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BASICS OF INSTRUMENTATION

BASICS OF INSTRUMENTATION. Presented by PULINANDI VIJAY KUMAR M.Tech , Assistant Professor DEPARTMENT OF MECHANICAL ENGINEERING VISAKHA INSTITUTE OF ENGINEERING & TECHNOLOGY. BAS I CS. What is Instrumentation? Basic Terminologies Process & its Control

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BASICS OF INSTRUMENTATION

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  1. BASICS OF INSTRUMENTATION Presented by PULINANDI VIJAY KUMAR M.Tech, Assistant Professor DEPARTMENT OF MECHANICAL ENGINEERING VISAKHA INSTITUTE OF ENGINEERING & TECHNOLOGY

  2. BASICS • What isInstrumentation? • Basic Terminologies • Process & itsControl • Field Instruments & itsprinciples • Valves & itsworking

  3. What isInstrumentation? • Instrumentation is about measurement and control. • Instrumentation engineering is the engineering specialization focused on the design and configuration of processsystems. • Instruments are devices which are used in measuring attributes of processsystems.

  4. BasicTerminologies • Process: Series of continuous or regularly recurring steps oractions intended to achieve a predetermined result,asin heat treating metal, or manufacturing acid. • Transducer(sensor): Element which converts one form of Energy to Otherform. • PrimaryTransducer: Transducer which converts the Process parameter to a form readable by SecondaryTransducer. Eg: Orificeplate • SecondaryTransducer: Transducer or transmitter which responds to a measured variable and converts it to a standardized transmission signal which is a function only of themeasurement. Eg: DPTransmitter

  5. • Signal: The signal is the event or phenomenon that conveys data from one point toanother. • Loop: A Loop is a combination of one or more interconnected instruments arranged to measure a process variable. It shall comprises the whole chain from Primary element to CorrectingElement. • Controller: A device that operates automatically by use of some established algorithm to regulate process variable(PV) according to the set point(SV). The controller input receives information about the status of the process variable and then provides an appropriate output signal(MV-manipulated variable) to the final control element(eg-valvesetc.,).

  6. Interlock: • It refers to the set of plant conditions(eg. Level of a tank, temp of furnace, position of furnace or a valve, flow of a fluid, etc) which are to be satisfied before operating(starting, stopping, opening ,closing ,etc)of any instrument orequipment. • ANALYSER: • Monitor pollutant gas emissions from industrialprocesses. • Gas analyzer is a gas comparator providing high linearity of signal transformation function. • WEIGHFEEDER: • Controller multiplies the signal from the load cell(belt load,kg/m) with that from the speed transducer(belt speed,m/s) to get the feedrate. • The controller then either changes the belt speed or belt load to get the set feedrate.

  7. CONVEYOR: Conveyors are used as components in automated distribution and warehousing. A belt conveyor consists of two or more pulleys, with a continuous loopof material - the conveyor belt - that rotates about them. One or both of the pulleys are powered, moving the belt and the material on the belt forward. The powered pulley is called the drive pulley while the unpowered pulley is called theidler. • BELTWEIGHER: • Material flowing over the belt may be weighed in transit using a beltweigher.A belweigher or belt weigher is a piece of industrial control equipment used to gauge the mass or flow rate of material travelling over a conveyorbelt.

  8. Process & itsControl • ProcessParameters: – – – – Pressure Level Temperature Flow

  9. PressureMeasurement • PRESSURE A force applied to or distributed over a surface. The pressure (P) of a force (F) distributed over an area(A) is defined as : P = F /A Standard Unit of Pressure is Pascal Other units of pressureare psi kg/cm2 bar atmosphere torr 1 Pa = 1N/m2

  10. PressureMeasurement • Primary Pressure MeasuringDevices: • Diaphragm • Bellows • Manometer • Pressure measurements can be divided into three different categories: • absolutepressure • gauge pressureand • differentialpressure

  11. GAUGE PRESSURE • Gauge pressure is the pressure relative to the local atmospheric or ambient pressure. • In measurements a gauge is used to record the pressure difference between the system and the atmospheric pressure. This is called gauge pressure and can be stated by the followingequation: • Pg=Pa+Po • where • Pg= gaugepressure • Po = atmosphericpressure • If the pressure of a system is below atmospheric, it is called vacuumpressure. • When pressure is measured by a gauge, the quantity obtained usually excludesthe • ambient atmospheric pressureand is therefore calledoverpressure, • Poverpressure = Pgauge

  12. ABSOLUTE PRESSURE If atmospheric pressure is included, then the resulting pressure is called absolutepressure Pabsolute = Patmospheric +Pgauge The absolute pressure is measured relative to the absolute zero pressure - the pressure that would occur at absolutevacuum. P=Pg+Po P=absolute pressure, Pg=gauge pressure, Po=atmosphericpressure. DIFFERENTIAL PRESSURE Differential pressure is the difference in pressure between twopoints.

  13. ATMOSPHERICPRESSURE • The atmospheric pressure is the pressure in the surrounding air at or "close" to the surface of theearth. • The atmospheric pressure varies with temperature and altitude above sealevel. • Atmospheric pressure is the pressure exerted at the surface of a body by a column of air in anatmosphere. • 1 atmosphere on Earth = 760 millimeters of mercury ( 760 Torr) and 101, 325Pascals. • STANDARD ATMOSPHERIC PRESSURE: • The Standard Atmospheric Pressure ( atm) is used as a reference for gas densities andvolumes. • The Standard Atmospheric Pressure is defined at sea- level at 273 oK (0 oC) and is 1.01325 bar or 101325 Pa ( absolute). The temperature of 293 oK ( 20oC) is also used.

  14. Types of Pressure MeasuringDevices Manometer BourdanGauge

  15. Contd.. PeizoElectric Strain GageTypes LVDT Capacitive

  16. LevelMeasurement • • Some of the most commonly used liquid-level measurement methodsare: • RFcapacitance • Conductance (conductivity) • Hydrostatic head/tankgauging • Radar • Ultrasonic

  17. LevelMeasurement Level Measurement using PressureTransmitter P =ρgh

  18. OpenTank ClosedTank

  19. RFCapacitance ConductiveType

  20. HydrostaticHead

  21. UltrasonicType RADARType

  22. FlowMeasurement • Principle: • – Flow is measured by measuring velocity through a known area.with this indirect method,the flow measured is the volume flow rate Q . • Q = A xV • Where A is the cross sectional area of thepipe • V is the fluidvelocity • Unit of low is m3/hr orlitres/hr

  23. FlowMeasurements • Types: – – – – Head Type Flowmeters MechanicalFlowmeters Electronic Flowmeters MassFlowmeters

  24. Different Type of HeadType Flowmeters • OrificePlate • Venturi • FlowNozzle • PitotTube • Elbow

  25. Orifice • • • • • Service: Clean Liquids, Gases Steam,(no slurries or corrosive) Scale: Square Root Accuracy: 1% Full Scale Permanent Pressure Loss:High Cost:Low Basic Equation:V=k*(h/D)0.5

  26. Venturi • • • • • • Service: Clean Liquids, Gases Steam Slurries and Dirty Fluids Scale: SquareRoot Accuracy: 1% FullScale Rangability:3:1 Permanent Pressure Loss: Low Cost:High

  27. Variable AreaMeters

  28. Mechanical Flowmeters

  29. ElectronicFlowmeters

  30. VortexFlowmeters

  31. UltrasonicFlowmeters

  32. TemperatureMeasurement • Temperature: – Webster’s defines temperature as “the degree of hotness or coldness measured on a definitescale. Various units of temperature are related as C= 5/9 (F –32) F = 9/5 (C ) +32 K = 273 +C R = 460 +F

  33. TemperatureMeasurement • Types of TempMeasurement: • RTD • Thermocouple • Thermistor • Thermopile • Pyrometer

  34. Temperatureterminology • Temperature ControlLoop • Temperature LoopIssues: – – • Fluid response slowly to change in input heat Requires advanced controlstrategies • FeedforwardControl Load Disturbance TIC Cold Water I/ P TT Steam Hot Water 35

  35. Temperature Measurement Technology Change in RESISTANCE with response to change in TEMPERATURE Thermistors RTD (discussedlater) • Example: • • • Thermistors Semi-conductors made from specific mixtures of pure oxides of nickel, manganese, copper, cobalt, and other metals sintered at very high temperature. Used with Wheatstone Bridge which amplifies small change in resistance - in a simple circuit with a battery and amicro-ammeter. • • Stability • Linearity • Slope ofOutput - - - Moderate Poor(Logarithmic) Negative 36

  36. TemperatureSensors • RTDs • What is an RTD? • –Resistance TemperatureDetector » Operation depends on inherent characteristic ofmetal (Platinum usually): electrical resistance to currentflow changes when a metal undergoes achangein temperature. » If we can measure the resistance in the metal, weknow the temperature! Platinum resistancechanges withtemperature Thin-film sensingelement Rosemount’s Wire-wound sensingelement Rosemount’s Series 68,58 Series78,88 Two common types of RTD elements:Series 65 37

  37. TemperatureSensors • RTDs • How does a RTD works? • Resistance changes are Repeatable • The resistance changes of the platinum wiring can be approximated byanideal curve -- the IEC751 350 Resistance(Ohms) International Resistance vs. TemperatureChart: IEC 751 300 250 200 150 100 50 0 0 200 400 600 Temperature(oC) -400 -200 800 IEC751 IEC 751 Constants are :- A = 0.0039083, B = - 5.775 x 10 -7, Ift>=0°C, C=0, If t<0, C = - 4.183 x 10-12 RT = R0 [1 + At + Bt2 +C(t-100)t3] = 103.90 Example: 38

  38. TemperatureSensors • Thermocouples • What is a Thermocouple? • Two dissimilar metals joined at a “Hot”junction • The wires are connected to an instrument (voltmeter) that measures the potential created by the temperature difference between the twoends. Process Temperature “40 millivolts!,” Tommy Seebeck yelled in a heated debate. Coldjunction Hotjunction + DT MV - The junction of two dissimilar metals creates a small voltage output proportional to temperature! 39

  39. TemperatureSensors • Thermocouples • How does a Thermocouple work? • – The measured voltage is proportional to thetemperature difference between the hot and cold junction! (T2 - T1)=T. Coldjunction Hotjunction + T MV Measurement Reference Junction T1 Heat Junction T2 - 80 Thermoelectric Voltage vs. TemperatureChart: Voltage(mV) 60 IEC 584 40 20 30 1.801 TYPE ETHERMOCOUPLE 0 -500 0 500 1000 -20 Temperature(oC) 40

  40. TemperatureSensors Thermocouples Types ofThermocouple Type J – Iron /Constantan • • • White,Red 0 to 760°C LeastExpensive TypeT – Copper / Constantan + - TypeK – Chromel /Alumel » » » Blue, Red -180 to 371 °C Highly resistant to corrosion from moisture + - » » » Yellow, Red 0 to 1150°C MostLinear + - 41

  41. TemperatureSensors • Comparison • Why choose RTD over Thermocouple? • Better Accuracy & Repeatability • RTD signal less susceptible to noise • Betterlinearity • RTD can be “matched” to transmitter (Interchangeability erroreliminated) • CJC error inherent with T/C’s; RTD’s lead wire resistance errors can beeliminated • BetterStability • T/C drift is erratic and unpredictable; RTD’s drift predictably • T/C’s cannot bere-calibrated • GreaterFlexibility • Special extension wires not needed • Don’t need to be careful with coldjunctions 42

  42. TemperatureSensors Comparison Why choose thermocouple over RTD? • Applications for HigherTemperatures • Above 1100°F Lower ElementCost • Cost is the same when considering temperature point performance requirements • Faster responsetime • Insignificant compared to response time for T-Well and process • Perceived as more rugged • Rosemount construction techniques produce extremely rugged RTD • • • • 43

  43. TemperatureSensors Comparison 44

  44. Sensoraccessories Thermowells • What is a thermowell (T-well)? • A unit that protects a sensor from process flow, pressure, vibrations, andcorrosion • Allows for sensor removal without process shutdown • Slows response time (by 5 times) • Why are there different material types ? • To handle different corrosiveenvironments • To handle different temperature and pressure limits 45

  45. ControlValves • The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point. The control valve regulates the rate of fluid flow as the position of the valve plug or disk is changed by force from theactuator. Control valves are valves used within industrial plants and elsewhere to control operating conditions such as temperature, pressure ,flow, and liquid level by fully or partially opening or closing in response to signals received from controllers that compare a "set point" to a "process variable" whose value is provided by sensors that monitor changes in such conditions. The opening or closing of control valves is done by means of electrical, hydraulic or pneumaticsystems. • • •

  46. CONTROLVALVES: They are basically pneumatically operated valves which require around 4 to 5 kg/cm2 of air pressure to operate thevalve. Pneumaticsignal CONTROLVALVE I / PConverter POSITIONER CURRENT SUPPLYAIR SUPPLYAIR

  47. Control ValveTypes

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