A sight flow indicator is a mechanically driven device.•Sight flow indicators are used for visual inspection ofthe process.•Three types of sight flow indicators are available, whichare the following: • Paddle • Flapper • Drip378
  379. 379. Paddle Type•Its design has a propeller inside its body.•It is only used for high flow rate applications.•A pressure drop in the paddle type indicator is higherthan the pressure in a drip or flapper type indicator.• It can be installed for flow directions that arehorizontal or vertical upward.•It is used when dark process fluids are present.379
  380. 380. Flapper Type•Bidirectional flappers are also available.•The flapper type sight flow indicator are used fortransparent or opaque solutions and gas services.•Flow direction can be horizontal or vertically upward.380
  381. 381. Drip Type•Its design is used when there is a dripping of fluid in avertically downward direction.•The drip type design is used for vertically downwardflows that are intermittent.•Assembly consists of a chamber, glass, gasket, endcovers, and bolts.381
  382. 382. Drag Plate•Flow produces a positive pressure on the plate.•The force is resisted by a null-balance supportingelement at the end of the support arm.•The signal is proportional to the square of the flowrate.382

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