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Basics Of Instrumentation



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Basics Of Instrumentation

  1. 1. BASICS • What is Instrumentation? • Basic Terminologies • Process & its Control • Field Instruments & its principles • Valves & its working
  2. 2. What is Instrumentation? • Instrumentation is about measurement and control. • Instrumentation engineering is the engineering specialization focused on the design and configuration of process systems. • Instruments are devices which are used in measuring attributes of process systems.
  3. 3. Basic Terminologies • Process: Series of continuous or regularly recurring steps or actions intended to achieve a predetermined result, as in heat treating metal, or manufacturing acid. • Transducer(sensor): Element which converts one form of Energy to Other form. • Primary Transducer: Transducer which converts the Process parameter to a form readable by Secondary Transducer. Eg: Orifice plate • Secondary Transducer: Transducer or transmitter which responds to a measured variable and converts it to a standardized transmission signal which is a function only of the measurement. Eg: DP Transmitter
  4. 4. • Signal: The signal is the event or phenomenon that conveys data from one point to another. • 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 Correcting Element. • 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-valves etc.,).
  5. 5. •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 or equipment. ANALYSER: •Monitor pollutant gas emissions from industrial processes. •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 feed rate. •The controller then either changes the belt speed or belt load to get the set feed rate.
  6. 6. CONVEYOR: Conveyors are used as components in automated distribution and warehousing. A belt conveyor consists of two or more pulleys, with a continuous loop of 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 the idler. 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 conveyor belt.
  7. 7. Process & its Control • Process Parameters: – Pressure – Level – Temperature – Flow
  8. 8. Pressure Measurement • 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 pressure are psi kg/cm2 bar atmosphere torr 1 Pa = 1 N/m2
  9. 9. Pressure Measurement • Primary Pressure Measuring Devices: – Diaphragm – Bellows – Manometer • Pressure measurements can be divided into three different categories: – absolute pressure – gauge pressure and – differential pressure
  10. 10. 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 following equation: Pg=Pa+Po where Pg= gauge pressure Po = atmospheric pressure • If the pressure of a system is below atmospheric, it is called vacuum pressure. •When pressure is measured by a gauge, the quantity obtained usually excludes the ambient atmospheric pressure and is therefore called overpressure, Poverpressure = Pgauge
  11. 11. ABSOLUTE PRESSURE If atmospheric pressure is included, then the resulting pressure is called absolute pressure Pabsolute = Patmospheric + Pgauge The absolute pressure is measured relative to the absolute zero pressure - the pressure that would occur at absolute vacuum. P=Pg+Po P=absolute pressure, Pg=gauge pressure, Po=atmospheric pressure. DIFFERENTIAL PRESSURE Differential pressure is the difference in pressure between two points.
  12. 12. ATMOSPHERIC PRESSURE •The atmospheric pressure is the pressure in the surrounding air at or "close" to the surface of the earth. •The atmospheric pressure varies with temperature and altitude above sea level. •Atmospheric pressure is the pressure exerted at the surface of a body by a column of air in an atmosphere. 1 atmosphere on Earth = 760 millimeters of mercury (760 Torr) and 101,325 Pascals. STANDARD ATMOSPHERIC PRESSURE : The Standard Atmospheric Pressure (atm) is used as a reference for gas densities and volumes. The Standard Atmospheric Pressure is defined at sea-level at 273o K (0o C) and is 1.01325 bar or 101325 Pa (absolute). The temperature of 293o K (20o C) is also used.
  13. 13. Types of Pressure Measuring Devices Manometer Bourdan Gauge
  14. 14. Contd.. Strain Gage Types Capacitive Peizo Electric LVDT
  15. 15. Level Measurement • Some of the most commonly used liquid-level measurement methods are: • RF capacitance • Conductance (conductivity) • Hydrostatic head/tank gauging • Radar • Ultrasonic
  16. 16. Level Measurement Level Measurement using Pressure Transmitter P = ρgh
  17. 17. Open Tank Closed Tank
  18. 18. RF Capacitance Conductive Type
  19. 19. Hydrostatic Head
  20. 20. RADAR Type Ultrasonic Type
  21. 21. Flow Measurement • 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 x V Where A is the cross sectional area of the pipe V is the fluid velocity Unit of low is m3 /hr or litres/hr
  22. 22. Flow Measurements • Types: – Head Type Flowmeters – Mechanical Flowmeters – Electronic Flowmeters – Mass Flowmeters
  23. 23. Different Type of Head Type Flowmeters • Orifice Plate • Venturi • Flow Nozzle • Pitot Tube • Elbow
  24. 24. 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
  25. 25. Venturi • Service: Clean Liquids, Gases Steam Slurries and Dirty Fluids • Scale: Square Root • Accuracy: 1% Full Scale • Rangability: 3:1 • Permanent Pressure Loss: Low • Cost: High
  26. 26. Variable Area Meters
  27. 27. Mechanical Flowmeters
  28. 28. Electronic Flowmeters
  29. 29. Vortex Flowmeters
  30. 30. Ultrasonic Flowmeters
  31. 31. Temperature Measurement • Temperature: – Webster’s defines temperature as “the degree of hotness or coldness measured on a definite scale. Various units of temperature are related as C = 5/9 (F – 32) F = 9/5 (C ) + 32 K = 273 + C R = 460 + F
  32. 32. Temperature Measurement • Types of Temp Measurement: – RTD – Thermocouple – Thermistor – Thermopile – Pyrometer
  33. 33. 35 Steam Cold Water Hot Water Load Disturbance TT TIC I/ P Temperature terminology Temperature Control Loop • Temperature Loop Issues: – Fluid response slowly to change in input heat – Requires advanced control strategies • Feedforward Control
  34. 34. 36 • Example: Thermistors • RTD (discussed later) • 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 a micro-ammeter. • Stability - • Linearity - • Slope of Output - Temperature Measurement Technology Change in RESISTANCE with response to change in TEMPERATURE Moderate Poor (Logarithmic) Negative
  35. 35. 37 Temperature Sensors RTDs • What is an RTD ? – RResistance TTemperature DDetector Platinum resistance changes with temperature Rosemount’s Series 78, 88 Rosemount’s Series 68, 58 Series 65 Two common types of RTD elements: Wire-wound sensing element Thin-film sensing element » Operation depends on inherent characteristic of metal (Platinum usually): electrical resistance to current flow changes when a metal undergoes a change in temperature. » If we can measure the resistance in the metal, we know the temperature!
  36. 36. 38 Temperature Sensors RTDs • How does a RTD works? – Resistance changes are Repeatable – The resistance changes of the platinum wiring can be approximated by an ideal curve -- the IEC 751 0 50 100 150 200 250 300 350 -400 -200 0 200 400 600 800 Resistance(Ohms) Temperature (o C) o C Ohms 0 100.00 10 103.90 20 107.79 30 111.67 International Resistance vs. Temperature Chart: IEC 751 IEC 751 IEC 751 Constants are :- A = 0.0039083, B = - 5.775 x 10 -7 , If t>=0°C, C=0, If t<0, C = - 4.183 x 10 -12 Example: RT = R0 [1 + At + Bt2 + C(t-100)t3 ] = 103.90
  37. 37. 39 ProcessProcess TemperatureTemperature Hot junction – Two dissimilar metals joined at a “Hot” junction Cold junction + - MV – The wires are connected to an instrument (voltmeter) that measures the potential created by the temperature difference between the two ends. DT “40 millivolts!,” Tommy Seebeck yelled in a heated debate. The junction of two dissimilar metals creates a small voltage output proportional to temperature! What is a Thermocouple ? Temperature Sensors Thermocouples
  38. 38. 40 • How does a Thermocouple work ? – The measured voltage is proportional to the temperaturetemperature differencedifference between the hot and cold junction! (T2 - T1) =∆T. + - MVHeat Hot junction Cold junction o C Millivolts 0 0.000 10 0.591 20 1.192 30 1.801 Thermoelectric Voltage vs. Temperature Chart: TYPE E THERMOCOUPLE ∆T -20 0 20 40 60 80 -500 0 500 1000 Voltage(mV) Temperature (o C) IEC 584 MeasurementMeasurement JunctionJunction TT22 ReferenceReference JunctionJunction TT11 Temperature Sensors Thermocouples
  39. 39. 41 Type J – Iron / Constantan • White, Red • 0 to 760 °C • Least Expensive Types of Thermocouple Type K – Chromel / Alumel » Yellow, Red » 0 to 1150 °C » Most Linear Type T – Copper / Constantan » Blue, Red » -180 to 371 °C » Highly resistant to corrosion from moisture + - + - + - Temperature Sensors Thermocouples
  40. 40. 42 • Better Accuracy & Repeatability – RTD signal less susceptible to noise – Better linearity – RTD can be “matched” to transmitter (Interchangeability error eliminated) – CJC error inherent with T/C’s; RTD’s lead wire resistance errors can be eliminated Why choose RTD over Thermocouple ? Better Stability – T/C drift is erratic and unpredictable; RTD’s drift predictably – T/C’s cannot be re-calibrated Greater Flexibility – Special extension wires not needed – Don’t need to be careful with cold junctions Temperature Sensors Comparison
  41. 41. 43 • Applications for Higher Temperatures • Above 1100°F • Lower Element Cost • Cost is the same when considering temperature point performance requirements • Faster response time • Insignificant compared to response time for T-Well and process • Perceived as more rugged • Rosemount construction techniques produce extremely rugged RTD Why choose thermocouple over RTD ? Temperature Sensors Comparison
  42. 42. 44 RANGE OFFER -200 to 500º C RTD 500 to 1100º C Thermocouple Type K >1100º C Special Thermocouple R, S or B Temperature SensorsTemperature Sensors ComparisonComparison
  43. 43. 45 • What is a thermowell (T-well) ? – A unit that protects a sensor from process flow, pressure, vibrations, and corrosion – Allows for sensor removal without process shutdown – Slows response time (by 5 times) Why are there different material types ? – To handle different corrosive environments – To handle different temperature and pressure limits Sensor accessories Thermowells
  44. 44. Control Valves • 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 the actuator. • 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 pneumatic systems.
  45. 45. CONTROL VALVES: They are basically pneumatically operated valves which require around 4 to 5 kg/cm2 of air pressure to operate the valve. I / P Converter POSITIONER CONTROL VALVE SUPPLY AIR Pneumatic signal CURRENT SUPPLY AIR
  46. 46. Control Valve Types
  47. 47. Valve Body Types • Diff. types of Valve Body: • Butterfly Valve • Globe Valve • Ball Valve • Plug type Valve • Needle Valve
  48. 48. Positioner & its accessories • Pneumatically operated valves depend on a positioner to take an input signal from a process controller and convert it to valve travel. • A pneumatic signal (usually 3-15 psig) is supplied to the positioner. The positioner translates this to a required valve position and supplies the valve actuator with the required air pressure to move the valve to the correct position. • Analog I/P Positioner—This positioner performs the same function as the one above, but uses electrical current (usually 4-20 mA) instead of air as the input signal.
  49. 49. Automation (ancient Greek: = self dictated), roboticization or industrial automation or numerical control is the use of control systems such as computers to control industrial machinery and processes, replacing human operators. The most commonly used automation systems are : • DCS - Distributed Control System • PLC - Programmable Logic Controller • SCADA – Supervisory Control And Data Acquisition System
  50. 50. DCS • Distributed control system (DCS) refers to a control system usually of a manufacturing system, process or any kind of dynamic systemdynamic system, in which the controller elements are not central in location (like the brain) but are distributed throughout the system with each component sub-system controlled by one or more controllers. The entire system of controllers are connected by a network for communication and monitoring. • DCS is a very broad term used in a variety of industries, to monitor and control distributed equipment. • A DCS typically uses computers (usually custom designed processors) as controllers and uses both proprietary interconnections and protocols for communication. Input & output modules form component parts of the DCS. The processor receives information from input modules and sends information to output modules. The input modules receive information from input instruments in the process (a.k.a. field) and output modules transmit instructions to the output instruments in the field. Computer buses or electrical buses connect the processor and modules through multiplexers/demultiplexers. Buses also connect the distributed controllers with the central controller and finally to the Human-Machine Interface (HMI) or control consoles.
  51. 51. ARCHITECTURE OF DCS Operator Workstation 1 Operator Workstation 2 Operator Workstation 3 Controller 1 Controller 2 Controller 3 Controller 4 Sensor 1 Actuator 1 Actuator 2 Sensor 3 Actuator 3 Sensor 4 Actuator 4 Database Input Module Output Module Sensor 2 Input Module Input Module Input Module Output Module Output Module Output Module
  53. 53. BASIC TERMINOLOGIES OF DCS HIS: Human Interface Station The HIS is mainly used for operation and monitoring-it displays process variables,control parameters and alarms necessary for users to quickly grasp the operating status of the plant. NIU: Node Interface Unit These are remote I/O units which all the Instruments are connected.these units in turn are connected to FCS through RIO bus. FCS: Field Control Station It is the main control unit which controls the plant.there can be more than one FCS which then communicate with each other and also communicate with the HIS from where the Operator is operating. Vnet: The Vnet real time control system BUS links station such as FCS,HIS,BCV andCGW. ETHERNET: Ethernet is used to link HIS,ENG and supervisory is also used for transferring data files to supervisory computers and for HIS data equalization. RL Bus: This a control system BUS(communication link) which connects Field control units,operators stations. CGW: Communication Gateway This unit links the Vnet control system BUS to an ETHERNET BUS BCV: Bus Converter The communication bus of one version of DCS may not communicate with the newer versions so BUS CONVERTER is used to convert the BUS to suitable mode. In our plant our existing RL BUS is converted to newer system bus Vnet by the Bus Converter kept in Engineering room near central control room.
  54. 54. Programmable Logic Controller (PLC) •Programmable Logic Controller (PLC) is a microprocessor based system that uses programmable memory to store instructions and implement functions such as logic, sequencing, timing, counting and arithmetic in order to control machines and processes. •Unlike Personal Computer, PLC does not contain peripherals, such as display or keyboard, that allow user to directly interact with PLC. In order to facilitate interaction, separate computer is provided, normally taking form of a standard PC. Through this external computer, operator can re-program PLC, provide set-points and view trends of process variables that are controlled and manipulated by PLC. PLC Actuator Process Sensor External Computer
  55. 55. Power Supply Communication Module Microprocessor + Memory Analogue Input (AI) Module Discrete Output (DO) Module Discrete Input (DI) Module Analogue Output (AO) Module Analogue Sensor Discrete Sensor Analogue Actuator Discrete Actuator Operator Workstation Programmable Logic Controller Architecture PLC
  56. 56. Communication Module Microprocessor Input Module External Computer Programmable Logic Controller Architecture PLC Output Module Actuator Process Sensor
  57. 57. PLC consists of the following components: • Microprocessor – This is the brain of PLC. It reads input signals, executes control program and communicates results (decisions) of control program as action signals to the outputs. • Memory – It stores control program that is to be executed at a prescribed rate. • Power Supply – This component is used to convert the mains AC voltage to the low DC voltage (e.g. from 240V AC to 5V DC). This unit powers the processor and the circuits in the input and output modules. • Input Module – This component receives information from external devices (sensors). It contains circuitry that provides electrical isolation and signal conditioning functionalities. Input module can be analogue input (AI) or discrete input (DI) module. AI module receives continuously changing signal whose amplitude is proportional to the current value of the measured process variable. DI module receives discrete/digital (ON/OFF) information from discrete sensors, for example push button (ON if button is pressed, OFF if button is not pressed). Note that DI is much more frequently used than AI. • Output Module – This module communicates control actions to external devices (actuators). It contains circuitry required to interface PLC with actuators (e.g. digital-to- analogue converter and power amplifier). Like input module, output module can be analogue output (AO) or discrete output (DO) module depending on the type of actuator used. • Communication Module – This component allows PLC to communicate with external devices using sophisticated multiple-bit digital communication protocols (e.g. Ethernet).
  58. 58. Programmable Logic Controller (PLC)
  59. 59. PLC Programming • Ladder Diagram - most common • Structure Text Programming (ST) • Functional Block Programming (FB) • Instruction List (IL) • Sequential Function Chart (SFC)
  60. 60. • SCADA system performs the following tasks • Collection of data from field devices, which can be sensors, actuators and controllers. • Transfer of field devices’ information via communication link to the central site (master station) • Execution of any necessary analysis and supervisory control calculations, all of which are taking place at the master stations. • Display process information on a number of operator screens. • Convey any required supervisory control actions back to the field devices. Supervisory Control and Data Acquisition (SCADA)

Editor's Notes

  • Has a very high temperature coefficients - produce large changes in resistance in response to change in temperature.
    Hence, particularly good for small span &amp; difficult to handle large spans.
    Has a Non-Linear temperature vs resistance curve.
    Stability greater than 300 degrees is a problem.
    Application - Temperature Control Circuits as safety device
    example: on windings of transformer to detect overheating
  • What are the most commonly sold types of T/C’s? (Type J and K)
  • Sensor elements for industrial applications must be packaged ruggedly. RMD standard sensors are all of mineral insulated cable design with 6 mm sheath diameter.
    RTD ( Resistance Temperature Detector) sensors using platinum resistance elements provide the most stable and reliable measurement over the range -200 to 500 C and should always be specified by choice. Sheath material is AISI 321 stainless steel. Model 65, using platinum film type element, has its’ range reduced to meet the requirements of the international specification IEC 751:1983 INDUSTRIAL PLATINUM RESISTANCE THERMOMETER SENSORS. ( including Amendments A1:1986 and A2:1995). Model 75, using a wire-wound element will meet IEC 751 over an extended temperature range. Grade B tolerance class is offered as standard with Grade A as an option.
    Thermocouples must be offered for temperatures in excess of 500 C. Up to 1100 C type K should be specified. Sheath material must be suitable for these high temperatures and Inconel is available as standard. Thermocouples meet the requirements of IEC 584 :1995 THERMOCOUPLES, and are available to tolerance class 2 as standard. Other tolerance classes, requiring specially selected materials, are available as specials.
    For temperatures above 1100 C thermocouples , such as types R or S, are available. These are always supplied as specials as the high temperature environment determines the many thermocouple and sheath material options that will be required.
  • Question - How is the transmitter powered?
    EXCERCISE: If the transmitter has a range of 0-200°C (and we know that the mA signal is linear with temperature), what must the mA signal be?
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