Process control overview

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Instrumentation Diploma

●Concept of Process Control●Pressure Measurements

●Level Measurements●Flow Measurements

●Temperature Measurements●Control Valves

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<b>Concept of Process Control</b>

<b>Moataz Sherif</b>

<b>Senior Instrumentation and Control Engineer</b>

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Concept of Process Control

●Process Control Definition

●Basic Elements of Control Loop

●Open Loop and Closed Loop Control●Closed Loop Control Modes

●Sensors and Transducers

●Standard Instrument Signals●Smart Transmitters

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Introduction

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Industrial Instrumentation

●Once we measure the quantity we are interested in, we usually transmit a signal representing this quantity to an indicating or computing device where either human or automated action then takes place.

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Industrial Instrumentation

●If the controlling action is automated, the computer sends a signal to a final controlling device which then influences the quantity being measured.

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Control System

● A system which responds to inputsignals from the process and/orfrom an operator and generatesoutput signals causing the processto operate in the desired manner.● The control system include

○ Input devices○ Controller

○ Final elements

<small>Definition from IEC 61511-1</small>

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Control System

Control system can be

●Programmable Electronic

Control system can used as a

●Basic Process Control System

●Safety Instrumented System

●Combined BPCS-SIS

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Basic Process Control System

●Basic Process Control System (BPCS) is a system which responds to input signals from the process, its associated equipment, other programmable systems and/or an

operator and generates output signals causing the process and its associated equipment to operate in the desired

manner but which does not perform any safety

instrumented functions with a claimed SIL ≥ 1

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Pneumatic Control System

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Programmable Logic Control (PLC)

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Turbo machinery Control System

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Boiler Control System

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Safety Instrumented System

SIS takes some other names●Trip and Alarm system

●Emergency Shutdown System (ESD)●Safety Shutdown System

●Safety Interlock System

●Safety Related Control System

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BPCS vs. SIS

●SIS is a protection layer located to prevent the Hazards from occurring.

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Safety Instrumented System

The system consists of ●Sensors

●Logic Solver

●Final control element

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Process Control Definition

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Process Control Definition

●A process is broadly defined as an operation that usesresources to transform inputs into outputs.

●It is the resource that provides the energy into the process forthe transformation to occur.

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Process Control Definition

●Each process exhibits a particular dynamic (time varying) behavior that governs the transformation.

●That is, how do changes in the resource or inputs over time affect the transformation.

●This dynamic behavior is determined by the physical

properties of the inputs, the resource, and the process itself.

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Process Control Definition

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●The manipulated variable (MV) is a measure of resource being fed into the process, for instance how much thermal energy.●A final control element (FCE) is the device that changes the

value of the manipulated variable.

●The controller output (CO) is the signal from the controller to the final control element.

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●The process variable (PV) is a measure of the process output

that changes in response to changes in the manipulated variable.●The set point (SP) is the value at which we wish to maintain the

process variable at.

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Process Control Definition

●Process control is the act of controlling a final control element to change the manipulated variable to maintain the process variable at a desired set point.

●A corollary to our definition of process control is a controllable process must behave in a predictable manner.

●For a given change in the manipulated variable, the process variable must respond in a predictable and consistent manner.

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Basic Elements of Control Loop

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Basic Elements of Process Control

●Controlling a process requires knowledge of four basic elements:

○the process itself

○the sensor that measures the process value

○the final control element that changes the manipulated variable

○the controller.

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Basic Elements of Process Control

●Input devices used to see what’s going on in the process●Control Systems make decisions based on process inputs,

operator inputs, and control software●Output devices control the process

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Basic Elements of Process Control

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Basic Elements of Process Control

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Open Loop and Closed Loop Control

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Open Loop Control

●The open-loop control is where output variable does not have any influence on the input variable.

●In open loop control the controller output is not a function of the process variable.

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Open Loop Control

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Open Loop Control

●the controller output is fixed at a value until it is changed by an operator.

●Many processes are stable in an open loop control mode and will maintain the process variable at a value in the absence of a

●Disturbances are uncontrolled changes in the process inputs or resources.

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Example for Open Loop Control

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Example for Open Loop Control

●A system consists of the "valve" with the output variable

"volumetric flow" and the input variable "control valve setting".●This system can be controlled by adjusting the control valve. This

allows the desired volumetric flow to be set.

●if the applied pressure fluctuates, the volumetric flow will also fluctuate.

●In this open system, adjustment must be made manually.

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Example for Open Loop Control

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Closed Loop Control

●process where the controlled variable is continuously monitored and compared with the reference variable. ●Depending on the result of this comparison, the input

variable for the system is influenced to adjust the output variable to the desired value despite any disturbing

influences.

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Closed Loop Control

●Closed loop control is also called feedback or regulatory control.●The output of a closed loop controller is a function of the error.●Error is the deviation of the process variable from the set point

and is defined as

E = SP - PV

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Example for Closed Loop Control

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Closed Loop Control

●The controller now passes a signal to the manipulating element dependent on the deviation.

●If there is a large negative deviation, that is the measured value of the volumetric flow is greater than the desired value the valve is closed further.

●If there is a large positive deviation, that is the measured value is smaller than the desired value, the valve is opened further.

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Example for Closed Loop Control

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Closed Loop Control

●Setting of the output variable is normally not ideal:

○If the intervention is too fast and too great, influence at theinput end of the system is too large. This results in greatfluctuations at the output.

○If influence is slow and small, the output variable will onlyapproximate to the desired value.

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Closed Loop Control Modes

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Closed Loop Control Modes

● Closed loop control can be, depending on the algorithm that determines the controller output:

■ Manual■ On-Off

■ Advanced PID (ratio, cascade, feedforward)■ or Model Based

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Manual Control Mode

●In manual control an operator directly manipulates the

controller output to the final control element to maintain adesired setpoint.

●Used in abnormal conditions when maintenance is requiredfor measuring instruments.

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Manual Control Mode

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On-Off Control Mode

●provides a controller output of on or off in response to error.

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On-Off Control Mode

●Upon changing the direction of the controller output, deadband isthe value that must be traversed before the controller output willchange its direction again.

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On-Off Control Mode

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PID Control Mode

●provides output that changes from 0 to 100% in response to error.

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PID Algorithm

● A proportional-integral-derivative controller (PID controller) is acommon feedback loop component in industrial control systems.

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PID Algorithm

●The PID can adjust process outputs based on the history and rateof change of the error signal, which gives more accurate and stablecontrol.

●PID controllers can be easily adjusted (or "tuned") to the desiredapplication.

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PID Algorithm

●K

: Proportional Gain - Larger Kp typically means faster response since the larger the error, the larger the feedback to compensate.●K

: Integral Gain - Larger Ki implies steady state errors are

eliminated quicker..

●K

: Derivative Gain - Larger Kd decreases overshoot, but slows down transient response.

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PID Algorithm

1- Proportional:

●To handle the immediate error, the error is multiplied by a constant Kp (for proportional), and added to the controlled quantity.

●Kp is only valid in the band over which a controller's output is proportional to the error of the system.

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PID Algorithm

2- Integral:

●To learn from the past, the error is integrated (added up) over a period of time, and then multiplied by a constant K

(making an average), and added to the controlled quantity.

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PID Algorithm

3- Derivative:

●To handle the future, the first derivative (the slope of the error)over time is calculated, and multiplied by another constant K

, andalso added to the controlled quantity.

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PID Interacting Algorithm

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PID Interacting Algorithm

●The series or "interacting" form, where the output of each part ofthe controller is used as the input for another part, so that separateP, D and I controllers are connected together in series.

●This is effectively how older pneumatic and analog electroniccontrollers worked. It is the more restricted form of the two.

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PID Non-interacting Algorithm

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PID Non-interacting Algorithm

●The parallel or "non-interacting" form, where the P, I and D parts of the controller are all given the same error input in parallel and their output is added together.

●This allows independent adjustment of the proportional, integral and derivative constants.

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PID response graph

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PID response graph – single-step change

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Cascade Control Mode

●Cascade control uses the output of a primary (master or outer) controller to manipulate the set point of a secondary (slave or inner) controller as if the slave controller were the final control element.

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Cascade Control Mode

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Cascade Control Mode

●The purpose of cascade control is to achieve greater stability of the primary process variable by regulating a secondary process variable in accordance with the needs of the first.

●An essential requirement of cascaded control is that the secondary process variable be faster-responding than the primary process

variable.

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Cascade Control Mode - Example 1

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Cascade Control Mode - Example 1

●heated air is used to evaporate water from a granular solid.

●The primary process variable is the outlet air exiting the dryer, which should be maintained at a high enough temperature

●This outlet temperature is fairly slow to react, as the solid material mass creates a large lag time.

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Cascade Control Mode - Example 1

●There are several parameters influencing the temperature of the outlet air.

●These include air flow, ambient air temperature, and variations in steam temperature.

●If any of these parameters were to suddenly change, the effect would be slow to register at the outlet temperature

●Correspondingly, the control system would be slow to correct for

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Cascade Control Mode - Example 1

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Cascade Control Mode - Example 1

● Installing a second temperature transmitter at the inlet duct of the dryer, with its own controller to adjust steam flow at the command of the

primary controller will be a great solution.

● Now, if any of the loads related to incoming air flow or temperature vary, the secondary controller (TC-1b) will immediately sense the change in dryer inlet temperature and compensate by adjusting steam flow through the heat exchanger. Thus, the “slave” control loop (1b) helps stabilize the “master” control loop (1a) by reacting to load changes long before any effect might manifest at the dryer outlet.

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Cascade Control Mode - Example 2

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Cascade Control Mode - Example 2

●The “secondary” or “slave” flow controller works to maintain

feedwater flow to the boiler at whatever flow rate is desired by the level controller. If feedwater pressure happens to increase or

decrease, any resulting changes in flow will be quickly countered by the flow controller without the level controller having to react to a consequent upset in steam drum water level.

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Cascade Control Mode - Example 2

●Thus, cascade control works to guard against steam drum levelinstability resulting from changes in the feedwater flow caused byfactors outside the boiler.

●As stated previously, the slave (flow) controller effectively shieldsthe master (level) controller from loads in the feedwater supplysystem, so that master controller doesn’t have to deal with thoseloads.

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Feedforward control

●It is based on that if all significant loads on a process variable are monitored, and their effects on that process variable are well-understood.

●A control system programmed to take appropriate action based on load changes will shield the process variable from any ill effect.

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Feedforward control

●The feedforward control system uses data from load sensors to predict when an upset is about to occur, then feeds that information forward to the final control element to

counteract the load change before it has an opportunity to affect the process variable.

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Feedforward control

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Feedforward control

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● A cascaded (slave) flow controller (FC) senses outgoing flow via a flow transmitter (FT) and works to maintain whatever rate of flow is “asked”

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Feedforward control

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● Being a purely feedforward control system, there is no level transmitter (LT) any more, just flow transmitters measuring the three loads.

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Feedforward control

● If all flow transmitter calibrations are perfect, the summing of flow ratesflawless, and the flow controller’s tuning robust, this level control systemshould control liquid level in the vessel by proactive effort (“thinkingahead”) rather than reactive effort (“after the fact”).

● Any change in the flow rate of ingredients A, B, and/or C is quicklymatched by an equal adjustment to the discharge flow rate. So long astotal volumetric flow out of the vessel is held equal to total volumetric

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Feedforward control

● In reality, this pure feedforward control system is impractical even if all instrument calibrations and control calculations are perfect. There are still loads unaccounted for: evaporation of liquid from the vessel, for example, or the occasional pipe fitting leak.

● Furthermore, since the control system has no “knowledge” of the actual liquid level, it cannot make adjustments to that level

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Feedforward control

● If an operator, for instance, desired to decrease the liquid level he or shewould have to manually drain liquid out of the vessel, or temporarilyplace the discharge flow controller in “manual” mode and increase theflow there (then place back into “cascade” mode where it follows theremote setpoint signal again).

● The advantage of proactive control and minimum deviation from setpointover time comes at a fairly high price of impracticality and inconvenience.

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Feedforward control

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● This “trim” signal should do very little of the control work in this system, the bulk of the liquid level stability coming from the feedforward signals provided by the incoming flow transmitters.

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Feedforward control

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