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...

Read More

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...

Read More

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...

Read More

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...

Read More

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...

Read More

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...

Read More

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...

Read More

Process Variables

SETPOINTThe setpoint is a value for a process variable that is desired to bemaintained. For example, if a process temperature needs to keptwithin 5 °C of 100 °C, then the setpoint is 100 °C. A temperaturesensor can be used to help maintain the temperature at setpoint.The sensor is inserted into the process, and a contoller compares thetemperature reading from the sensor to the setpoint. If the temperaturereading is 110 °C, then the controller determines...

Read More

PROCESS VARIABLE

A process variable is a condition of the process fluid (a liquid or gas)that can change the manufacturing process in some way. In the example of you sitting by the fire, the process variable was temperature. In the example of the tank in Figure 7.1, the process variable is level. Common process variables include: Pressure Flow Level Temperature Density Ph (acidity or alkalinity) Liquid interface (the relative amounts of different liquids that...

Read More

The Control Loop

Imagine you are sitting in a cabin in front of a small fire on a coldwinter evening. You feel uncomfortably cold, so you throw anotherlog on the fire. Thisis an example of a control loop. In thecontrol loop, a variable (temperature) fell below the setpoint (yourcomfort level), and you took action to bring the process back into thedesired condition by adding fuel to the fire. The control loop willnow remain static until the temperatureControl loops...

Read More

The Importance of Process Control

Refining, combining, handling, and otherwise manipulating fluids to profitably produce end products can be aprecise, demanding, and potentially hazardous process. Small changes in a process can have a large impacton the end result. Variations in proportions, temperature, flow, turbulence, and many other factors must becarefully and consistently controlled to produce the desired end product with a minimum of raw materials andenergy. Process control...

Read More

Introduction To Process Control

Control in process industries refers to the regulation of all aspects of the process. Precise control of level,temperature, pressure and flow is important in many process applications.Process control refers to the methods that are used to control processvariables when manufacturing a product. For example, factors suchas the proportion of one ingredient to another, the temperature of thematerials, how well the ingredients are mixed, and the pressure...

Read More

VFD- VFD OPERATION

Understanding the basic principles behind VFD operation requires understanding the three basic sections of the VFD: the rectifier, dc bus, and inverter.The voltage on an alternating current (ac) power supply rises and falls in the pattern of a sine wave(see Figure 1). When the voltage is positive, currentflows in one direction; when the voltage is negative,the current flows in the opposite direction. This typeof power system enables large amounts...

Read More

Variable Frequency Drives (VFD)

A variable-frequency drive (VFD) (also termed adjustable-frequency drive, variable-speed drive, AC drive, micro drive or inverter drive) is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage.VFDs are used in applications ranging from small...

Read More

What is a Relay

         A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one...

Read More

Rotary Encoder

A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code.There are two main types: absolute and incremental (relative). The output of absolute encoders indicates the current position of the shaft, making them angle transducers. The output of incremental encoders provides information about the motion of...

Read More