Instruments used in industry are of 5 major classes:indicating,transmitting,controlling,final control & alarming type.Same instrument can have more than one functions like indicating,alarming,recording& transmitting.
1.Indicating:Meters of all sort,LED
2.Transmitting:DP Transmitter
3.Controlling:Controllers,micro computer
4.Final Control:control valve,damper,fan,motor,pumps
5.Alarm:smoke detectors,home security system,buffers,LED
Process Instrumentation is broadly classified as field instrumentation & control instrumentation.Field instrumentation mainly deals with instruments to indicate,transmit,alarm &final controlling.Improvement occur continously in this field .Control instrumentation deals with control systems like DCS( Distributed Control System ),SCADA( Supervisory Control And Data Acquisition ),PLC( Programmable Logic Control ),SIS( Safety Instrumented System ).
Actually there are 2 types of process industries:continous & batch type.Continous type of process industries include chemical,water treatment,power,oil & gas industry.In continous process industries the process variables ( Pressure,Temperature,Flow,Level etc) tend to approach a desired set value(Set Point).Batch type process industries include pharmaceutical,food & beverages industry.In batch type industries together with the process value approaching the set point value,all physical events are synchronized with sequence of events in process reciepe.
DCS is mainly used in plants of process industries where closed loop control is prominent like refinery & is interested in process trends.It is also process oriented & hardwired system which is always connected to data source.SCADA is mainly used in plants of process industries where long distance material transfer is prominent like oil & gas transmission,water supply & distribution,raiways,power transmission & distribution.It is also a data gathering oriented & process event driven system.PLC is used in plants where sequential control & motion control is prominent like machine control,packaging & material handling.SIS is the result of guidelines for safe operations of industrial installations in the petrochemical,oil and gas production devoloped by industrial groups,safety certifying agencies and insurance companies for more stingent regulation and legislation.
Before moving into the details of each process variable,process control & Control instrumentation,we will first go through functional(operation) & characteristics(performance) features of an instrument with some of the basic terms used in instrumentation field.
One of the main function of instrumentation is measurement without which no control is possible.Measurements have got to be one of the most important equipment in any processing plant. Any decision made on what the plant should do is based on what the measurements tell us. Measurement is actually the process of comparison between a predefined quantity( standard) & the quantity to be measured(measurand).Standard of measurement is the physical representation of a unit of measurement.The various standards of measurement are
1.International : at International Bereau Of Standards
2.Primary : at National labs
3.Secondary :Basic reference standard used in industrial labs
4.Working standard : Used commercially
The various measurement units doesnt require special mention like
1.Fundamental
2.Supplementary
3.Derived
Measurement system application are
1.used to regulate trade
2.used in monitoring function
3.caliberation of instruments
4.used as automatic feedback control system
FUNCTIONAL ELEMENTS OF AN INSTRUMENT/INSTRUMENT SYSTEM
Fuctional element of an instrument system provide how an instrument operates.
1.Primary Sensing Element => Sensor + Transducer
Sensor: sense the various physical variables related to measurement like force,length,temp,acceleration,velocity,pressure,frequency,capacitance,resistance & time
Transducer: convert one form of energy into another mainly electrical.
2.Signal Conditioning Element =>Variable Conversion & Variable Manipulation
Variable Conversion: Ex:Analog to Digital Convertor
Variable Manipulation: Ex:Amplifier,Filter,Isolation
Transmitter => Primary Sensing Element & Signal Conditioning Element
O/P of Transmitter =>4-20 mA,3-15 psi,0-10 V
3.Data Transmission Element => Telemetry system,Fiber Optic Cables
O/P of a transmitter corresponding to process variable measured value is transmitted via data transmission element to monitoring,recording,storage or controlling instrument.
4.Data Presentation Element =>Pointer moving on a scale,recording of a pen moving on chart.
5.Data Storage Element =>Printer,Digital Memory
If its a automatic feedback control system,The transmitter value is passed to a control system too.
.....to be continued.
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