1.Process Technician TrainingProcess ControlInstrumentsPackage number 1 1

2. Process Control InstrumentationProcess Control Instrumentation is a wide rangingsubject.While only dealing with the subject in a necessarilybrief manner the packages are of some considerablelengthFor this reason the subject has been broken into twoseparate and distinct packages.It is recommended that the packages are studied intheir correct sequence2 3. Process Control InstrumentationThe first package deals with the principles of Flow,Level, Temperature and Pressure and how they arecontrolledThe second package deals with the Controllers andthe ancillaries that complement themThe package also deals with some of the morecomplicated control systems such as PLCs and DCSs3 4. Process Control InstrumentationThere are many video clips within the packages.For the clips to activate and play the PowerPointpresentation must be viewed in View Show from theSlide Show drop-down menuSlide Show can be selected from the Toolbar at thetop of the page. View Show can be selected from themenu.4 5. Units in this CourseFirst PackageUnit 1 IntroductionUnit 2 Pressure MeasurementUnit 3 Level MeasurementUnit 4 Temperature MeasurementUnit 5 Flow MeasurementsSecond PackageUnit 6 Ancillary Control EquipmentUnit 7 ControllersUnit 8 Control LoopsUnit 9 Overview of: -Distributive Control Systems (DCS) -Programmable Logic Controllers (PLC)5 6. Process Control InstrumentationFirst PackageUnit 1IntroductionUnit 2Pressure MeasurementUnit 3Level MeasurementUnit 4Temperature MeasurementUnit 5Flow Measurements6 7. ObjectivesThe objectives of this course are that the participant: Knows the four variables being measured and controlled Understands what a Process Instrument is. Understands how an instrument functions Gains an understanding of the many types of instruments Learns of the many applications that control systems can be used in7 8. Introduction to Process Control InstrumentsInstrumentation plays an important part in theefficient operation of any processing or productionplant.Instruments enable the plant to operate smoothlyand safely with a minimum of operating staff.This course will give a basic understanding of howthe instruments function.8 9. Introduction to Process Control Instruments A process is the changing of a raw material into a finished product. As raw materials flow through the process equipment they are subjected to various conditions. These conditions may alter the composition of the raw material and may be the chemical structure.9 10. Introduction to Process Control InstrumentsIt is important that the process conditions areaccurately controlled at all times.The controlling is done by instruments.An instrument cannot think. It can only respond toparameters that are set by the Process DepartmentIt is the Technicians that tell the instruments what todo.10 11. Introduction to Process Control Instruments 11 12. Introduction to Process Control Instruments 12 13. Introduction to Process Control InstrumentsThe Technician who knows the instruments isthe master.The instruments are his slaves or servants.When the instruments are properly used, theprocess equipment operates correctly.When the process equipment operates correctlythe finished products are up to specification.When the process equipment is running steadilythe company is making money.An upset unit is costing money 13 14. Introduction to Process Control InstrumentsProcess ControlA simple example of a Manual process control.TechnicianThe process is temperature control. The indicator is athermometer 15. Introduction to Process Control InstrumentsProcess ControlThe correcting unit is the gas control valve.The controller is the Technician who uses his ownjudgment to keep the water temperature constant.Manual control has its uses: It is cheap to install and maintain It is simple to operate.15 16. Introduction to Process Control InstrumentsSuch a control system is seldom used in industrybecause:The Technician must remain in position at all times.It cannot be used if the Technician is placed in adangerous area.The process may change faster than the Techniciancan react.A mistake by the Technician can have dangerousresults. 16 17. Introduction to Process Control Instruments These problems are avoided by using automatic control. Modern household appliances now use automatic control to make work easier. For example:- Refrigerators and air conditioners use automatic temperature control. Air conditioners use automatic temperature control. An electric water heater uses automatic heating and water control and a switch that will shut off when the water boils.17 18. Introduction to Process Control InstrumentsThis is a simple automatic controller.The boiler now has the loop closed and no Technicianis required. 19. Introduction to Process Control InstrumentsTo install the automatic system the following items wereadded:The temperature transmitter (T.T) which senses thetemperature of the hot water and changes it to astandard signal.A signal line from the transmitter to the automaticcontroller (the signal may be either pneumatic orelectrical)A controller which keeps the temperature of the hotwater at a position set by the Technician (set point). 19 20. Introduction to Process Control InstrumentsThe controller adjusts the correcting unit (automaticcontrol valve) using an output signal line similar to theinput line from the transmitter.The controller may provide alarm signals to alert theTechnician if the system fails. It may also shut off thegas if the water starts to boil.20 21. Introduction to Process Control Instruments Agroup of instruments that are used to control a particular process is a Control Loop Control loops can be very simple like the one illustrated in the following video clip which is a simple temperature control on the steam into a heater Othersystems can be quite complex and involve a wide range of instruments that all talk to one another to control a system Agood example of this is the level control system on the CPF Inlet Separators - these will be dealt with later.21 22. Introduction to Process Control Instruments 22 23. Introduction to Process Control InstrumentsControl Loop:A Control Loop is an active system that keeps aprocess variable within maximum and minimumvalues by continuous measurement and continuouscorrective actions.A level control loopcontrolling the level in avessel by restricting thepump discharge23 24. Introduction to Process Control Instruments A process Control Loop may contain many control Instruments and can be very complex. However, if each part or device that is in the complex unit is taken one by one the system becomes much easier to understand. The following series of video clips describe a very simple type of process control and will give an understanding of how changes made to a process take effect24 25. Introduction to Process Control Instruments 25 26. Introduction to Process Control Instruments 26 27. Introduction to Process Control Instruments 27 28. Introduction to Process Control Instruments 28 29. Introduction to Process Control Instruments 29 30. Introduction to Process Control Instruments 30 31. Introduction to Process Control InstrumentsThe previous video clips were simplified and describeda very basic system the control of steam being usedto heat water.Some of the terms used in the video are terms incommon use in this type of industryThe following is a list of a few of these terms and theirmeanings 31 32. Introduction to Process Control InstrumentsBasic Definitions Instrument Any device for measuring, indicating,controlling, recording and adjusting a physicalor chemical property e.g. flow pressure,acidity, weight, gas concentration, etc Instrumentation A complete set of instruments used to control a process e.g. refining, oil/gas production, LNG, LPG, etc IndicatorA device which shows a measured value tothe operator LagThe time taken between an adjustment beingmade and the process responding to it 32 33. Introduction to Process Control InstrumentsBasic DefinitionsRecorderA device which continuously recordsmeasurements, either electronically or on anink chart. It is used to show productionfigures, etc.Process LoopA group of instruments used to control a singleprocess variable e.g. pressure, flow, level, etc.Process The word for a manufacturing unit e.g.refining, liquefying gas, etc.MeasuredThe value of the property being controlled by aVariable or single process loop e.g. pressure, flow, level,Process etc.Variable (MV) 33 34. Introduction to Process Control InstrumentsBasic DefinitionsDesired ValueThe value required by the operator.or Set Point(SP)Error Signal The difference between the measured(ES) variable and the set point - should be zero for good control.Controller A device, either pneumatic or electrical / electronic, which adjusts the error signal to zero.Correcting Unit A device which works on the command of the(Final Controlcontroller. It is used to adjust the measuredElement)value to obtain a zero error signal e,g, controlvalve etc.34 35. Introduction to Process Control InstrumentsBasic DefinitionsTransmission A method of standardising signals sent from various parts of the plant.TransmitterA device which takes a measurement and changes it into a standard signal.Transducer A device which changes one form of energy to another; particularly from electrical to pneumatic.DisturbanceA change in the process that is not anticipatedLoad A planned change in throughput of a unitChange 35 36. Introduction to Process Control InstrumentsNote: The instruments use in a processing facility canvary greatly, depending on the age of the installation.They may be air (pneumatic), liquid (hydraulic) orelectric/electronic in operation.The way the information is shown or recorded may besimple, like a clock or thermometerIn other cases it may be by the latest informationtechnology displaying the information on a personalcomputer screen (video display unit).36 37. Introduction to Process Control Instruments Process Variables A process variable is a process that we can measure and change. There are many process variables in a Processing Facility. However there are only four main variables in a facility: Pressure Flow Level Temperature We will concentrate on these four process variables. 37 38. Introduction to Process Control Instruments 38 39. Introduction to Process Control InstrumentsPressureIt is pressure that pushes fluids through pipesand equipment.It can be considered that pressure is the most importantprocess variable.It is therefore important that you have a very goodunderstanding of pressure.Without that understanding you will find it difficult to followcourses that come later in your training.39 40. Introduction to Process Control InstrumentsPressurePressure (P) is defined as the Force (F) applied divided by Area (A).Pressure and GasesThe diagram shows a force (F)applied to a piston pressing ona liquid in a cylinder.The liquid is consideredincompressible and the pressure of the liquid on the walls of the cylinderis the same in all directions. This gives the formulaP=F A 41. Introduction to Process Control InstrumentsPressure on a GasThis diagram shows a force (F)applied to a piston pressing ongas in the cylinder. The gas iscompressible.The volume of the gas willdecrease until the pressureof the gas on the walls of the cylinder equals the pressureapplied by the piston.This gives the formula F = P A 42. Introduction to Process Control InstrumentsPressure UnitsThere is no agreed standard for pressuremeasurement in the petrochemical industry.Some companies use Imperial Units (USA), someuse International Standard Metric Units (ISO)Some use both.The Technician needs to understand both systems andbe able to convert from one to another.42 43. Introduction to Process Control InstrumentsPressure UnitsImperialISO ForcePoundNewton Area Square InchSquare Metre Pressure Pounds per square inch Newtons per squarecalled PSI metre called Pascal 43 44. Introduction to Process Control InstrumentsPressure UnitsThere are tables for changing from one system to another. Anexample is the conversion of psi to kPa.Conversion: 1 psi =6.89 kPaThe Pascal is a very small unit so the KILOPASCAL (kPa) isoften used. The bigger unit is the BAR. The bar is the mostcommon ISO unit. 44 45. Introduction to Process Control Instruments Pressure Conversions:100 kPa = 1 barNote: On very old installations the kilogram per centimetresquare is still used.For all general purposes. 1 kg/cm2=1 barVery small pressures are measured using the height of acolumn of liquid. The liquids used most are water (H20) andmercury (Hg). 45 46. Introduction to Process Control InstrumentsPressureAbsolute, Gauge and Atmospheric PressureThe price of oil or gas depends on the quantity (mass) ofthe product.The quantity of oil or gas in a given volume depends onthe pressure.For this measurement, absolute pressure must be used.Absolute PressureThis is the pressure above a total vacuum (there are noparticles of matter in a total vacuum). 46 47. Introduction to Process Control InstrumentsPressureGauge PressureThis is the pressure measured by a gauge. Gaugepressure is the pressure above that of the surroundingatmosphere.Atmospheric PressureThe pressure of the air all around you.This is not constant, it depends on things like theweather and the altitude of the plant.47 48. Introduction to Process Control InstrumentsPressureThe equation linking the above pressures together isAbsolute Pressure = Gauge Pressure + AtmosphericPressure.Because atmospheric pressure can vary, a standardatmospheric pressure of 1.013 Bar or 14.70 psi. is usedGauge pressure is written as psig.Absolute pressure is written as psia.48 49. Introduction to Process Control Instruments Pressure Example A pressure gauge indicates 11.4 psi. To find the absolute pressure if the atmospheric pressure 14.65 psi. Solution The absolute pressure is equal to atmospheric pressure plus the gauge pressure (AP = GP + Atmospheric Pressure) AP = 11.4 + 14.65 = 26.05 Therefore the Absolute Pressure is 26.05 psi49 50. Introduction to Process Control InstrumentsUnits of FlowThis simplified diagram shows a tanker being loaded from astorage tank. The amount of oil loaded must be accuratelymeasured to know how much it costs. The total flow(quantity) of oil into the tanker can be measured in twoways.By volume, in barrels or cubic metres.By mass, in metric or imperial tonnes. 51. Introduction to Process Control Instruments Units of Flow For control purposes the rate of flow (how fast the ship is loaded) is also measured. Rate of flow units can also be given in either volumetric or mass units. Rate of flow by Volume (Volumetric)Barrel / HourCubic Feet / MinuteCubic Metres / Second Rate of flow by MassTonnes / HourKilograms / SecondPounds / Minute51 52. Introduction to Process Control Instruments Units of Flow The petrochemical industry uses many different units and there is no common standard. The following list gives some of the units and their conversion VOLUME Barrel (bbl)= 42 US gallons = 34.97 Imperial gallons Cubic foot (ft3)= 0.0929 m3 Cubic metres (m3) = 10.76 ft3 Cubic metres (m3) = 1000 litres 1 litre = 1000 cubic centimetres (millilitres)52 53. Introduction to Process Control Instruments Units of Flow MASS Pound (lb)= 0.454 kg Kilogram (kg) = 2.2 lb Imperial Tonne= 2240 lb (long tonne) Metric Tonne= 1000 kg American Tonne= 2000 lb (short tonne) Long Tonne= 1.016 Metric tonne Metric Tonne= 0.984 long tonneNote: It is not necessary to memorise conversions.Conversion tables will be available at your facility. 53 54. Introduction to Process Control InstrumentsUnits of Flow VELOCITY ft/sec ft/min metre/sec metre/min 160 0.304818.29 0.0166710.0050800.3048 0.032811.9685 0.010.600 3.281196.85 1 60 0.5473.2810.01667 1 54 55. Introduction to Process Control InstrumentsUnits of FlowABBREVIATIONS Bopd = Barrels oil per day Blpd = Barrels liquid per day Bcpd = Barrels condensate per day Scfpd= Standard cubic feet per day MScdpf = Thousand standard cubic feet per day MMScfpd= Million standard cubic feet per day Nm3pd= Normalised cubic metres per day 1/m or Llt/m = Litres per minute 5000 ml/s= 500 millilitres per second Note: Sometimes the p (per) is omitted in the abbreviation. Standard and normalised refer to a standard temperature and pressure. Common standards are: 14.7 psi at 680F and 1.013 Bar at 150C. 55 56. Introduction to Process Control Instruments 56 57. Introduction to Process Control Instruments 57 58. Introduction to Process Control InstrumentsTemperatureThere are different scales for measuring temperatures. The diagramcompares the two common temperature scales; Fahrenheit (Imperial) andCelsius (ISO). 59. Process Control Instrumentation 59 60. Process Control Instrumentation 60 61. Process Control Instrumentation 61 62. Process Control Instrumentation 62 63. Process Control Instrumentation 63 64. Process Control Instrumentation 64 65. Introduction to Process Control Instruments Temperature The fixed points for both scales are the temperature at which ice melts and water boils at standard pressure. A temperature in Fahrenheit can easily be changed to Celsius and vice versa. The conversion equations depend on the number of divisions in each scale. Fahrenheit has 180 divisions between the freezing and boiling points of water but Celsius has only 100 divisions. Therefore, the ratio is 180/100 or 9:5. This gives:0 C = 5/9 (0F 32)or 0F = 9/5 0C + 32 There are tables available for Fahrenheit Celsius conversions. 65 66. Pressure MeasurementIntroductionThe object of this unit is to describe the commondevices used to measure pressure. 66 67. Pressure Measurement67 68. Pressure Measurement68 69. Pressure MeasurementThe Bourdon Tube Pressure GaugeThe Bourdon tube gauge is the most common pressureindicator in the petrochemical industry. It shows thepressure in a clear, simple way. 70. Pressure MeasurementThe Bourdon Tube Pressure GaugeThe previous diagram showed a typical Bourdon gauge.It consists of the following parts: The Bourdon tube itself. This is a metal tube shaped like a C. It has an oval cross sectional area. It is sealed at one end. The sealed end is free to move. A linkage and pinion to turn the pointer. A scale to indicate the pressure.70 71. Pressure Measurement The Operation of a Bourdon Tube Pressure Gauge When a pressure is applied to the inside of the tube it will try to straighten. The closed end (the tip) will move and the linkage moves the pinion which moves the pointer. The movement of the pointer shows how much pressure is applied to the Bourdon tube. The Bourdon gauges come in all shapes and sizes and can measure from about 0-15 psig (0-1 bar) to 0-10,000 psig (0-700 bar) depending on the stiffness of the material used.71 72. Pressure MeasurementThe Bourdon Tube Pressure GaugeThere are also other types of Bourdon tubes: Spiral Bourdon tubes Helical Bourdon tubesThese perform the same function as the simple C typeBourdon tube except that they provide more movementand are more accurate.72 73. Pressure Measurement Bourdon Tube Pressure Gauge - Spiral Bourdon Tube This diagram shows a spiral Bourdon tube. It is used to indicate low pressures. When pressure is applied the spiral unwinds and the free end moves to indicate the pressure. 74. Pressure Measurement Bourdon Tube Pressure Gauge - Helical Bourdon Tube This diagram shows a Helical Bourdon tube. This is usually used to indicate high pressures. When pressure is applied the helix unwinds and the free end moves to indicate the pressure applied. 75. Pressure Measurement A helix coil is used for low pressure applications. They expand to a greater degree than the Bourdon tube75 76. Pressure MeasurementBellowsBellows are tubes with thin wallsmade of brass, stainless steel,etc. The thin walls arecorrugated. This improves theirability to expand and contract.When pressure is applied (eitherto the outside or the inside), thecorrugated walls expand orcontract. This movement is usedto indicate pressure. Bellowsunits are used in various ways.These are the three mostcommon methods 77. Pressure Measurement Diaphragms A diaphragm is a stiff corrugated disc which is flexible under pressure. A single diaphragm is often used as a seal to protect a gauge from corrosive liquids. A typical example is given in the illustration. 78. Pressure Measurement Diaphragms Diaphragms are also used to make high pressure bellows (a diaphragm stack). A typical example is shown 79. Pressure Measurement Capsules Capsules are made of two diaphragms welded onto a metal ring and filled with a fluid. Different mechanical and electrical methods are used to show the differential pressure across the capsule. The diagram shows a capsule used in a pneumatic differential pressure transmitter. 80. Pressure MeasurementThe Strain GaugeThe strain gauge is a resistor which has been deposited into aflexible bar. As the bar is bent the resistor will change in lengthand thus its resistance. The changes in resistance are detectedand electronically changed to a pressure signal. The method isused in electrical transmitters. 81. Pressure Measurement Vibrating (Resonant) Wire The vibrating wire is the operating method used in some pressure transmitters. The diagram shows the basic construction. 82. Pressure MeasurementVibrating (Resonant) Wire - OperationThe frequency of vibration of a wire depends on itstension. The tension of the vibrating wire is changing bythe pressure applied to the diaphragm. The electronicsunit vibrates the wire and measures the change invibration frequency caused by pressure moving thediaphragm.The electronics unit changes the pressure applied to thediaphragm into an electrical output signal.82 83. Pressure Measurement Electrical Pressure Sensing Methods The old mechanical methods of detecting pressure are slowly being replaced by electrical methods. Electrical methods are more accurate and cheaper. The following gives a simple explanation of the principle involved. The Piezo Electric Effect Certain crystals, such as quartz, produce a voltage across them when a pressure is applied. This voltage is simplified electronically and displayed digitally on a multimeter.83 84. Pressure MeasurementDifferential PressureA differential pressure is the difference in pressurebetween two measuring pointsThe differential pressure is used in a process tomeasure the pressure drop across a resistance to a flowThis resistance could be an orifice of a known size andthe pressure differential can be used to calculate a flowrate this principle is used in an orifice plate.A pressure differential across a filter is used todetermine the fouling across the filter and is used toknow when to change an element 84 85. Differential Pressure Not all Differential Pressure Indicators have a dial face. This is the Differential Pressure Indicator across the diesel filter in the Kutubu Refinery. The pressure is read on a linear scale. 85 86. Pressure MeasurementPressure SwitchPressure switches are devices that open or closeelectrical circuits when they sense a pre-set pressure.The electrical circuits can then be used to open orclose valves to relieve pressure in a system.The switches can be used to switch on pumps orcompressors to maintain pressure in a system. 86 87. Pressure Measurement87 88. Pressure Measurement88 89. Pressure Measurement89 90. Level Measurement Introduction This unit will describe the common methods and devices used to measure liquid levels in process equipment.90 91. Level MeasurementTypes of Level Measuring DevicesThere are two main types of level measuring devices. Direct level measuring devices. Indirect level measuring devices91 92. Level Measurement Direct Level Measuring Devices Direct methods allow the operator to actually see the liquid level or to take a direct measurement of the levels of liquid in a vessel. You can see how much liquid you have in your windscreen washer tank by looking at the level through the wall of the tank. You can see how much acid you have in your car battery by looking at the level through the wall of the battery. You can measure the level of oil in your car engine by looking at the dipstick. You physically measure the oil level. All of the above are direct level measuring devices. 92 93. Level MeasurementIndirect Level Measuring DevicesYou cannot measure or see how much petrol you havein the tank of your car.An instrument measures the level and shows you howmuch petrol there is on a indicator on the dashboard (thepetrol gauge).This is an example of an indirect level measuringdevice.93 94. Level MeasurementDirect Level Measuring Devices - The Dip StickThe Dip Stick is the only true measurement of level.It is still used by operators and ships captains to check thatthe instrumentation which measures the level of a liquid in atank is correct. 95. Level MeasurementDirect Level Measuring Devices - The Dip StickThe Dip Stick is a long calibrated ruler.The depth of the liquid in the tank is indicated by a WET markwhen the stick is removed. It is the same principle as checkingthe oil level of a car.Because there may be rubbish at the bottom of the tank thelevel may be taken from a bottom level datum line.A datum line is a base line from which things can bemeasured.There is also a top datum line which is used to measure thespace between the liquid and the top of the tank. 95 96. Direct Level Measurement The Dip Tape The Dip tape (see Figure 3.2) is a development of the dip stick. It is used to find the level in large tanks. The tape is calibrated like the dip stick. The tape is run out until the weight touches the bottom of the tank. It is then pulled up. The wet mark of the tape indicates the level of the liquid. By using a special water finding paste on the bottom of the tape you can detect the level of water that could be below the oil in the tank.96 97. Level MeasurementDirect Level Measuring Devices - The Dip Tape HANDLEWINDER 98. Level MeasurementThe Sight GlassThis is the indicator used byoperators in the plant to seeinside of a vessel.The sight glass is connectedto the side of a vessel andthe level is seen by lookingthrough the glass.A high pressure sight glassis illustrated 99. Indirect Level MeasurementLevel Transmitter BridleVentTransmitter /Control BoxvesselDisplacerLiquid levelLevel ColumnDrain 99 100. Level MeasurementSight GlassesBAMagnetic Sight-glasses. A on the Inlet Separator, CPF and B is on aLiquid KO pot on Gobe South Compressors100 101. Level MeasurementTypical Level Control system in a process area Level Transmitter Level Switches Level ColumnBridle Sight Glass 101 102. Level MeasurementIndirect Level Measuring DevicesIndirect methods product mechanical or electrical outputsignals which indicate changes in level.Simple FloatsFigure 3.4 shows a simple float level indicator. It is stillused by water departments and on chemical tanks onolder oil platforms.It is cheap to install and easy to operate.102 103. Level Measurement Indirect Level Measuring Devices Simple Floats Operation The float and counter weight are connected together by a wire on pulleys. The system is in balance with the float on the surface of the liquid. If the level rises, the float rises and the counter weight falls to the new balance point. If the level falls the counter weight rises. 103 104. Level Measurement The counter weight has a pointer which indicates the level scale on the outside of the tank. This scale is the reverse to FLOAT normal. The pointer shows full when the counter weight is at the bottom of the scale and empty when it is at the top. The scale can be made very large so that it can be seen from the ground by the operator.104 105. Level MeasurementIndirect Level Measuring DevicesSimple Float Operation (cont)The simple float is not very accurate and can be verydifficult to read.If the surface of the liquid is moving then the float startsto swing.This problem is solved by fitting special devices insidethe tank as shown in the following slide. 105 106. Level MeasurementIndirect Level Measuring DevicesSimple FloatsOperationA is a guided wiresystem. CB is a Still Pipe Asystem where thefloat is in a slottedpipe and connectswith the groundlevel displayC is another StillBPipe but the floatconnects with atransmitter whichsends a signal tothe control room 107. Level MeasurementIndirect Level Measuring DevicesSimple Float Operation - Guide Wire System (Figure A)This is the cheapest system. The float is held in place bywires which are are fixed to the bottom by a concrete block.The wires are kept tight by a spring.The float is connected by a wire. The wire runs through apulley system and through a pipe to the indicating unit Thepipe is supported on brackets fixed to the tank. Theindicating unit is the counterweight and the level is indicatedby a mechanical counter.107 108. Level MeasurementIndirect Level Measuring DevicesSimple Floats Operation - Still Pipe System (B and C) This is a more expensive but more accurate method. The float is contained inside a still pipe (a steel pipe withholes in it). The level inside the pipe does not move so itgives very accurate measurements of level. Figure B shows the older mechanical indication method. Figure C shows the modern method where the system iselectronically controlled and the level measurement is sentas an electronic signal to the control room.108 109. Level MeasurementIndirect Level Measuring DevicesHydrostatic Tank Gauging(HTG) Pressure indicatorMany of the modern oilstorage tank facilities (tankfarms) use hydrostatic tankgauging to indicate the level Transmitterin a tank.HTG is good because there isno equipment inside the tank.A Hydrostatic Tank Gauginginstallation on the Skim Tank atIt is cheaper to install andthe CPFmaintain than the floatinstallations. 110. Level Measurement110 111. Level MeasurementIndirect Level Measuring DevicesHydrostatic Tank Gauging (HTG)The higher the level of a liquid in a tank, the higher thepressure on the bottom of the tank.An outlet near the bottom of the tank is under morepressure than an outlet near the top of the tank.The greater the pressure the further the outflowstream will reach. 111 112. Level MeasurementIndirect Level Measuring DevicesHydrostatic Tank Gauging (HTG)The pressure on the bottom of the tank only dependson the level of the liquid in the tank; not the volume or theshape of the tank.No matter what the shape of the tank, the pressure atthe bottom of the tank is the same.Using this principle, a pressure sensor at the bottom ofthe tank can gauge the level of the liquid in the tank.The higher the pressure, the higher the level of theliquid in the tank 112 113. Level MeasurementIndirect Level Measuring DevicesDisplacers and Local Level ControlThe displacer is a locally mounted device whichcontrols the level in a vessel.It is used on remote sites where it is too expensive toreturn signals to the control room.The most common types in use are manufactured byFisher or Masoneilan.The diagram Figure 3.7 shows a Fisher device (TheLevel-Trol). 113 114. Level Measurement114 115. Level Measurement Indirect Level Measuring DevicesDisplacers and LocalConnecting RodLevel ControlThe Displacer unit isconnected to both the Torque Tubevessel and the control valve.Displacer 116. Level MeasurementDisplacers and Local Level ControlOperation The weight of the displacer changes as the level of theliquid rises or falls in the displacer housing. The displacer hangs on the torque tube via theconnecting rod. The changing weight of the displacermakes the torque tube twist or untwist. The twisting motion of the torque tube moves a flapperagainst a nozzle. This sends a control signal to thepneumatic control valve. The pneumatic control valve opens and closes to controlthe flow of liquid into the tank. This keeps the level ofliquid in the tank constant at the set point. 116 117. Level MeasurementDisplacers and Local Level ControlThis forms a self contained local control loop as shown in thefigure below 118. Level MeasurementAir Bubble MethodThe Air Bubble method is one of the oldest and simplestmeans used to indicate level and/or transmit a signal. Thediagram shows a simplified layout of the method. 119. Level Measurement119 120. Level Measurement Air Bubble Method - Operation An inert gas (air or nitrogen) is passed down the bubbler tube. There is just enough gas pressure to push the bubbles out of the bottom of the tube when the liquid is at the maximum level in the vessel. When the vessel is full the pressure gauge or transmitter will read a maximum back pressure. This back pressure is equal to the hydrostatic head (H), (the pressure of the liquid above the zero level). At zero level there will be no back pressure so the gauge or transmitter will read zero. No back pressure means the liquid level is at zero; the tank is nearly empty.120 121. Level MeasurementAir Bubble Method - Operation (cont) The back pressure between zero and maximum levels isproportional to the liquid level in the vessel. Thepressure gauge or transmitter can be calibrated toindicate the liquid level. The gas pressure is adjusted by the regulator to give asteady flow of gas down the bubbler tube. The gas flowis indicated on the Rotamater. This is a very accurate method of showing liquid levelusing modern instrument systems. 121 122. Level MeasurementLevel SwitchesA level switch is the last safety device when controlling level.If the level controller stops working the vessel can overfill. Thiscan be dangerous.A level switch uses a float to operate a switch to shut downfilling pumps in an emergency. A typical example is shown. 123. Level Measurement Level Switches Figure 3.10 shows a pneumatic level switch. When the level of liquid is low the float hangs down. The operating screw on the end of the flexible shaft holds the flapper tight against the nozzle. The output signal is a maximum so the pumps continue to fill the vessel. If the level rises and lifts the float the screw on the end of the flexible shaft moves down. The flapper moves away form the nozzle and the output signal falls to zero. This shuts down the pumps so no more liquid comes into the vessel. 123 124. Level MeasurementOther Methods of Level MeasurementThis unit has introduced some common methods of measuringlevels used on most installations.There are many other methods using various types of hightechnology.These will be special for only one or two installations.They will have to be learnt on the job. A few examples are: a) Radar, ultrasonic, gamma and infrared detectors b) Capacitive sensors c) Resistive sensors 124 125. Temperature Measurement Introduction This unit will describe the common methods and devices used to measure temperature. It will also describe when and where these devices are used and how they are protected.125 126. Temperature MeasurementFilled Thermal ElementsThermal filled elements operate by the expansion and contraction offluids or vapours in a closed tube.The simplest of these devices is the mercury filled thermometer. Liquid in glass thermometers are not strong enough for use on the plant. Stronger systems have to be used. 127. Temperature MeasurementFilled Systems One common kind of temperature measuring device used in industry is the filled system. However, it is not made of glass like a hospital thermometer. These systems use steel bulbs and stems. The stem has a bourdon tube at the end. The liquid or gas in the device expands and contracts as the temperature changes. The expansion and contraction of the fluid in the system is changed to pressure. An increase in pressure expands the bourdon tube which moves the pointer to the scale. 127 128. Temperature MeasurementFilled Systems The liquid filled system is normally used in process plant applications. 129. Temperature MeasurementBi-Metal Strip ThermometersLiquid and gas filled systems use the expansion of fluids tomeasure temperature. Some temperature measuringdevices use the expansion of solids to measuretemperature. One kind of solid expansion thermometer isthe bi-metal strip illustrated in the diagramCold Hot 130. Temperature MeasurementBi-Metal Strip ThermometersTwo strips of metal, brass and invar, are tightly bondedtogether and fixed at one end. When the strip is heated thebrass expands much more than the invar and the strip bendsas shown. This action is used to make a dial thermometer asshown. The most common type is the Rototherm. 131. Temperature MeasurementBi-Metal Strip Thermometers - OperationThe bi-metal strip is shaped into helix.The helix is fixed at one end.The other end of the helix is free to rotate the shaftwhich is fixed to it.The heat applied to the bi-metal strip at the fixed endcauses the helix to unwind and turn the pointer on thescale.131 132. Temperature Measurement Thermocouple When two different metals are welded together at their ends a junction is formed. This is called a thermocouple. If this junction is heated a small electrical emf (electromotive force) is produced that causes a current to flow. This current can be measured by attaching a meter to the free ends of the metal strips as seen in figure 4.5 (next slide). The strength of the current can be used to show changed in temperature on the thermocouple. 132 133. Temperature Measurement Thermocouple 134. Temperature MeasurementTemperature Measurement Devices The thermocouple is used to sense the process variable and transmit the signal to the controller electrically.Filled thermal bulband capillary tubingResistance bulb Thermocouple and protective well134 135. Temperature Measurement Radiation Temperature Detectors (Pyrometers) Temperature measuring devices such as a bi-metallic strip or a thermometer must be in contact with the substance or thing which they are measuring. Radiation temperature detectors (pyrometers) are non-contact devices. They are used to measure the temperature of something which is difficult to reach, eg gas turbine combustion chambers. They are also used to measure very high temperatures (above 15000C). All the other devices would melt at these temperatures. 135 136. Temperature MeasurementRadiation Temperature Detectors (Pyrometers)The heat from the objects is focused by lenses onto a sensor.Its the same as when you use a magnifying glass to focus theheat from the sun in order to start a fire. The output from thesensor is electronically processed by the amplifier to give areading in degrees. This device can also transmit a signal to thecontrol room if required. 137. Temperature MeasurementResistance Temperature Detector (RTD)The device indicates temperature by measuring thechange in the electrical resistance of a metal.When metals get hotter their resistance increases.This increase in resistance is almost linear. In otherwords, the resistance increases at the same rate asthe temperature.When the resistance is measured it gives anaccurate indication of temperature.There are other temperature sensors in use butthese are of more interest to instrument technicians. 137 138. Temperature MeasurementResistance Temperature Detector (RTD) 139. Temperature Measurement139 140. Temperature Measurement140 141. Temperature MeasurementThermowellsThe thermowell is a devicefitted into a flow line so thatThermocouplethe temperature of a fluid canbe measured without shuttingdown the process.A thermowell is placed in aflow line when the line isbuilt.The thermometer orthermocouple is fitted into thethermowell. Thermowell 142. Temperature MeasurementThermowellsMost vessels and pipes in process and productionplants contain liquids or gases under pressure.The thermowell protects the temperature sensor fromdamage from pressure and also from fluid flow.The heat in a fluid takes longer to transfer through athermowell, so changes in temperature take longer toshow.Different methods are used to speed up heat transfer.Sometimes the space between the probe and thethermowell is filled with a liquid which conducts heat well.142 143. Temperature MeasurementThermowellsSometimes the probe is placed in a corrugatedaluminum cover to give a direct metal contact between theprobe and the thermowell.When a thermowell is filled with heat conducting liquid asmall amount of air has to be left as a gas cap at the topof the well to allow for thermal expansion of the liquid.As the conducting liquid expands with the increase intemperature the liquid compresses the gas cap.This prevents the pressure generated by the expansionfrom damaging the instruments in the well. 143 144. Flow Measurements Introduction This unit will describe how the flow in a process is used to control the other process variables. It will also describe how flow is measured. 144 145. Flow Measurement Flow Measurement Flow measurement means measuring how much material moves past a given point in a given time. For example, the petrol in the pump at the service station flows at about 20 litres per minute. Therefore the rate of flow is 20 litres per minute. In a process system its very important to know the rate of flow through different process equipment.145 146. Flow MeasurementThe rate of flow affects how well the process works.When we know how much is flowing we can decideif it is too much or too little.We can then change the flow to what we want it tobe, in other words set it at the desired value.146 147. Flow Measurements Flow Measurement The flow must be controlled to Control the Process We use flow control to control other variables in a Process such as: Pressure Temperature Level 147 148. Flow MeasurementsRate of Flow MeasurementDevices for measuring the rate of flow do not have tobe very accurate.It is the change in the rate of flow that is important toa Technician.Flow measurement devices are often used to give aflow signal directly to a controller. In this case they arecalled Flow Indicator Controllers (FICs)If they sent a signal to the Control Room or a locallymounted recorder they would be called Flow RecorderControllers (FRCs)148 149. Flow MeasurementsRate of Flow Measurement - Flow BasicsIt is pressure that pushes fluids through a pipe.For a flow to occur there must be a pressure drop (decreasein pressure) between the ends of the pipe.The downstream pressure is less than the upstream pressuretherefore the direction of the flow is from upstream (highpressure) to downstream (low pressure).Upstream means where the fluid is coming from.Downstream means where the fluid is going to.149 150. Flow MeasurementFlow BasicsThe flow is produced by the difference in pressureacross the ends of the pipe.If there is a big difference in pressure then the rate offlow will be fast.If there is a small difference in pressure than the rateof flow will be slow.The difference in pressure is called the Differentialpressure.The term Differential Pressure is a commonexpression and one that you need to understand 150 151. Flow Measurements Flow Basics Friction and Types of Flow The walls of pipes are not perfectly smooth. The frictional force at the walls will cause the fluid to go slower at the edge than at the centre. There are two types of flow: Laminar Flow Turbulent Flow 151 152. Flow MeasurementsRate of Flow Measurement - Flow Basics 153. Flow Measurements Flow Basics Friction and Types of Flow Laminar Flow Laminar flow occurs when the fluid flow rate is slow. The velocity (speed) of the fluid through the pipe is much higher in the centre of the pipe than at the edges. The fluid next to the walls of the pipe flows more slowly because the fluid is rubbing against the pipe. The liquid is slowed down by friction. 153 154. Flow MeasurementsFlow Basics Friction and Types of FlowTurbulent FlowTurbulent flow occurs when the fluid flow rate is high.The velocity of the fluid through the pipe is nearly thesame across the pipe.The flow is a little slower at the edges because of thefriction between the fluid and the wall of the pipe.154 155. Flow MeasurementsFlow Basics Calculating the Rate of FlowThere must be a differential pressure across the endsof the pipe in order for fluid to flow.If the differential pressure and the size of the pipe isknown, we can calculate how fast the fluid is flowingthrough the pipe (the rate of flow).The calculation is complicated. An easier method hadto be found to calculate the flow through the pipe.The easier method is to put a restriction in the pipe. Arestriction is something which blocks part of the flow.155 156. Flow MeasurementFlow Basics Calculating the Rate of FlowThere are three main devices used to make restrictionsin a pipe: Orifice Plate Venturi Tube Flow NozzleThe restriction produces a differential pressure across it.In other words the pressure downstream of the restrictionis lower than the pressure upstream of the restriction. 156 157. Flow MeasurementFlow Basics Calculating the Rate of FlowThe pressure difference is due to the increase invelocity as the process fluid flows through the restriction.When the velocity of the flow increases, the pressure atthat point in the line decreases.By knowing the differential pressure, the internaldiameter of the pipe and the size of the hole inrestriction, we can calculate the rate of flow. The instruments do the calculation for us. 157 158. Flow Measurement 158 159. Flow measurement 159 160. Flow MeasurementOrifice Plate RestrictionsThe illustration shows a sideview of an orifice plate fittedinto a pipe.The pressure downstream ofthe orifice is lower than thepressure upstream. TheInstrument measures thisdifferential pressure.The measurement can be usedto calculate the rate of flow atthat point in the pipe. 161. Flow Measurement Orifice Plate Restrictions All orifice plates are marked with the orifice size. The side of the plate which goes upstream (inlet) is also clearly marked. 162. Flow Measurement Differential Pressure Cell The upstream side of the plate is at higher pressure than the flow on the downstream side of the plate. The difference between the two pressures is called the Differential Pressure or the DP. The Orifice plate is held between two Orifice flanges learn toDPC - Differential Pressure Cell recognize these 162 163. Flow MeasurementOrifice plates installed at the Agogo/Moran facility.This type of orifice plate can be removed while the line is in use. A normalorifice plate requires that the process be shut down as the line is separatedwhen the plate is removed 163 164. Flow MeasurementsVenturi Tube RestrictionsAnother type of restriction device is the venturitube.If the fluid in a pipe is flowing under very lowpressure the restriction by an orifice plate could stopthe flow.In these cases a venturi tube is used.These devices are very expensive.164 165. Flow Measurement Venturi Tube Restrictions A venturi tube works on the same principle as an orifice plate. Instruments measure the pressure differential across the restriction. However, the shape of the venturi tube allows the fluid to pass through it easily. 166. Flow Measurement Flow Nozzle Restrictions A third type of restriction device is the flow nozzle. The flow nozzle is a combination of the orifice plate and the venturi tube. The pressure loss across the nozzle is more than across the venturi, but it is less than across an orifice plate. The flow nozzle is less expensive than a venturi, but more expensive than an orifice plate. 166 167. Flow MeasurementFlow Nozzle RestrictionsFlow nozzles are good for liquids with high flow rates.Orifice plates are usually used for gases with high flow rates. 168. Flow MeasurementFlow Straighteners (Straightening Vanes)All flow measuring devices which use a restriction needa stream-lined flow.Flow measuring devices must not be placed in pipesnear things that disturb the flow; elbows, valves, etc.If this is not possible then the flow has to be stream-lined (made to flow smoothly).The flow is stream-lined with flow straighteners. 168 169. Flow MeasurementsFlow Straighteners (Straightening Vanes)A flow straightener is a cylinder filled with many smallpipes. This device is place in the pipeline upstream ofthe flow measuring device. It causes the fluid to flowsmoothly and evenly which means the measuring devicecan get a more accurate measurement. 170. Flow MeasurementCalibration of Differential DevicesCalibrating a differential-pressure, flow measuringdevice is a skilled job. An instrument technician will usefigures given by the design engineer to do this.For control purposes the actual measurement of flowneed not be exact.Its the changes in the rate of flow which areimportant. 170 171. Flow Measurements Variable Area Meters These are simple devices used to indicate small rates of flow. They are used by a Technician in the field.Typical uses are: In seal-oil and lubrication-oil flowlines on large rotating machinese.g. diesel engines and gasCcompressors. B In cooling water lines formachines and processes.AAA The diagram shows a variablearea meter or Rotameter. 172. Flow MeasurementsVariable Area Meters - OperationThe Rotameter is fitted vertically into the flow line. Theflow of the fluid is from the bottom to the top of the cylinder.The cylinder is bigger at the top than at the bottom.When there is no flow, the float is at the bottom of thecylinder (position A)When the flow increases, the increased pressure makesthe float rise.It will rise to a position where the flow pressure on thefloat equals the weight of the float, (position B). 172 173. Flow MeasurementsVariable Area Meters - OperationIf the flow gets faster there is more pressure on the floatand it will rise higher (position C).The flow rate indicated depends on the size of thedevice. It is pre-calibrated by the manufacturerThe Technician reads the flow rate from the transparentscale using the top of the float as a marker.173 174. Flow MeasurementPositive Displacement MetersPositive displacement flow-measurement meters arevery accurate. They are also called quantity meters.Basic PrincipleThe meter traps a known fixed volume of fluid andtransfers it from the inlet to the outlet.The number of fixed volumes of fluid transferred (ormoved) is a measure of flow.174 175. Flow MeasurementPositive Displacement MeterLook at the drawingThe bucket holds 12 litres.The man moves 10 bucketsof water from the inlet tank tothe outlet tank in one minute.Therefore, we can say therate of flow is 10 x 12 litrebuckets a minute or 120 litresper minute.175 176. Flow MeasurementsPositive Displacement Meters - Basic PrincipleA positive displacement metre works on the sameprinciple as the man with the bucket. However, a positivedisplacement meter is much faster and more reliable thana man with a bucket.Flow meters that use thisbasic principle are:Reciprocating piston metersRotating vane metersLobed impeller meters 177. Flow MeasurementsPositive Displacement MetersReciprocating Piston MeterEach time the piston moves up and down in the cylinder afixed amount of fluid is pushed out of the outlet.The valves are arranged to work in time with the piston soone side of the cylinder is filled as the other side is emptied. 178. Flow Measurements Positive Displacement Meters - Rotating Vane Meter The rotating vane meter is another type of positive displacement meter. Each time a vane moves past the outlet it pushes out a measured volume of liquid, e.g. 2 deciliters. This type of meter is used on petrol pumps at service stations. The meter counts how many times the vanes go around on the cam and you pay for this amount of petrol.178 179. Flow MeasurementsPositive Displacement MetersRotating Vane Meter 180. Flow MeasurementsPositive Displacement Meters - Lobed ImpellerAnother kind of positive displacement meter is a lobedimpeller meter. Each rotation of the impeller pushes ameasured quantity of fluid through the meter. 181. Flow MeasurementsVelocity Meters (Semi-Positive Displacement)The velocity meter measures the speed of flow. then calculates the volume of flow using calibration figures. ItThe calibration figures are placed in the electronics units memory by the operator or instrument technician.These calibration figures depend on the type of liquid flowing through the meter.181 182. Flow MeasurementsVelocity Meters(Semi-Positive Displacement)The magnet rotates with the rotor.The pickup coil gets a signal from themagnet (pulse) each time the rotorcompletes a rotation.The number of pulses is counted by anelectronics unit.The electronic unit then displays thetotal quantity of flow.Note: If the type of fluid flowingthrough this meter changes, then thecalibration figures in the meterselectronic memory have to bechanged. 183. Flow MeasurementVortex Meters Vortex Meters are used on fuel gas systems such as gas from the Test Separator183 184. Flow Measurement Micro-motion Flow Transmitter A micro-motion meter measures a flow by sensing the vibrations between two parallel loops that the flow causes. The higher the flow the greater the vibration and distortion between the loops. This type of meter is very accurate and reliable184 185. Flow Measurement 185 186. Increased Density Density changes Vibration actionDecreased DensityIncreasedIncreased DensityDensity186 187. Flow Measurement Turbine MeterTurbine meters areused extensively ina process facility.This one is at theCPF Export PumpStation. Others areat the valve stationsalong the exportpipeline and at theMarine Terminal -Kumul 187 188. Flow MeasurementThe following video clips have been included to let theparticipant gain a knowledge of the components of aTurbine meter.Although the video is aimed at the maintenance of theequipment the information is applicable to a ProcessTechnician as wellThe rotating element is much smaller than you wouldexpect.Even in large meters such as those that are used on thepipeline an appreciation can be gained of how easily theblades of the rotor could become damaged and worn.188 189. Flow Measurement 189 190. Flow Measurement 190 191. Flow Measurement 191 192. Flow Measurement 192 193. InstrumentationThis completes this first package on InstrumentationThe second package deals with the Control Valves andthe ancillary equipment which complements them.Sophisticated control systems such as PLCs andTECs are also dealt with in that package.For comments and suggestions contact Len Dallow or Peter Cannell onldallow@picknowl.com.au or petercannell1943@bigpond.com193