MULTI VARIABLE LOOPS/Advanced Type

Multivariable loops are control loops in which a primary controllercontrols one process variable by sending signals to a controller of adifferent loop that impacts the process variable of the primary loop.For example, the primary process variable may be the temperature ofthe fluid in a tank that is heated by a steam jacket (a pressurized steamchamber surrounding the tank). To control the primary variable(temperature), the primary (master) controller...

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Single Control Loops

PRESSURE CONTROL LOOPSPressure control loops vary in speed—that is, they can respond tochanges in load or to control action slowly or quickly. The speedrequired in a pressure control loop may be dictated by the volume ofthe process fluid. High-volume systems (e.g., large natural gas storagefacilities) tend to change more slowly than low-volume systems(Figure 7.21).FLOW CONTROL LOOPSGenerally, flow control loops are regarded as fast loops that respondto...

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Controller Tuning

Controllers are tuned in an effort to match the characteristics of thecontrol equipment to the process so that two goals are achieved:is the foundation of process control measurement in that electricity: The system responds quickly to errors. The system remains stable (PV does not oscillate around  the SP).GAINController tuning is performed to adjust the manner in which a controlvalve (or other final control element) responds to a change in...

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Controllers

The actions of controllers can be divided into groups based upon thefunctions of their control mechanism. Each type of contoller hasadvantages and disadvantages and will meet the needs of differentapplications. Grouped by control mechanism function, the threetypes of controllers are: Discrete controllers Multistep controllers Continuous controllersDISCRETE CONTROLLERSDiscrete controllers are controllers that have only two modes orpositions: on...

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ISA Standards Symbology

The Instrumentation, Systems, and Automation Society (ISA) is one ofthe leading process control trade and standards organizations. The ISAhas developed a set of symbols for use in engineering drawings anddesigns of control loops (ISA S5.1 instrumentation symbolspecification). You should be familiar with ISA symbology so that youcan demonstrate possible process control loop solutions on paper toyour customer.IDENTIFICATION LETTERSIdentification letters...

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Components of Control Loops

This section describes the instruments, technologies, and equipment used to develop and maintain processcontrol loops. In addition, this section describes how process control equipment is represented in technicaldrawings of control loops.PRIMARY ELEMENTS/SENSORSIn all cases, some kind of instrument is measuring changes in theprocess and reporting a process variable measurement. Some of thegreatest ingenuity in the process control field is apparent in sensingdevices. Because sensing devices are the first element in the controlloop to measure the...

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Process Signals

SIGNALSThere are three kinds of signals that exist for the process industry totransmit the process variable measurement from the instrument to acentralized control system.1. Pneumatic signal2. Analog signal3. Digital signalPneumatic SignalsPneumatic signals are signals produced by changing the air pressurein a signal pipe in proportion to the measured change in a processvariable. The common industry standard pneumatic signal range is3–15 psig. The...

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