ch. 1 introduction to industrial instrumentation

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  • 1.I&CCHAPTER 1INTRODUCTION TO INDUSTRIAL INSTRUMENTATION 1

2. CHAPTER HIGHLIGHTS Instrumentation is the science of automated measurement and control.Applications of this science be plentiful in modern research, industry, and everyday living.This chapter explains some of the fundamental principles of i d t i l i t i i l f industrial instrumentation. t ti2 3. In Oil & Gas industry, the first step, naturally, is y, p, y, measuring the process variables such as; pressure, flow, level , temperature, analytical, etc. fl l l l i lOnce the process variable measured, transmit a p , signal representing this quantity to an indicating or computing d device where either human or h h h automated action then takes place.If the controlling action is automated, the computer sends a signal to a final controlling device which then influences the quantity being measured. 3 4. Both the measurement device and the final control device connect to some physical system which we call the process To show this as a general block process. diagram:4 5. INSTRUMENTATION TERMS AND THEIR DEFINITIONS: Process: The physical system we are attempting to control or measure Examples: oil refinery unit measure. unit, water filtration system, steam boiler, power generation unit unit. Process Variable (PV): The specific quantity we are measuring in a process. Examples: pressure, level, temperature, flow, electrical conductivity, pH, position, speed, vibration. Setpoint (SP): The value at which we desire the process variable to be maintained at. In other words, words the target value of the process variable target variable. 5 6. Primary Sensing Element (PSE): A device that directly senses the process variable and translates that di l h i bl d l h sensed quantity into an analog representation (electrical voltage, current, resistance; mechanical force, motion, ) a p o oup , , bou do ub , etc.). Examples: thermocouple, RTD, bourdon tube, potentiometer, electrochemical cell. Transducer (Converter/ Relay): A device that converts one standardized instrumentation signal into g another standardized instrumentation signal, and/or performs some sort of processing on that signal signal. Examples: I/P converter, P/I converter, square-root extractor. 6 7. Transmitter: A device that translates PSE signal into a standardized instrumentation signal such as; d d d l h pneumatic 3-15 psi, electrical 4-20 mA DC, Fieldbus digital signal packet, etc.,Lower- and Upper-range values (LRV & URV): e a ues o p ocess easu e e t dee ed The values of process measurement deemed to be 0% and 100% of a transmitters calibrated range. For example, if a temperature transmitter is calibrated to measure a range of temperature starting at 3000C and ending at 5000C; the 300 0C degrees would be LRV and the 500 0C degrees would d ld b d th d ld be URV.7 8. o Zero and Span: alternative descriptions to LRV and URV for the 0% and 100% points of an instruments calibrated range. Zero the beginning-point of an instruments range (equivalent to LRV), Span the width of its range (URV LRV). Spano Controller: A device that receives a process variable (PV) signal from transmitter, then compares that signal to the desired value (SP), and calculates an appropriate output signal value to be sent to a final control element such as an electric motor or control valve valve. 8 9. Final Control Element (FCE): A device that receives signal from the controller to directly influence the process. Examples: variable-speed electric motor, control valve, valve electric heater. heaterManipulated Variable (MV): The output signal generated b the controller. This is the signal d by h ll h h l commanding manipulating the final control element to influence the process.Automatic mode: When the controller generates an g output signal based on the relationship of process variable (PV) to the setpoint ( ) ( ) p (SP).Manual mode: When the controllers decision-making ability is bypassed to let a human operator directly determine the output signal sent to FCE.9 10. ESSENTIAL ELEMENTS OF A WATER LEVEL CONTROL SYSTEM, SHOWING TRANSMITTER, CONTROLLER, AND CONTROL VALVE:10 11. EXAMPLE: WASTEWATER DISINFECTION11 12. EXAMPLE: CHEMICAL REACTOR TEMPERATURE CONTROL12 13. OTHER TYPESOF INSTRUMENTSIndicators Indicators provide a human- readable indication of an instrument signal.Indicators give a convenient way of seeing what I di t i i t f i h t the output of the transmitter is without having to connect test equipmentIndicators may be located far from their respective transmitters, providing readouts in locations more convenient than the location of the transmitter itself. 13 14. 14 15. NUMERICAL AND BARGRAPH PANEL-MOUNTED INDICATOR15 16. LESS SOPHISTICATED STYLE OF PANEL-MOUNTED INDICATOR SHOWS ONLY A NUMERIC DISPLAY16 17. FIELD-MOUNTED INDICATORS17 18. RECORDERS Chart recorder or a trend recorder used to draw a graph of process variable(s) over time. h f i bl ( ) tiRecorders usually have indicators for showing the instantaneous value of the instrument signal(s) simultaneously with the historical values.18 19. CIRCULAR CHART RECORDER USES A ROUND SHEET OF PAPER19 20. STRIPCHART RECORDER ON THE RIGHT AND APAPERLESS CHART RECORDER ON THE LEFT20 21. EXAMPLE OF A TYPICALTREND SHOWING THE RELATIONSHIP BETWEENPROCESS VARIABLE, SETPOINT, AND CONTROLLER OUTPUT IN AUTOMATIC MODE, AS GRAPHED BY A RECORDER:21 22. 22 23. PROCESS SWITCHES AND ALARMS Process switch is used to turn on and off with varying process conditions conditions.Usually, switches are used to activate alarms to alert human operators to take special action.In other situations, switches are directly used as control devices.23 24. THEFOLLOWING P&ID OF A COMPRESSED AIR CONTROL SYSTEM SHOWS BOTH USES OF PROCESS SWITCHES:24 25. ALARM MODULE25 26. ANNUNCIATORS Process alarm switches may be used to trigger a special type of indicator device known as an annunciator.An annunciator is an indicator lights designed to secure a human operator s attention by blinking and sounding an audible operators buzzer when a process switch actuates into an abnormal state.The alarm state may be then acknowledged by an operator pushing a button, causing the alarm light to remain on (solid) rather than blink, and silencing the buzzer.The indicator light does not turn off until the actual alarm condition (the process switch) has returned to its regular state.26 27. ANANNUNCIATOR LOCATED ON A CONTROLPANEL FOR A LARGE ENGINE-DRIVEN PUMP27 28. A SIMPLE LOGIC GATE CIRCUIT ILLUSTRATES THE ACKNOWLEDGMENT LATCHING FEATURE(HERE IMPLEMENTED BY AN S R LATCH S-RCIRCUIT) COMMON TO ALL PROCESS ALARM ANNUNCIATORS:28 29. INSTRUMENT CALIBRATION Most instruments contain a f ilit f M t i t t t i facility for making t ki two adjustments. dj t t These are 1. 2. The RANGE adjustment. The ZERO adjustment.In order to calibrate the instrument an accurate gauge is required. required This is likely to be a SECONDARY STANDARD STANDARD. Instruments calibrated as a secondary standard have themselves been calibrated against a PRIMARY STANDARD.PROCEDURE An input representing the minimum gauge setting should be applied. The output should be adjusted to be correct. Next the maximum signal is applied. The range is then adjusted to give the required output. This should be repeated until the gauge is correct at the minimum and maximum values.29 30. CALIBRATION ERRORS RANGE AND ZERO ERRO After obtaining correct zero and range for the instrument, a calibration graph should be produced. This involves plotting the indicated reading against the correct reading from the standard g g gauge. This should be done in about ten steps with increasing p g signals and then with reducing signals. Several forms of error could show up. If the zero or range is still incorrect the error will appear as shown shown.30 31. HYSTERESIS and NON LINEAR ERRORS Hysteresis is produced when the displayed values are too small for increasing signals and too large for decreasing signals. signals This is commonly caused in mechanical instruments by loose gears and linkages and friction. It occurs widely with things involving magnetisation and demagnetisation. The Th calibration may b correct at th maximum and minimum lib ti be t t the i d i i values of the range but the graph joining them may not be a straight line (when it ought to be). This is a non linear error. The inst ment ma ha e some adj stments fo this and it instrument may have adjustments for may be possible to make it correct at mid range as shown.31 32. TRANSMITTERS A transmitter sends representative signal of the value of measured variable from the sensor to the indicator or controller. This has the advantage of keeping hazardous process fluids outside the control room and allows the use of a common signal range. Transmitter picks up the measurement provided by the sensor and converts it to a standard signal range. Sensors and transmitters are combined in to one device. The two most common types of transmission used in industry are. 1.Pneumatic, and2.Electronic 32 33. Pneumatic System Air systems operate in the range of 3 to 15 psi (0.2 to 1.0 Bar) and make use of small bore piping to transmit the signal around the plant. The main advantages of this system are: 1. Freedom to a certain extent from the fear of electrical power failures 2. Abilit 2 Ability to t a transmit a d send signals th o gh it and e d ig al through hazardous areas without the fear of explosion. 3. Noise immunity from external sources Unfortunately as the transmission distances increase y lags the measurement system increase and some distortion of the signal occurs. 33 34. Electronic Electronic systems make use of cables to send current signals in range of 4-20 mA around the use of a li zero used in current t f live di t transmission, allows i i ll for an easy method to detect a loss of signal due to cab e damage. u t e the current s the same cable da age. Further t e cu e t is t e sa e at all points the lo

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