I&C CHAPTER 1

INTRODUCTION TO INDUSTRIAL

INSTRUMENTATION

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CHAPTER HIGHLIGHTS

Instrumentation is the science of automated

measurement and control.

Applications of this science be plentiful in modern Applications of this science be plentiful in modern

research, industry, and everyday living.

This chapter explains some of the fundamental

i i l f i d t i l i t t ti principles of industrial instrumentation.

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In Oil & Gas industry, the first step, naturally, is y, p, y,

measuring the process variables such as; pressure,

fl l l l i l flow, level , temperature, analytical, …etc.

Once the process variable measured, transmit a p ,

signal representing this quantity to an indicating or

d h h hcomputing device where either human or

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

3measured.

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 call the process. To show this as a general block

diagram:

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INSTRUMENTATION TERMS AND THEIR DEFINITIONS:

Process: The physical system we are attempting to control or measure Examples: oil refinery unit control or measure. Examples: oil refinery unit, water filtration system, steam boiler, power generation unitgeneration 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 Setpoint (SP): The value at which we desire the process variable to be maintained at. In other words the “target” value of the process variable

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words, the target value of the process variable.

Primary Sensing Element (PSE): A device that

di l h i bl d l h directly senses the process variable and translates that

sensed quantity into an analog representation (electrical

voltage, current, resistance; mechanical force, motion,

etc.). Examples: thermocouple, RTD, bourdon tube, ) a p o oup , , bou do ub ,

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 performs some sort of processing on that signal.

Examples: I/P converter, P/I converter, square-root

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extractor.

Transmitter: A device that translates PSE signal

d d d l hinto a standardized instrumentation signal such as;

pneumatic 3-15 psi, electrical 4-20 mA DC, Fieldbus

digital signal packet, etc.,

Lower- and Upper-range values (LRV & URV):

The values of process measurement deemed to be e a ues o p ocess easu e e t dee ed 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

d ld b LRV d th 500 0C d ld 7

degrees would be LRV and the 500 0C degrees would

be URV.

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 the width of its range (URV LRV).

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 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

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such as an electric motor or control valve.

Final Control Element (FCE): A device that receives signal from the controller to directly influence the signal from the controller to directly influence the process. Examples: variable-speed electric motor, control valve electric heatervalve, electric heater.

Manipulated Variable (MV): The output signal d b h ll h h lgenerated by the controller. This is the signal

commanding “manipulating” the final control element to influence the process.

Automatic mode: When the controller generates an goutput signal based on the relationship of process variable (PV) to the setpoint (SP).( ) p ( )

Manual mode: When the controller’s decision-making ability is bypassed to let a human operator directly

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ability is bypassed to let a human operator directly determine the output signal sent to FCE.

ESSENTIAL ELEMENTS OF A WATER LEVEL CONTROL SYSTEM, SHOWINGTRANSMITTER, CONTROLLER, AND CONTROL VALVE: TRANSMITTER, CONTROLLER, AND CONTROL VALVE:

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EXAMPLE: WASTEWATER DISINFECTION

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EXAMPLE: CHEMICAL REACTOR TEMPERATURE CONTROL

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OTHER TYPES OF INSTRUMENTS

IndicatorsIndicators

Indicators provide a human- readable indication of an instrument signal.

I di t i i t f i h t Indicators give a convenient way of seeing what the output of the transmitter is without having to connect test equipment

Indicators may be located far from their Indicators may be located far from their respective transmitters, providing readouts in locations more convenient than the location of the locations more convenient than the location of the transmitter itself.

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NUMERICAL AND BARGRAPH PANEL-MOUNTED INDICATOR

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LESS SOPHISTICATED STYLE OF PANEL-MOUNTED INDICATOR SHOWSONLY A NUMERIC DISPLAY

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FIELD-MOUNTED INDICATORS

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RECORDERS

Chart recorder or a trend recorder used to draw a

h f i bl ( ) ti graph of process variable(s) over time.

Recorders usually have indicators for showing the Recorders usually have indicators for showing the

instantaneous value of the instrument signal(s)

simultaneously with the historical values.

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CIRCULAR CHART RECORDER USES A ROUND SHEET OF PAPER

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STRIP CHART RECORDER ON THE RIGHT AND APAPERLESS CHART RECORDER ON THE LEFTPAPERLESS CHART RECORDER ON THE LEFT

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EXAMPLE OF A TYPICAL “TREND” SHOWING THE RELATIONSHIP BETWEEN

PROCESS VARIABLE SETPOINT AND CONTROLLER OUTPUT IN AUTOMATIC MODE PROCESS VARIABLE, SETPOINT, AND CONTROLLER OUTPUT IN AUTOMATIC MODE,

AS GRAPHED BY A RECORDER:

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PROCESS SWITCHES AND ALARMS

Process switch is used to turn on and off with

varying process conditions varying process 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.

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THE FOLLOWING P&ID OF A COMPRESSED AIR CONTROLSYSTEM SHOWS BOTH USES OF PROCESS SWITCHES: SYSTEM SHOWS BOTH USES OF PROCESS SWITCHES:

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ALARM MODULE

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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 human operator s attention by blinking and sounding an audible

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 The indicator light does not turn off until the actual alarm

condition (the process switch) has returned to its regular state.

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AN ANNUNCIATOR LOCATED ON A CONTROLPANEL FOR A LARGE ENGINE-DRIVEN PUMPPANEL FOR A LARGE ENGINE DRIVEN PUMP

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A SIMPLE LOGIC GATE CIRCUIT ILLUSTRATES THE ACKNOWLEDGMENT

LATCHING FEATURE (HERE IMPLEMENTED BY AN S R LATCHLATCHING FEATURE (HERE IMPLEMENTED BY AN S-R LATCH

CIRCUIT) COMMON TO ALL PROCESS ALARM ANNUNCIATORS:

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INSTRUMENT CALIBRATIONM t i t t t i f ilit f ki t dj t tMost instruments contain a facility for making two adjustments.

These are1. The RANGE adjustment.2. The ZERO adjustment.

In order to calibrate the instrument an accurate gauge isrequired This is likely to be a SECONDARY STANDARDrequired. This is likely to be a SECONDARY STANDARD.Instruments calibrated as a secondary standard havethemselves been calibrated against a PRIMARY STANDARD.

PROCEDUREPROCEDURE An input representing the minimum gauge setting should be

applied. The output should be adjusted to be correct. Nextthe maximum signal is applied. The range is then adjusted togive the required output. This should be repeated until thegauge is correct at the minimum and maximum values.

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CALIBRATION ERRORSRANGE 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 gauge. This should be done in about ten steps with increasing g g p gsignals 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 shownas shown.

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HYSTERESIS and NON LINEAR ERRORSHYSTERESIS and NON LINEAR ERRORS Hysteresis is produced when the displayed values are too

small for increasing signals and too large for decreasingsignals This is commonly caused in mechanical instrumentssignals. This is commonly caused in mechanical instrumentsby loose gears and linkages and friction. It occurs widely withthings involving magnetisation and demagnetisation.Th lib ti b t t th i d i i The calibration may be correct at the maximum and minimumvalues of the range but the graph joining them may not be astraight line (when it ought to be). This is a non linear error.The inst ment ma ha e some adj stments fo this and itThe instrument may have some adjustments for this and itmay be possible to make it correct at mid range as shown.

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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 control room and allows the use of a common signal range. Transmitter picks up the measurement provided by the sensor and converts 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

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Pneumatic SystemPneumatic 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 thesignal around the plant.

The main advantages of this system are:1. Freedom to a certain extent from the fear of electrical

power failures2 Abilit to t a it a d e d ig al th o gh2. Ability to transmit and send signals through

hazardous areas without the fear of explosion.3. Noise immunity from external sources3. Noise immunity from external sources

Unfortunately as the transmission distances increaseylags the measurement system increase and somedistortion of the signal occurs.

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Electronic Electronic systems make use of cables to send

current signals in range of 4-20 mA around the use f li d i t t i i ll of a live zero used in current transmission, allows

for an easy method to detect a loss of signal due to cable damage. Further the current is the same at cab e da age. u t e t e cu e t s t e sa e at all points the loop.

Electrical signals do not suffer from lag and signal di i bl h l i i distortion problems when long transmission distances are encountered. Unfortunately they can Suffer from noise problems and all signals are lost Suffer from noise problems and all signals are lost when the power fails unless an Uninterruptible power Supply (UPS) Is available. The main

bl i h h l i l Si l i h l i problem with the electrical Signals is the explosion risk in hazardous areas.

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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 analogue current 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 i t i maintain.

Smart field instruments may not only address this issues but can provide further benefits the features of Smart 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.

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HIGHWAY ADDRESSABLE REMOTE TRANSDUCER (HART) PROTOCOLHIGHWAY ADDRESSABLE REMOTE TRANSDUCER (HART) PROTOCOL

HART field communications protocol is widely accepted in theindustry as the standard for digitally enhanced 4-20 mAcommunication with smart field instruments. The HART protocolwas designed specifically for use with intelligent measurementand control instruments that traditionally communicate using 4-and control instruments that traditionally communicate using 420 mA analogue signals HART preserves the 4-20 mA signal andenables two–way digital communications to occur withoutdi t bi th i t it f th 4 20 A i l U lik thdisturbing the integrity of the 4 -20 mA signal. Unlike otherdigital communication technologies the HART protocol maintainscompatibility with existing 4 -20 mA systems and in doing soprovides users with uniquely compatible solution the HARTprotocol permits the process variable to be transmitted by the 4 -20 mA analogue signal and additional information about other20 mA analogue signal and additional information about othervariables parameters , device configuration , calibration anddevice diagnostics to be transmitted digitally at the same time .

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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 superimpose digital communications at a low level on top of the 4 20 mA signal.

This enables two-way field communications to take place and makes it possible for additional information beyond just the normal process variable to 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).

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As with most protocol-based systems the manufacturer tries to “tie-you” into his equipment The therefore if you use a HART based you into his equipment. The therefore if you use a HART based system you cannot attach Honeywell equipment and expect it to work. Hopefully with the final introduction of the Field-bus foundation protocol this will change and a greater flexibility will emerge.p g g 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.

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Intelligent (microprocessor based) measurementSMART INSTRUMENTS

Intelligent (microprocessor based) measurement.Digital data communication Includes diagnostic information as well as ProcessPopular in hybrid 4-20 mA mode.

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