IntroductionWhat is an automatic process control? Control in which regulating and switching operations are performed automatically in response to predetermined conditions

IntroductionWhy automatic process control is important? Automatic control systems enable us to operate our processes in a safe and profitable manner. Control systems achieve this "safe and profitable" objective by continually measuring process variables such as temperature, pressure, level, flow and concentration - and taking actions such as opening valves, slowing down pumps and turning up heaters - all so that the measured process variables are maintained at operator specified set point values

IntroductionProcess Control TerminologyProcess Variable (PV): What you are trying to control: temperature, pressure, flow, composition, pH, etc. Also called the measurement.Set-Point (SP): The set-point is where you would like the process variable to be. Error (e): Error = SP - PV. In auto mode, the controller uses the error in its calculation to find the output that will get you to the setpoint.

IntroductionProcess Control TerminologyController Output (CO): Output of the controller. In auto mode the controller calculates the output based on its calculation using the error signal (difference between SP and PV). In manual mode, the user sets the output.Manipulated Variable (MV): Process condition or variable that can be changed to cause the process variable to change.Disturbance (D): process condition that tends to change the value of the process variable.

IntroductionComponents of a Control Systemcontroller seeks to maintain the measured process variable (PV) at set point (SP) in spite of unmeasured disturbances (D). The major components of a control system include a sensor, a controller and a final control element.

IntroductionComponents of a Control SystemSensor: Device which is used to measure process variable e.g. flow orifice, Bourdon tube, thermocouple.Controller: Device that keeps process variable at the setpoint.Final control element: Device which directly changes the manipulated variable e.g. control valves, pump, compressor.

Control ArchitectureFeedback controlFeedforward with feedback controlCascade controlSplit-range controlRatio controlOverride control

Feedback ControlControllerFinal ControlElement (FCE)Measurement Sensor/Transmitterset point, SPmeasured processvariable signal, PVcontroller errore(t) = (SP-PV)controller output signal (CO)processvariablemanipulatedvariable (MV) Disturbance, DFeedback Control Block Diagram

Feedback ControlGas pipeline 1Gas pipeline 2Without control system, flowrate through this line depends on the pressure difference between these 2 gas pipelines.

Feedback ControlGas pipeline 1Gas pipeline 2FCCOPVSP

Feedback ControlWhen to use feedback control?Normally feedback control is the first choice when you want to apply automatic control to your system because of its simplicity.

Feedback ControlAdvantages of feedback controlCan handle unforeseen disturbances such as frictional and pressure losses. Ensures the desired performance by altering the inputs immediately once deviations are observed regardless of what caused the disturbance. By analyzing the output of a system, unstable processes may be stabilized. Feedback controls do not require detailed knowledge of the system and, in particular, do not require a mathematical model of the process. Feedback controls can be easily duplicated from one system to another.

Feedback ControlDisadvantages of feedback controlNot suitable for system having large dead time.Can only take input from one sensor.The controller responds to the disturbance after its occurrence, so perfect control of the system is theoretically impossible.Feedback control does not take predictive control action towards the effects of known disturbances.

Cascade ControlCascade Control Block Diagram

Cascade ControlPIflow to other usersLCL SetpointPVCOHeader pressure varies as other line valves move, disturbing our feed rate Exit flow

Cascade ControlPIflow to other usersLCL SetpointPV1CO1 = SP2Header pressure varies as other line valves move, disturbing our feed rate Exit flowFCCO2PV2Setpoint

Cascade ControlWhen to use cascade control?When a number of various disturbances might be expected in a single control loop. When a fast response is required and it cannot be achieved by using only single control loop.

Cascade ControlRequirements for Cascade ControlThe secondary control loop must be faster than the primary control loop.The secondary control loop receives the maximum disturbance.The secondary loop must include the worst expected disturbances.The secondary loop must have a direct influence on the primary loop.Secondary loop must be measured and controllable.

Cascade ControlAdvantages of Cascade ControlBetter control of the primary variable.Primary variable less affected by disturbances.Faster recovery from disturbances.

Disadvantages of Cascade ControlCost of measurement of secondary variable.Additional complexity.

Feedforward with Feedback ControlFeedforward with Feedback Control Block DiagramControllerFinal ControlElementProcessMeasurement Sensor/Transmitter+-set point, SPmeasured process variablesignal, PVcontroller errore(t) = (SP-PV)controller output signal (CO)manipulatedvariable processvariableDisturbance, DDisturbanceSensor/TransmitterFeedforwardComputation ElementDisturbance ProcessCO feedforward

Feedforward with Feedback ControlBFW from headerSteam to headerPV1L setpointLCSteam Drum2-Element Cascade ControlFCCOF setpoint

Feedforward with Feedback ControlBFW from headerSteam to headerPV1L setpointCOLCSteam Drum3-Element ControlFC

Feedforward with Feedback ControlWhen to use feedforward control?Concern is focused on one specific disturbance that is responsible for repeated, costly disruptions to stable operation.Reject disturbance(s) before it disturbs / disrupts the system. There is no practical secondary process variable which can be established.

Feedforward with Feedback ControlRequirements for feedforward control?Disturbances must be measured.A mathematical model of process is required.

Feedforward with Feedback ControlAdvantages of feedforward controlCorrective action taken BEFORE PV deviates from setpoint.

Disadvantages of feedforward controlCost of measurement of disturbance.Additional complexity.Not suitable for a system which has a number of disturbances.

Split-Range ControlSplit-Range Control Loop Block DiagramControllerFinal ControlElement 1 (FCE1)Final ControlElement 2 (FCE2)ProcessMeasurement Sensor/Transmitter+-set point, SPmeasured process variablesignal, PVcontroller errore(t) = (SP-PV)controller output signal (CO)CO 1CO 2manipulatedvariable 1processvariablemanipulatedvariable 2Splitter

Split-Range ControlPCGas to compressorGas to flareController Output0%100%0%100%% valve openingAB0-50%50-100%% opening valve A% opening valve B

Split-Range ControlPCGas from headerGas to flareController Output0%100%0%100%% valve openingAB0-50%50-100%% opening valve A% opening valve B

Split-Range ControlWhen to use split-range control Split-range control is used in situations where more than one manipulated variable is required.

Split-Range ControlAdvantages of split-range controlCan control over the wide rangeCan handle more than one disturbances at the same time.

Disadvantages of split-range controlAdditional cost for final control element.Difficulty in controller tuning.

Ratio ControlControllerFinal ControlElement 1 (FCE1)ProcessMeasurement Sensor/Transmitter+-set point, SPmeasured process variable signal, PV1controller errore(t) = (SP-PV)controller output signal (CO)manipulatedvariable processvariablemeasured process variable signal, PV2RatioRatio Control Block DiagramMeasurementSensor/Transmitter

Ratio ControlStorage TankProduct 1Product 2FTFCRYMixed ratioFTPV 1PV 2COSP

Ratio ControlWhen to use ratio control The ratio control architecture is used to maintain the flow rate of one stream in a process at a defined or specified proportion relative to that of another.

Ratio ControlRequirement for ratio control A ratio controller architecture requires that the signal from each flow sensor/transmitter change linearly with flow rate. Transmitter Signal4 mA20 mAFluid flowrate0 MMSCFD50 MMSCFD

Override ControlControllerFinal ControlElement (FCE)ProcessMeasurement Sensor/Transmitterset point, SP1measured process variablesignal, PVcontroller errore(t) = (SP-PV)controller output signal (CO1)processvariablemanipulatedvariable (MV) Disturbance, DControllerset point, SP2controller errore(t) = (SP-PV)controller output signal (CO2)Selectorselected COOverride Control Block Diagram

Override ControlFired HeaterFuel gasHeated process fluidPC>FC

Override ControlWhen to use override controlWhen there is a constraint in operation or equipment.

Advanced ControlFired HeaterAir BlowerHeated process fluidFuel gas from headerMax.Min.O2RY>FCY

Summary

Control ArchitecturePhilosophyAdvantagesDisadvantagesFeedbackControl adjusts error after it occurs- simple to design - no process model needed- correct error after it has already occurred. - usually takes input from one sensorFeedforwardControl corrects error before it occurs.- correct possible error before it occurs. - Ideally can produce a perfect control where there is never offset.- accurate process model is needed. - accurate measurement is needed. - increase complexity to the control system - additional costs required. RatioControl connects two flows to maintain a constant ratio.- generate defined ratio for given two streams. - simple model- flow sensor/transmitter has to change linearly with the flowrate.CascadeController output from one control is a setpoint of the other control loop.- responds quickly to high frequency changes - incorporate multiple sensor reading together.- add complexity to the process control - tuning model is difficult due to multiple setpoints and parameters.Override Control action from one controller takes priority - prevent operation or equipment from operating over its constraints.- make system unstable due to integral action. External feedback is needed.Split-rangeOne controller adjusts more than one manipulated variables.-can control over the wide range-can handle many disturbances-make controller tuning more difficult.