Ch. 1 introduction to industrial instrumentation

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Ch. 1 introduction to industrial instrumentation

  1. 1. I&C CHAPTER 1 INTRODUCTION TO INDUSTRIAL INSTRUMENTATION 1
  2. 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 ti 2
  3. 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 l  Once 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. 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. 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. 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. 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 transmitter’s 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. 8. o Zero and Span: alternative descriptions to LRV and URV for the 0% and 100% points of an instrument’s calibrated range.  “Zero” the beginning-point of an instrument’s range (equivalent to LRV),  “Span” the width of its range (URV − LRV). Span o 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. 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. heater  Manipulated 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 controller’s decision-making ability is bypassed to let a human operator directly determine the output signal sent to FCE. 9
  10. 10. ESSENTIAL ELEMENTS OF A WATER LEVEL CONTROL SYSTEM, SHOWING TRANSMITTER, CONTROLLER, AND CONTROL VALVE: 10
  11. 11. EXAMPLE: WASTEWATER DISINFECTION 11
  12. 12. EXAMPLE: CHEMICAL REACTOR TEMPERATURE CONTROL 12
  13. 13. OTHER TYPES OF INSTRUMENTS Indicators  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 equipment  Indicators may be located far from their respective transmitters, providing readouts in locations more convenient than the location of the transmitter itself. 13
  14. 14. 14
  15. 15. NUMERICAL AND BARGRAPH PANEL-MOUNTED INDICATOR 15
  16. 16. LESS SOPHISTICATED STYLE OF PANEL-MOUNTED INDICATOR SHOWS ONLY A NUMERIC DISPLAY 16
  17. 17. FIELD-MOUNTED INDICATORS 17
  18. 18. RECORDERS  Chart recorder or a trend recorder used to draw a graph of process variable(s) over time. h f i bl ( ) ti  Recorders usually have indicators for showing the instantaneous value of the instrument signal(s) simultaneously with the historical values. 18
  19. 19. CIRCULAR CHART RECORDER USES A ROUND SHEET OF PAPER 19
  20. 20. STRIP CHART RECORDER ON THE RIGHT AND A PAPERLESS CHART RECORDER ON THE LEFT 20
  21. 21. EXAMPLE OF A TYPICAL “TREND” SHOWING THE RELATIONSHIP BETWEEN PROCESS VARIABLE, SETPOINT, AND CONTROLLER OUTPUT IN AUTOMATIC MODE, AS GRAPHED BY A RECORDER: 21
  22. 22. 22
  23. 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. 24. THE FOLLOWING P&ID OF A COMPRESSED AIR CONTROL SYSTEM SHOWS BOTH USES OF PROCESS SWITCHES: 24
  25. 25. ALARM MODULE 25
  26. 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 operator’s 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. 27. AN ANNUNCIATOR LOCATED ON A CONTROL PANEL FOR A LARGE ENGINE-DRIVEN PUMP 27
  28. 28. A SIMPLE LOGIC GATE CIRCUIT ILLUSTRATES THE ACKNOWLEDGMENT LATCHING FEATURE (HERE IMPLEMENTED BY AN S R LATCH S-R CIRCUIT) COMMON TO ALL PROCESS ALARM ANNUNCIATORS: 28
  29. 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. 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. 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. 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, and 2. Electronic 32
  33. 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. 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 loop.  Electrical signals do not suffer from lag and signal distortion problems when long transmission di i bl h l i i distances are encountered. Unfortunately they can Suffer from noise problems and all signals are lost when the power fails unless an Uninterruptible power Supply (UPS) Is available. The main problem with the electrical Signals is the explosion bl ih h l i l Si l i h l i risk in hazardous areas. 34
  35. 35. Digital  The traditional favorite means communicating a process signal from the field to the control room is via the 4-20 mA analogue current loop loop.  This is a fast reliable industry standard but it leaves a lot to be desired in terms of maintenance capabilities, performance and diagnostics.  Analogue field instruments can be expensive to install and maintain. i t i  Smart field instruments may not only address this issues but can provide further benefits the features of Smart transmitters issues but can provide further benefits.  The features of Smart transmitters can provide substantial benefits to users in terms of time and labor savings as well as providing an increase in plant operating safety. 35
  36. 36. HIGHWAY ADDRESSABLE REMOTE TRANSDUCER (HART) PROTOCOL  HART field communications protocol is widely accepted in the industry as the standard for digitally enhanced 4-20 mA communication with smart field instruments. The HART protocol was designed specifically for use with intelligent measurement and control instruments that traditionally communicate using 44 20 mA analogue signals HART preserves the 4-20 mA signal and enables two–way digital communications to occur without disturbing th i t di t bi the integrity of th 4 -20 mA signal. U lik other it f the 20 A i l Unlike th digital communication technologies the HART protocol maintains compatibility with existing 4 -20 mA systems and in doing so provides users with uniquely compatible solution the HART protocol permits the process variable to be transmitted by the 4 20 mA analogue signal and additional information about other variables parameters , device configuration , calibration and device diagnostics to be transmitted digitally at the same time . 36
  37. 37.    HART makes use the technic of frequency shift keying (FSK) standard to superimpose digital communications at a low level on top of the 4 -20 mA 20 signal. This enables two-way field communications to take place and makes it possible for additional information beyond just the normal process variable to be communicated to from a smart field instrument the HART protocol allows a host application (master) to get two or more digital updates per second from a field device which is not fast enough for most applications. HART is a master slave protocol, that means the field (slave) device only speaks when spoken to by master sends out command signal (C) and the slave sends back a response (R). 37
  38. 38.   As with most protocol-based systems the manufacturer tries to “tieyou you” into his equipment. The therefore if you use a HART based equipment system you cannot attach Honeywell equipment and expect it to work. Hopefully with the final introduction of the Field-bus foundation p protocol this will change and a g g greater flexibility will emerge. y g The HART protocol permits all digital communication with field devices in installation saving are possible with the multi-drop networking capabilities of HART. 38
  39. 39. SMART INSTRUMENTS  Intelligent  Digital (microprocessor based) measurement measurement. data communication  Includes  Popular diagnostic information as well as Process in hybrid 4-20 mA mode. 39

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