Instrumentation Diploma ● Concept of Process Control ● Pressure Measurements ● Level Measurements ● Flow Measurements ● Temperature Measurements ● Control Valves ● Distributed Control System (DCS) 2 Concept of Process Control Moataz Sherif Senior Instrumentation and Control Engineer Concept of Process Control ● Introduction ● 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 4 Introduction 5 Industrial Instrumentation ● Instrumentation is the science of automated measurement and control. ● The first step, naturally, is measurement. If we can’t measure something, it is really pointless to try to control it. 6 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.
7 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. 8 Control System ● A system which responds to input signals from the process and/or from an operator and generates output signals causing the process to operate in the desired manner. ● The control system include ○ Input devices ○ Controller ○ Final elements Definition from IEC 61511-1 9 Control System Control system choice depends on ● Purpose. ● Equipment Under Control (EUC).
10 Control System Control system can be Control system can used as a ● Pneumatic ● Basic Process Control ● Electric System ● Electronic ● Safety Instrumented ● Programmable System Electronic ● Combined BPCS-SIS 11 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 12 Pneumatic Control System 13 Programmable Logic Control (PLC) 14 Turbo machinery Control System 15 Boiler Control System 16 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 17 BPCS vs. SIS ● SIS is a protection layer located to prevent the Hazards from occurring. 18 Safety Instrumented System The system consists of ● Sensors ● Logic Solver ● Final control element 19 Process Control Definition 20 Process Control Definition ● A process is broadly defined as an operation that uses resources to transform inputs into outputs. ● It is the resource that provides the energy into the process for the transformation to occur.
21 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. 22 Process Control Definition 23 Terminology: ● 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. 24 Terminology: ● 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.
25 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. 26 Basic Elements of Control Loop 27 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.
28 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 29 Basic Elements of Process Control 30 Basic Elements of Process Control 31 Open Loop and Closed Loop Control 32 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. 33 Open Loop Control 34 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.
● Disturbances are uncontrolled changes in the process inputs or resources. 35 Example for Open Loop Control 36 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. 37 Example for Open Loop Control 38 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.
39 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 40 Example for Closed Loop Control 41 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. 42 Example for Closed Loop Control 43 Closed Loop Control ● Setting of the output variable is normally not ideal: ○ If the intervention is too fast and too great, influence at the input end of the system is too large. This results in great fluctuations at the output. ○ If influence is slow and small, the output variable will only approximate to the desired value.
44 Closed Loop Control Modes 45 Closed Loop Control Modes ● Closed loop control can be, depending on the algorithm that determines the controller output: ■ Manual ■ On-Off ■ PID ■ Advanced PID (ratio, cascade, feedforward) ■ or Model Based 46 Manual Control Mode ● In manual control an operator directly manipulates the controller output to the final control element to maintain a desired setpoint. ● Used in abnormal conditions when maintenance is required for measuring instruments. 47 Manual Control Mode 48 On-Off Control Mode ● provides a controller output of on or off in response to error. 49 On-Off Control Mode ● Upon changing the direction of the controller output, deadband is the value that must be traversed before the controller output will change its direction again.
50 On-Off Control Mode 51