Process Technician Training Process Control InstrumentsPackage number 1 1
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 sequence 2
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 PLC’s and DCS’s 3
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 the“Slide Show” drop-down menu“Slide Show” can be selected from the Toolbar at thetop of the page. “View Show” can be selected from themenu. 4
Units in this CourseFirst Package Unit 1 Introduction Unit 2 Pressure Measurement Unit 3 Level Measurement Unit 4 Temperature Measurement Unit 5 Flow MeasurementsSecond Package Unit 6 Ancillary Control Equipment Unit 7 Controllers Unit 8 Control Loops Unit 9 Overview of: - Distributive Control Systems (DCS) - Programmable Logic Controllers (PLC) 5
Process Control InstrumentationFirst Package Unit 1 Introduction Unit 2 Pressure Measurement Unit 3 Level Measurement Unit 4 Temperature Measurement Unit 5 Flow Measurements 6
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 in 7
Introduction to Process Control Instruments Instrumentation plays an important part in the efficient operation of any processing or production plant. Instruments enable the plant to operate smoothly and safely with a minimum of operating staff. This course will give a basic understanding of how the instruments function. 8
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
Introduction to Process Control Instruments It is important that the process conditions are accurately controlled at all times. The controlling is done by instruments. An instrument cannot ‘think’. It can only respond to parameters that are set by the Process Department It is the Technicians that tell the instruments what to do. 10
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Introduction to Process Control Instruments The Technician who knows the instruments is the master. The instruments are his slaves or servants. When the instruments are properly used, the process equipment operates correctly. When the process equipment operates correctly the finished products are up to specification. When the process equipment is running steadily the company is making money. An upset unit is costing money 13
Introduction to Process Control InstrumentsProcess ControlA simple example of a Manual process control. TechnicianThe process is temperature control. The indicator is athermometer
Introduction to Process Control Instruments Process Control The correcting unit is the gas control valve. The controller is the Technician who uses his own judgment to keep the water temperature constant. Manual control has its uses: It is cheap to install and maintain It is simple to operate. 15
Introduction to Process Control Instruments Such a control system is seldom used in industry because: The Technician must remain in position at all times. It cannot be used if the Technician is placed in a dangerous area. The process may change faster than the Technician can react. A mistake by the Technician can have dangerous results. 16
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
Introduction to Process Control Instruments This is a simple automatic controller. The boiler now has the loop closed and no Technician is required.
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
Introduction to Process Control Instruments The controller adjusts the correcting unit (automatic control valve) using an output signal line similar to the input line from the transmitter. The controller may provide alarm signals to alert the Technician if the system fails. It may also shut off the gas if the water starts to boil. 20
Introduction to Process Control Instruments A group 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 Other systems can be quite complex and involve a wide range of instruments that all ‘talk’ to one another to control a system A good example of this is the level control system on the CPF Inlet Separators - these will be dealt with later. 21
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Introduction to Process Control Instruments Control Loop: A Control Loop is an active system that keeps a process variable within maximum and minimum values by continuous measurement and continuous corrective actions. A level control loop controlling the level in a vessel by restricting the pump discharge 23
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 effect 24
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Introduction to Process Control Instruments The previous video clips were simplified and described a very basic system – the control of steam being used to heat water. Some of the terms used in the video are terms in common use in this type of industry The following is a list of a few of these terms and their meanings 31
Introduction to Process Control InstrumentsBasic Definitions Instrument Any device for measuring, indicating, controlling, recording and adjusting a physical or 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 Indicator A device which shows a measured value to the operator Lag The time taken between an adjustment being made and the process responding to it 32
Introduction to Process Control InstrumentsBasic DefinitionsRecorder A device which continuously records measurements, either electronically or on an ink chart. It is used to show production figures, etc.Process Loop A group of instruments used to control a single process variable e.g. pressure, flow, level, etc.Process The word for a manufacturing unit e.g. refining, liquefying gas, etc.Measured The value of the property being controlled by aVariable or single process loop e.g. pressure, flow, level,Process etc.Variable (MV) 33
Introduction to Process Control InstrumentsBasic DefinitionsDesired Value The 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 Control controller. It is used to adjust the measuredElement) value to obtain a zero error signal e,g, control valve etc. 34
Introduction to Process Control InstrumentsBasic Definitions Transmission A method of standardising signals sent from various parts of the plant. Transmitter A 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. Disturbance A change in the process that is not anticipated Load A planned change in throughput of a unit Change 35
Introduction to Process Control Instruments Note: The instruments use in a processing facility can vary greatly, depending on the age of the installation. They may be air (pneumatic), liquid (hydraulic) or electric/electronic in operation. The way the information is shown or recorded may be simple, like a clock or thermometer In other cases it may be by the latest information technology displaying the information on a personal computer screen (video display unit). 36
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
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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
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
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
Introduction to Process Control Instruments Pressure Units There is no agreed standard for pressure measurement in the petrochemical industry. Some companies use Imperial Units (USA), some use International Standard Metric Units (ISO) Some use both. The Technician needs to understand both systems and be able to convert from one to another. 42
Introduction to Process Control InstrumentsPressure Units Imperial ISO Force Pound Newton Area Square Inch Square Metre Pressure Pounds per square inch Newtons per square called PSI metre called Pascal 43
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
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
Introduction to Process Control Instruments Pressure Absolute, Gauge and Atmospheric Pressure The price of oil or gas depends on the quantity (mass) of the product. The quantity of oil or gas in a given volume depends on the pressure. For this measurement, absolute pressure must be used. Absolute Pressure This is the pressure above a total vacuum (there are no particles of matter in a total vacuum). 46
Introduction to Process Control Instruments Pressure Gauge Pressure This is the pressure measured by a gauge. Gauge pressure is the pressure above that of the surrounding atmosphere. Atmospheric Pressure The pressure of the air all around you. This is not constant, it depends on things like the weather and the altitude of the plant. 47
Introduction to Process Control Instruments Pressure The equation linking the above pressures together is Absolute Pressure = Gauge Pressure + Atmospheric Pressure. Because atmospheric pressure can vary, a standard atmospheric pressure of 1.013 Bar or 14.70 psi. is used Gauge pressure is written as psig. Absolute pressure is written as psia. 48
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 psi 49
Introduction to Process Control InstrumentsUnits of Flow This simplified diagram shows a tanker being loaded from a storage tank. The amount of oil loaded must be accurately measured to know how much it costs. The total flow (quantity) of oil into the tanker can be measured in two ways. By volume, in barrels or cubic metres. By mass, in metric or imperial tonnes.
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 / Hour Cubic Feet / Minute Cubic Metres / Second Rate of flow by Mass Tonnes / Hour Kilograms / Second Pounds / Minute 51
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
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 tonne Note: It is not necessary to memorise conversions. Conversion tables will be available at your facility. 53
Introduction to Process Control Instruments Units of Flow VELOCITY ft/sec ft/min metre/sec metre/min 1 60 0.3048 18.29 0.01667 1 0.005080 0.3048 0.03281 1.9685 0.01 0.600 3.281 196.85 1 60 0.547 3.281 0.01667 1 54
Introduction to Process Control InstrumentsUnits of Flow ABBREVIATIONS 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
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Introduction to Process Control InstrumentsTemperatureThere are different scales for measuring temperatures. The diagramcompares the two common temperature scales; Fahrenheit (Imperial) andCelsius (ISO).
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
Pressure Measurement Introduction The object of this unit is to describe the common devices used to measure pressure. 66
Pressure MeasurementThe Bourdon Tube Pressure Gauge The Bourdon tube gauge is the most common pressure indicator in the petrochemical industry. It shows the pressure in a clear, simple way.
Pressure Measurement The Bourdon Tube Pressure Gauge The 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
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
Pressure Measurement The Bourdon Tube Pressure Gauge There are also other types of Bourdon tubes: Spiral Bourdon tubes Helical Bourdon tubes These perform the same function as the simple ‘C’ type Bourdon tube except that they provide more movement and are more accurate. 72
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.
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.
Pressure Measurement A helix coil is used for low pressure applications. They expand to a greater degree than the Bourdon tube 75
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
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.
Pressure Measurement Diaphragms Diaphragms are also used to make high pressure bellows (a diaphragm stack). A typical example is shown
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.
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.
Pressure Measurement Vibrating (Resonant) Wire The vibrating wire is the operating method used in some pressure transmitters. The diagram shows the basic construction.
Pressure Measurement Vibrating (Resonant) Wire - Operation The frequency of vibration of a wire depends on its tension. The tension of the vibrating wire is changing by the pressure applied to the diaphragm. The electronics unit vibrates the wire and measures the change in vibration frequency caused by pressure moving the diaphragm. The electronics unit changes the pressure applied to the diaphragm into an electrical output signal. 82
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
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
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
Pressure Measurement Pressure Switch Pressure switches are devices that open or close electrical circuits when they sense a pre-set pressure. The electrical circuits can then be used to open or close valves to relieve pressure in a system. The switches can be used to switch on pumps or compressors to maintain pressure in a system. 86
Level Measurement Introduction This unit will describe the common methods and devices used to measure liquid levels in process equipment. 90
Level Measurement Types of Level Measuring Devices There are two main types of level measuring devices. Direct level measuring devices. Indirect level measuring devices 91
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
Level Measurement Indirect Level Measuring Devices You cannot measure or see how much petrol you have in the tank of your car. An instrument measures the level and shows you how much petrol there is on a indicator on the dashboard (the petrol gauge). This is an example of an indirect level measuring device. 93
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.
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
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
Level Measurement Direct Level Measuring Devices - The Dip Tape HANDLE WINDER
Level MeasurementThe Sight GlassThis is the indicator used byoperators in the plant to ‘see’inside 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
Level Measurement Sight Glasses B AMagnetic Sight-glasses. A on the Inlet Separator, CPF and B is on aLiquid KO pot on Gobe South Compressors 100
Level MeasurementTypical Level Control system in a process area Level Transmitter Level Switches Level Column Bridle Sight Glass 101
Level Measurement Indirect Level Measuring Devices Indirect methods product mechanical or electrical output signals which indicate changes in level. Simple Floats Figure 3.4 shows a simple float level indicator. It is still used by water departments and on chemical tanks on older oil platforms. It is cheap to install and easy to operate. 102
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
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
Level Measurement Indirect Level Measuring Devices Simple Float Operation (cont) The simple float is not very accurate and can be very difficult to read. If the surface of the liquid is moving then the float starts to swing. This problem is solved by fitting special devices inside the tank as shown in the following slide. 105
Level MeasurementIndirect Level Measuring DevicesSimple FloatsOperation“A” is a guided wiresystem. C“B” is a Still Pipe Asystem where thefloat is in a slottedpipe and connectswith the groundlevel display“C” is another Still BPipe but the floatconnects with atransmitter whichsends a signal tothe control room
Level MeasurementIndirect Level Measuring DevicesSimple Float Operation - Guide Wire System (Figure A) This is the cheapest system. The float is held in place by wires 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 a pulley system and through a pipe to the indicating unit The pipe is supported on brackets fixed to the tank. The indicating unit is the counterweight and the level is indicated by a mechanical counter. 107
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 with holes in it). The level inside the pipe does not move so it gives very accurate measurements of level. Figure B shows the older mechanical indication method. Figure C shows the modern method where the system is electronically controlled and the level measurement is sent as an electronic signal to the control room. 108
Level MeasurementIndirect Level Measuring Devices Hydrostatic Tank Gauging (HTG) Pressure indicator Many of the modern oil storage tank facilities (tank farms) use hydrostatic tank gauging to indicate the level Transmitter in a tank. HTG is good because there is no equipment inside the tank. A Hydrostatic Tank Gauging installation on the Skim Tank at It is cheaper to install and the CPF maintain than the float installations.
Level Measurement Indirect Level Measuring Devices Hydrostatic Tank Gauging (HTG) The higher the level of a liquid in a tank, the higher the pressure on the bottom of the tank. An outlet near the bottom of the tank is under more pressure than an outlet near the top of the tank. The greater the pressure the further the outflow stream will reach. 111
Level Measurement Indirect Level Measuring Devices Hydrostatic Tank Gauging (HTG) The pressure on the bottom of the tank only depends on the level of the liquid in the tank; not the volume or the shape of the tank. No matter what the shape of the tank, the pressure at the bottom of the tank is the same. Using this principle, a pressure sensor at the bottom of the tank can gauge the level of the liquid in the tank. The higher the pressure, the higher the level of the liquid in the tank 112
Level Measurement Indirect Level Measuring Devices Displacers and Local Level Control The displacer is a locally mounted device which controls the level in a vessel. It is used on remote sites where it is too expensive to return signals to the control room. The most common types in use are manufactured by Fisher or Masoneilan. The diagram Figure 3.7 shows a Fisher device (The Level-Trol). 113
Level Measurement Indirect Level Measuring DevicesDisplacers and Local Connecting RodLevel ControlThe Displacer unit isconnected to both the Torque Tubevessel and the control valve. Displacer
Level Measurement Displacers and Local Level Control Operation The weight of the displacer changes as the level of the liquid rises or falls in the displacer housing. The displacer hangs on the torque tube via the connecting rod. The changing weight of the displacer makes the torque tube twist or untwist. The twisting motion of the torque tube moves a flapper against a nozzle. This sends a control signal to the pneumatic control valve. The pneumatic control valve opens and closes to control the flow of liquid into the tank. This keeps the level of liquid in the tank constant at the set point. 116
Level MeasurementDisplacers and Local Level ControlThis forms a self contained local control loop as shown in thefigure below
Level Measurement Air Bubble Method The Air Bubble method is one of the oldest and simplest means used to indicate level and/or transmit a signal. The diagram shows a simplified layout of the method.
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
Level Measurement Air Bubble Method - Operation (cont) The back pressure between zero and maximum levels is proportional to the liquid level in the vessel. The pressure gauge or transmitter can be calibrated to indicate the liquid level. The gas pressure is adjusted by the regulator to give a steady flow of gas down the bubbler tube. The gas flow is indicated on the Rotamater. This is a very accurate method of showing liquid level using modern instrument systems. 121
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.
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
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
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
Temperature Measurement Filled Thermal Elements Thermal filled elements operate by the expansion and contraction of fluids 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.
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
Temperature MeasurementFilled Systems The liquid filled system is normally used in process plant applications.
Temperature Measurement Bi-Metal Strip Thermometers Liquid and gas filled systems use the expansion of fluids to measure temperature. Some temperature measuring devices use the expansion of solids to measure temperature. One kind of solid expansion thermometer is the bi-metal strip illustrated in the diagram Cold Hot
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.
Temperature Measurement Bi-Metal Strip Thermometers - Operation The 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 shaft which is fixed to it. The heat applied to the bi-metal strip at the fixed end causes the helix to unwind and turn the pointer on the scale. 131
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
Temperature MeasurementTemperature Measurement Devices The thermocouple is used to sense the process variable and transmit the signal to the controller electrically. Filled thermal bulb and capillary tubing Resistance bulb Thermocouple and protective well 134
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
Temperature MeasurementRadiation Temperature Detectors (Pyrometers)The heat from the objects is focused by lenses onto a sensor.It’s 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.
Temperature MeasurementResistance Temperature Detector (RTD) The device indicates temperature by measuring the change in the electrical resistance of a metal. When metals get hotter their resistance increases. This increase in resistance is almost linear. In other words, the resistance increases at the same rate as the temperature. When the resistance is measured it gives an accurate indication of temperature. There are other temperature sensors in use but these are of more interest to instrument technicians. 137
Temperature MeasurementResistance Temperature Detector (RTD)
Temperature MeasurementThermowellsThe thermowell is a devicefitted into a flow line so that Thermocouplethe 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
Temperature Measurement Thermowells Most vessels and pipes in process and production plants contain liquids or gases under pressure. The thermowell protects the temperature sensor from damage from pressure and also from fluid flow. The heat in a fluid takes longer to transfer through a thermowell, so changes in temperature take longer to show. Different methods are used to speed up heat transfer. Sometimes the space between the probe and the thermowell is filled with a liquid which conducts heat well. 142
Temperature Measurement Thermowells Sometimes the probe is placed in a corrugated aluminum cover to give a direct metal contact between the probe and the thermowell. When a thermowell is filled with heat conducting liquid a small amount of air has to be left as a gas cap at the top of the well to allow for thermal expansion of the liquid. As the conducting liquid expands with the increase in temperature the liquid compresses the gas cap. This prevents the pressure generated by the expansion from damaging the instruments in the well. 143
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
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 it’s very important to know the rate of flow through different process equipment. 145
Flow Measurement The rate of flow affects how well the process works. When we know how much is flowing we can decide if it is too much or too little. We can then change the flow to what we want it to be, in other words set it at the desired value. 146
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
Flow Measurements Rate of Flow Measurement Devices for measuring the rate of flow do not have to be very accurate. It is the change in the rate of flow that is important to a Technician. Flow measurement devices are often used to give a flow signal directly to a controller. In this case they are called Flow Indicator Controllers (FIC’s) If they sent a signal to the Control Room or a locally mounted recorder they would be called Flow Recorder Controllers (FRC’s) 148
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
Flow Measurement Flow Basics The flow is produced by the difference in pressure across the ends of the pipe. If there is a big difference in pressure then the rate of flow will be fast. If there is a small difference in pressure than the rate of flow will be slow. The difference in pressure is called the ‘Differential pressure’. The term ‘Differential Pressure’ is a common expression and one that you need to understand 150
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
Flow MeasurementsRate of Flow Measurement - Flow Basics
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
Flow Measurements Flow Basics – Friction and Types of Flow Turbulent Flow Turbulent flow occurs when the fluid flow rate is high. The velocity of the fluid through the pipe is nearly the same across the pipe. The flow is a little slower at the edges because of the friction between the fluid and the wall of the pipe. 154
Flow Measurements Flow Basics – Calculating the Rate of Flow There must be a differential pressure across the ends of the pipe in order for fluid to flow. If the differential pressure and the size of the pipe is known, we can calculate how fast the fluid is flowing through the pipe (the rate of flow). The calculation is complicated. An easier method had to be found to calculate the flow through the pipe. The easier method is to put a restriction in the pipe. A restriction is something which blocks part of the flow. 155
Flow Measurement Flow Basics – Calculating the Rate of Flow There are three main devices used to make restrictions in a pipe: Orifice Plate Venturi Tube Flow Nozzle The restriction produces a differential pressure across it. In other words the pressure downstream of the restriction is lower than the pressure upstream of the restriction. 156
Flow Measurement Flow Basics – Calculating the Rate of Flow The pressure difference is due to the increase in velocity as the process fluid flows through the restriction. When the velocity of the flow increases, the pressure at that point in the line decreases. By knowing the differential pressure, the internal diameter of the pipe and the size of the hole in restriction, we can calculate the rate of flow. The instruments do the calculation for us. 157
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.
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.
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
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
Flow Measurements Venturi Tube Restrictions Another type of restriction device is the venturi tube. If the fluid in a pipe is flowing under very low pressure the restriction by an orifice plate could stop the flow. In these cases a venturi tube is used. These devices are very expensive. 164
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.
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
Flow MeasurementFlow Nozzle RestrictionsFlow nozzles are good for liquids with high flow rates.Orifice plates are usually used for gases with high flow rates.
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
Flow Measurements Flow Straighteners (Straightening Vanes) A flow straightener is a cylinder filled with many small pipes. This device is place in the pipeline upstream of the flow measuring device. It causes the fluid to flow smoothly and evenly which means the measuring device can get a more accurate measurement.
Flow Measurement Calibration of Differential Devices Calibrating a differential-pressure, flow measuring device is a skilled job. An instrument technician will use figures given by the design engineer to do this. For control purposes the actual measurement of flow need not be exact. It’s the changes in the rate of flow which are important. 170
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 flow lines on large rotating machines e.g. diesel engines and gas C compressors. B In cooling water lines for machines and processes. A A A The diagram shows a variable area meter or Rotameter.
Flow Measurements Variable Area Meters - Operation The Rotameter is fitted vertically into the flow line. The flow 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 the cylinder (position A) When the flow increases, the increased pressure makes the float rise. It will rise to a position where the flow pressure on the float equals the weight of the float, (position B). 172
Flow Measurements Variable Area Meters - Operation If the flow gets faster there is more pressure on the float and it will rise higher (position C). The flow rate indicated depends on the size of the device. It is pre-calibrated by the manufacturer The Technician reads the flow rate from the transparent scale using the top of the float as a marker. 173
Flow Measurement Positive Displacement Meters Positive displacement flow-measurement meters are very accurate. They are also called quantity meters. Basic Principle The meter traps a known fixed volume of fluid and transfers it from the inlet to the outlet. The number of fixed volumes of fluid transferred (or moved) is a measure of flow. 174
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
Flow Measurements Positive Displacement Meters - Basic Principle A positive displacement metre works on the same principle as the man with the bucket. However, a positive displacement meter is much faster and more reliable than a man with a bucket. Flow meters that use this basic principle are: Reciprocating piston meters Rotating vane meters Lobed impeller meters
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.
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
Flow MeasurementsPositive Displacement MetersRotating Vane Meter
Flow Measurements Positive Displacement Meters - Lobed Impeller Another kind of positive displacement meter is a lobed impeller meter. Each rotation of the impeller pushes a measured quantity of fluid through the meter.
Flow Measurements Velocity Meters (Semi-Positive Displacement) The velocity meter measures the speed of flow. then calculates the volume of flow using calibration figures. It The calibration figures are placed in the electronics unit’s memory by the operator or instrument technician. These calibration figures depend on the type of liquid flowing through the meter. 181
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.
Flow MeasurementVortex Meters Vortex Meters are used on fuel gas systems such as gas from the Test Separator 183
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 reliable 184
Increased Density Density changes Vibration action Decreased Density Increased Increased Density Density 186
Flow Measurement Turbine Meter Turbine meters are used extensively in a process facility. This one is at the CPF Export Pump Station. Others are at the valve stations along the export pipeline and at the Marine Terminal - Kumul 187
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
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 PLC’s andTEC’s are also dealt with in that package. For comments and suggestions contact Len Dallow or Peter Cannell on firstname.lastname@example.org or email@example.com 193