JABATAN KEJURUTERAAN MEKANIKAL POLITEKNIK SEBERANG PERAI process control & instrumentation neoh hock seng KHUSHAIRY AHMAD NAWAWI mody namics
PROCESS CONTROL & INSTRUMENTATION Neoh Hock Seng Khushairy Ahmad Nawawi 2023 Mechanical Engineering Department ©All rights reserved for electronic, mechanical, recording, or otherwise, without prior permission in writing from Politeknik Seberang Perai.
eBook PSP | Process Control & Instrumentation ii All rights reserved No part of this publication may be translated or reproduced in any retrieval system, or transmitted in any form or by any means, electronic, mechanical, recording, or otherwise, without prior permission in writing from Politeknik Seberang Perai. Published by Politeknik Seberang Perai Jalan Permatang Pauh, 13500 Permatang Pauh Pulau Pinang Editor Ruhil Naznin Azaman Content Reviewer Syafirul Ikmar Shaharudin Cover Designer Azilah Abd Rahim Tel : 04-538 3322 Fax : 04-538 9266 Email : [email protected] Website : www.psp.edu.my FB : politeknikseberangperai Ig : politeknikseberangperai eISBN 978-967-2774-44-0 Neoh Hock Seng, editor Khushairy Ahmad Nawawi, editor PROCESS CONTROL & INSTRUMENTATION/ Editor Neoh Hock Seng, Khushairy Ahmad Bin Nawawi 2023 Politeknik Seberang Perai, Electronic books Power Plant - - Closed systems - - Open system - - Government publications - - Malaysia - -
iii eBook PSP | Process Control & Instrumentation Acknowledgment The authors would like to express their gratitude to the Centre of Technology for Teaching and Learning (CTTL) unit of Seberang Perai Polytechnic for having been allowed to undertake the task of writing this e-book. Neoh Hock Seng, Khushairy Ahmad Nawawi
eBook PSP | Process Control & Instrumentation iv Preface Currently, there is a lack of reference materials suitable for Malaysian polytechnic students in process control and instrumentation. Most of the reference books recommended are geared toward engineers and are too theoretical to be useful for technicians. Hopefully, this eBook addresses this problem. This e-book provides a broad overview of this field to students. The materials focus on important principles and practical aspects which will be useful and relevant in the workplace. The understanding gained from this book should equip aspiring technicians to gain the necessary knowledge at the beginning of their careers.
v eBook PSP | Process Control & Instrumentation Table of Content 01 Introduction To Process Control Systems Definition and Objectives of Process Control Systems Manual and Automatic Process Control Systems Standard Terms Relevant to Process Control Systems Self Assessment 1 2 3 4 5 02 Process Control Loops Single Variable Loops Pressure Control Loop Level Control Loop Flow Control Loop Temperature Control Loop Multivariable loops Feedforward and Feedback Loops Cascade Control Split Range Control SCADA Systems Computerized Process Control Systems Self Assessment 15 16 16 17 18 19 19 20 22 23 24 25 26 03 Components of Control Loops and ISA Symbology Sensors Transducers Converters Transmitter Signals Indicators Recorders Controllers ISA Symbology Self Assessment 33 34 35 36 37 38 38 39 40 40 41 References 50
01 INTRODUCTION TO PROCESS CONTROL SYSTEMS This chapter introduces the scope and objectives of process control systems. The basic concepts and common terminologies associated with process control systems are discussed.
2 eBook PSP | Process Control & Instrumentation Definition and Objectives of Process Control Systems Process Process refers to the methods of changing or refining raw materials to create end products. The raw materials in liquid or gaseous form are mixed, heated, filtered, measured, transferred, stored, and handled to produce an end product. This is common in the oil and gas, chemical, pharmaceutical, water treatment, and power industries. Process Control Process control refers to the methods used to control process variables (temperature, pressure, flow rate, and level) when manufacturing a product. For instance, the temperature of materials or operating pressure can determine the quality of the end product. Process control ensures that the magnitudes of the process variables are within acceptable ranges or setpoints. Objectives of Process Control Systems There are 3 objectives for implementing process control: Reduce variability. Accurate process control to increase efficiency. Ensure safety. Variability occurs when there is a lack of precise control of a process variable. For instance, when the temperature of a liquid used to produce paint varies considerably from the setpoint, it will affect the quality of the paint. In process control, the reduction in variability ensures high-quality products at an optimum cost. Accurate control of process variables is important to maintain process efficiency. Certain process variables must be maintained at certain magnitudes to ensure maximum efficiency. Efficiency enables resources to be optimized to produce high-quality products. Safety in process control is an important issue. For instance, a boiler with poor control of the inlet flow of air during combustion and outflow of air from the exhaust can result in implosion. This will threaten the safety of workers in the surrounding area. Therefore,
eBook PSP | Process Control & Instrumentation 3 precise control of the inflow and outflow of the air at a desired pressure is required for safety. Manual and Automatic Process Control Systems Manual Control Before the advent of automation, process control was implemented manually. For instance, Figure 1.1 shows a person manually adjusting the valve to release excess water from the tank. In this case, the manual operation enabled the water level to revert to its desired level or setpoint. Process control systems that require human intervention to control process variables are called manual control systems. Figure 1.1 Manual Control of a Valve Process control systems that require minimal or no human intervention are called automatic control systems. For example, Figure 1.2 shows the control valve automatically adjusting its opening based on the signal received from the flow controller. The optimal adjustment of the valve opening ensures that the flow rate achieves its set point. This process is automated to enable continuous operation without human intervention.
4 eBook PSP | Process Control & Instrumentation Figure 1.2 Automatic Control of a Valve Standard Terms Relevant to Process Control Systems Process Variable The process variable refers to the condition of the process fluid (liquid or gas) that can change the manufacturing process. The main process variables include level, pressure, temperature, and flow rate. Other process variables include density, pH, mass, turbidity, and conductivity. Set Point, r The set point is the desired value for a process variable and is to be maintained. For instance, if the desired room temperature is 250C, 250C is the setpoint temperature. If the water level in a tank is to be maintained at 1 m, 1 m is the setpoint level. Open Loop Process Control System In an open loop process control system, the process variable is not compared to any setpoint, and the control action is taken without regard to the process variable conditions. The absence of a feedback element in Figure 1.3 depicts this. No corrective action is performed.
eBook PSP | Process Control & Instrumentation 5 Figure 1.3 Open Loop Process Control Systems Closed Loop Process Control System A closed-loop process control system involves the comparison of a measured process variable with the setpoint. Corrective action is taken if there is a deviation from the setpoint. Figure 1.4 shows a closed-loop process control system in which the feedback element measures the process variable or output. The feedback signal, f, which represents the measured variable, is compared with the setpoint. If there is a deviation between the measured variable and the setpoint, the error signal is produced. Based on the resulting error, the controller produces the manipulated variable to the actuator to reduce the deviation. Figure 1.4 Closed Loop Process Control Systems Manipulated Variable, m. The manipulated variable directly affects the output of a process control system. It is adjusted to bring back the process variable to the setpoint. For example, the manipulated variable modulates the opening of a valve to adjust the flow rate of liquid until the setpoint flow rate is reached.
6 eBook PSP | Process Control & Instrumentation Figure 1.5 shows the corrective action in process control systems. The measured process variables are fed to the controller which compares these values with the setpoints. The controller then produces a manipulated variable which brings back the process variables to the setpoints. Figure 1.5 Corrective Action in Process Control Systems Error, e. Error is the difference between the measured process variable and the setpoint. Mathematically, the error, e = r ± f. Errors can be either positive or negative. For example, if the measured variable (temperature) of a furnace is 900C and the setpoint temperature is 1000C, the error is positive 100C. There are three important aspects of errors in process control systems viz: -error magnitude, duration of error, and the rate of change of error. Offset Offset is the sustained deviation of the process variable from the setpoint. For instance, if the temperature of a furnace for the duration of its operation is 990C and the setpoint temperature is 1000C, an offset of 10C has been produced. Feedback Element The feedback element measures the output or process variable of the process control system. The feedback signal, f, from the feedback element is compared with the setpoint. The difference produces an error signal. Examples of feedback elements are temperature, flow rate, pressure, and level sensors. Controller The controller calculates errors in the process control systems. The errors are produced by the difference between the measured process variable and the setpoint. It produces the manipulated variable sent to the actuators for corrective actions.
eBook PSP | Process Control & Instrumentation 7 Actuators and Final Control Elements These elements directly perform corrective action. For instance, a valve is actuated by a servo motor to modulate its opening to achieve the desired flow rate of liquid to a tank. The valve in this case is the final control element and the servo motor is the actuator. Output The output refers to the value of the process variable measured during operation. It is also called the controlled variable. The main problem with the output is that it deviates from the setpoint. Load Disturbance Load disturbance is an undesired change that can affect the process variable. For example, in Figure 1.6, the addition of cold fluid will lower the temperature of the fluid (lower than the setpoint temperature of 1000C) in the tank. In this case, the cold fluid is the load disturbance. Figure 1.6 Load Disturbance
8 eBook PSP | Process Control & Instrumentation SELF ASSESSMENT Question 1. What are the advantages of reducing variability in process control? A. Assured safety. B. Increase the reaction rate of the process. C. Increase the efficiency of the process. D. Ensure consistently high-quality end products. Question 2. Which tasks are associated with process control? i. Measure ii. Compare iii. Quality control iv. Calculation v. Adjustment A. i, ii, iii B. ii, iii and iv. C. i, ii and v D. ii, iv and v Question 3. Which variables are considered process variables in a process control system? i. Flow rate ii. Temperature iii. Pressure iv. Level A. i and ii B. ii, iii and iv C. i, iii and iii D. i, ii, iii and iv
eBook PSP | Process Control & Instrumentation 9 Question 4. In an open-loop controlled system, the input __________________. A. Has no control over the output. B. Has good control over the output. C. Has optimum control over the output. D. All of the above. Question 5. In a closed loop-controlled system, the input _________________________. A. Has no control over the output. B. Has good control over the output. C. Both A and B. D. None of the above. Question 6. Set point __________________________. A. Monitors the output of the process control system. B. Refers to the output of the process control system. C. Is the desired output value of the process control system. D. Is the amplified output of the process control system. Question 7. _________ is the deviation from the set point due to load disturbance. A. Offset. B. Rate of change of error. C. Error. D. Feedback.
10 eBook PSP | Process Control & Instrumentation Question 8. ____________ is a continuous error due to the inability of a process control system to maintain the measured variable at the setpoint. A. Pressure. B. Offset. C. Load disturbance. D. Deviation. Question 9. The main function of a controller is _________________________. A. To reduce error by sending a modulating signal to an actuator. B. To eliminate errors by sending a modulating signal to an actuator. C. To reduce error by monitoring the output of a process control system. D. To eliminate disturbances by amplifying the sensor signals. Question 10. The function of a feedback element is to _________________________. i. Conditions the output signals to signal forms suited to the controller. ii. The output of the feedback element takes multiple forms such as pressure and currents. iii. Determines the manipulated variable signals. iv. The output from the feedback element is compared to the set point value in the controller. A. i, iii B. i, ii, iii C. iii, iv D. i. ii, iv Question 11. This factor is used to maintain the controlled variable at a desired value. A. Offset. B. Set Point. C. Controlled Variable. D. Error.
eBook PSP | Process Control & Instrumentation 11 Question 12. In a closed-loop controlled system, the variable that is regularly controlled at the output is __________________________. A. Set Point. B. Controlled Variable. C. Sensor Amplification. D. Manipulated Variable. For questions 13 to 16, match each sentence to the correct term. A. Load disturbance. B. Manual control. C. Setpoint. D. Manipulated Variable. . Questions 13. A process control system that directly involves human intervention. Questions 14. An undesired change in a factor that affects the process control system. Questions 15. This factor is changed to maintain the measured variable at the setpoint. Questions 16. A value or range of values for a process variable must be maintained to keep the process operating properly.
12 eBook PSP | Process Control & Instrumentation For questions 17 to 19, match each sentence to the correct term. A. Closed-loop, automatic control. B. Closed-loop, manual control. C. Open-loop, automatic control. Questions 17. An operator turns off the heater coil when the temperature transmitter outputs a certain reading. Questions 18. A controller turns off the heater coil at set intervals, regardless of the process temperature. Questions 19. A thermocouple measures temperature and sends the result to a controller to be compared to the setpoint. Subsequently, the controller turns off the heater coil. Questions 20. A liquid level needs to be maintained within 5 cm of 120 cm in a tank. A pressure transmitter monitors the liquid level using a pressure reading and sends the result to the controller. The controller compares the level reading with the setpoint and adjusts the liquid flow at the inlet and outlet pipes of the tank. i. What is the process variable? ii. What is the manipulated variable? iii. What is the measured variable? iv. What is the setpoint?
eBook PSP | Process Control & Instrumentation 13 ANSWER: 1. D 2. C 3. D 4. A 5. B 6. C 7. C 8. B 9. B 10. D 11. B 12. B 13. B 14. A 15. D 16. C 17. B 18. C 19. A 20 i. Liquid level ii. The flow of liquid to the tank iii. Pressure iv. 120 cm
02 PROCESS CONTROL LOOPS This chapter discusses how the process control systems can be implemented through the single and multivariable loop methods. Specialized control systems originating from these basic methodologies are also discussed in terms of current applications in the industry.
16 eBook PSP | Process Control & Instrumentation Single Variable Loop Figure 2.1 shows the block diagram involving a single variable loop. The feedback element measures the process variable, c, and sends a feedback signal, f, to the controller. In the controller, the feedback is compared with the setpoint, r. If any deviation occurs, corrective action is taken to enable the magnitude of the process variable to reach the set point. In this case, a single process variable is involved. Figure 2.1 Single Variable Loop In the following sections, single-variable loops involving process variables such as pressure, flow, level, and temperature will be discussed. Pressure Control Loop Figure 2.2 shows a process control system involving a pressure control loop. The pressure transmitter measures the discharge pressure of the pump. The pressure controller modulates the opening and closing of the relief valve to adjust the pressure of the process fluid being pumped. Pressure control loops vary in speed. They can respond to load changes quickly or slowly. The speed required in a pressure control loop is dictated by the volume of the process fluid. High-volume systems tend to change more slowly than low-volume systems.
eBook PSP | Process Control & Instrumentation 17 Figure 2.2 Pressure Control Loop Level Control Loop The speed of change in level control loops depends on the size and shape of the process vessel and the flow rate of fluids at the inflow and outflow pipes. The final control elements are usually control valves on the input connection to the tank. Figure 2.3 shows the level control loop for a fluid level control system. The level transmitter measures the fluid level in the tank and sends a feedback signal to the controller. The controller emits a modulating signal to the control valve to control the inflow of the process fluid to the tank to maintain the setpoint level. Figure 2.3 Level Control Loop
18 eBook PSP | Process Control & Instrumentation Flow Control Loop Flow control loops are regarded as fast loops and respond to changes quickly. The flow control equipment must have fast responses and sampling times. The problem faced is that the flow transmitters are rather sensitive and can produce noise and fluctuations in control signals. To overcome this problem, the flow transmitters have damping functions that reduce the noise. Filters are also installed between the flow transmitters and controllers to further reduce noise. In practice, temperature measurement is also taken with flow measurements. The temperature compensation is accounted for in the flow rate calculation. Figure 2.4. Flow Control Loop Figure 2.4 shows a flow rate control loop. The flow transmitter measures the flow rate of the process fluid discharged from the pump. The measured flow rate is fed to the controller. The controller adjusts the opening of the valve to enable the flow rate at the discharge to reach the setpoint flow rate.
eBook PSP | Process Control & Instrumentation 19 Temperature Control Loop Temperature control loops tend to be relatively slow in that it takes time to change the temperature of process fluids. Resistance temperature detectors and thermocouples are commonly used as temperature sensors. The final control elements are usually the fuel valve to a burner or a valve to a heat exchanger. Figure 2.5 shows a heating system involving the temperature control loop. The temperature transmitter measures the temperature in the tank and sends a feedback signal to the controller. The controller adjusts the opening of the steam valve to maintain the process fluid temperature in the tank at the set point temperature. Figure 2.5. Temperature Control Loop Multivariable Control Loop Multivariable loops are control loops in which a primary controller (master) controls one process variable by sending signals to a secondary controller (slave) that impacts the process variable of the primary loop. Figure 2.6 illustrates the concept of multivariable loops. The primary process variable is the temperature of the fluid in the tank. The fluid is heated by the steam jacket. The secondary controller controls the steam pressure. The primary controller manipulates the setpoint of the secondary controller to maintain the setpoint temperature of the fluid. In practice, the secondary loop is tuned before the primary loop. This is because the adjustments to the secondary loop will impact the primary loop. However, tuning the primary loop will not impact the secondary loop.
20 eBook PSP | Process Control & Instrumentation Figure 2.6. Multivariable Control Loop Feedforward and Feedback Control Loop Figure 2.7. Feedforward Loop
eBook PSP | Process Control & Instrumentation 21 Feedforward control systems anticipate load disturbances and control them before affecting the process variable. For feedforward control to work, the user must have a mathematical understanding of how the manipulated variable will impact the process variable. Figure 2.7 depicts the concept of the feedforward loop. The controller adjusts the steam valve based on the feedback from the flow transmitter. In other words, the opening or closing of the steam valve depends on the amount of cold fluid that passes through the tank. The advantage of feedforward control is the prevention rather than the correction of errors. However, it is difficult to account for all load disturbances. Factors such as external temperature, humidity, consistency of raw materials, and moisture can become load disturbances and cannot all be effectively accounted for. Figure 2.8. Feedforward and Feedback Loop Since feedforward systems cannot account for all load disturbances, they are often combined with feedback systems. Controllers with summing functions are used in these combined systems to sum the inputs from both the feedforward and feedback loop and then the combined signal is sent to the final control element. Figure 2.8 shows a combined system in which the flow and temperature transmitters provide information to control the hot steam valve.
22 eBook PSP | Process Control & Instrumentation Cascade Control Cascade control is a control system in which a secondary control loop (slave) is set up to control a variable that is a major source of disturbance for another primary control loop (master). The controller of the primary loop determines the setpoint of the summing controller in the secondary loop. Figure 2.9 shows the concept of cascade control. Figure 2.9. Cascade Control Figure 2.10. Cascade Control
eBook PSP | Process Control & Instrumentation 23 Figure 2.10 shows a cascade control system to control the temperature of fluid in the tank. The inner control loop (Temperature Controller 2 and Thermocouple 2) detects the disturbance from the pump. The disturbance is in the form of a variation of temperature of the heat transfer fluid resulting from the pump action. Temperature controller 1 receives the temperature reading from thermocouple 1. The output from thermocouple 1 becomes the setpoint for temperature controller 2. The input to temperature controller 2 (part of the inner control loop) is the temperature reading of thermocouple 2 and the output from temperature controller 1. The output from temperature controller 2 is the manipulated variable signal to modulate the opening of the control valve to enable the correct volume and flow of the heating fluid to maintain the set point temperature of the liquid in the tank. The advantage of cascade control systems is that they quickly respond to load disturbances. Split Range Control Figure 2.11. Split Range Control Split range control is a method that makes it possible to control several valves by using a single controller. Figure 2.11 shows a pressure control system for a separator tank in the split range mode. The pressure transmitter measures the pressure in the separator tank and sends the measurement to the controller. The measured pressure is compared with
24 eBook PSP | Process Control & Instrumentation the setpoint pressure. If the measured pressure is lower than the setpoint pressure, the controller will open the gassing valve. On the other hand, if the measured pressure is higher than the setpoint pressure, the degassing valve is opened. In practice, the opening and closing of the valves are split according to the signal range of the controller. Figure 2.12 illustrates this mode of operation. At 0 to 50 percent controller output, the gassing valve (valve 1) is opened, and the degassing valve (valve 2) is closed. At 50 to 100 percent controller output, the gassing valve (valve 1) is closed, and the degassing valve (valve 2) is opened. The opening range of the valve for each case is from 0 to 100 percent. For instance, if the controller output is 20 percent, valve 1 is opened at 60 percent, and valve 2 is closed. Figure 2.12. Split Range Control for Valves SCADA Systems SCADA (Supervisory Control and Data Acquisition) systems are supervisory control systems in which a supervisory computer system controls multiple processes. Each process under control possesses its controller. The SCADA system optimizes the process control by calculating new setpoints for each controller. The controllers are called ‘Programmable Logic Controllers‘(PLCs).
eBook PSP | Process Control & Instrumentation 25 Figure 2.13. SCADA Systems Figure 2.13 depicts the SCADA system. The supervisory computer reads the measured flow rate of the liquid through the pump and the liquid level in the tank. The flow rate of the liquid and liquid level in the tank is measured by the flow and level transmitters respectively. These measured values are sent to the PLSs which act as controllers. The measured values are compared with their respective setpoints. Should deviations occur, the supervisory computer calculates new setpoints for each process. PLC1 adjusts the pump speed to enable the flow rate of the liquid to reach the setpoint flow rate. Likewise, PLC2 adjusts the opening of the valve to enable the liquid level in the tank to reach the setpoint level. Feedforward and Feedback Control Loop Figure 2.14 depicts the concept of computerized process control systems. The computer acts as the controller and is interfaced with the analog-to-digital converter (ADC) and digital to analogue converter (DAC). The ADC converts the analogue signal from the sensors to the equivalent digital form to be processed by the computer. The DAC converts the digital signal from the computer to the analogue control signal which is sent to the final control elements. The multiplexers are used to selectively activate the channels of the ADC and DAC. Figure 2.15 shows the difference between the digital and analogue signal forms. The digital signal is in the discrete form and is either in the ‘high’ or ‘low’ value. The analogue signal changes its value concerning time.
26 eBook PSP | Process Control & Instrumentation Figure 2.14. Computerized Process Control Systems Figure 2.15. Digital and Analogue Signals Figure 2.16 shows a computerized temperature and pressure control system for a chemical mixing process. The ratio of the chemical mix is dependent upon the pressure and temperature in the tank. The pressure and temperature sensors are activated by the input multiplexers. The measured values in the form of analogue signals are converted to their digital equivalent by the ADC component. The digital signals are interpreted by the computer as measured temperature and pressure values and compared with their respective setpoints. Subsequently, the computers would send the manipulated signal to the valves. Since the output of the computer is in digital form, the DAC converts the digitized output from the computer to its analogue equivalent to modulate the openings of the valves.
eBook PSP | Process Control & Instrumentation 27 Figure 2.16. Computerized Process Control Systems for Chemical Mixing
28 eBook PSP | Process Control & Instrumentation SELF ASSESSMENT Question 1. The elements that are involved in the signal flow from the input to the output of the Closedloop control systems are ________________________________. i. Final Control Elements ii. Controller iii. Feedback Elements iv. Process. A. i, ii B. i, ii, iii C. i, ii, iv D. i, ii, iii, iv Question 2. Figure 2.16 shows a closed loop control system. What is its controlled variable? Figure 2.16 A. Temperature. B. Level. C. Flow Rate. D. Pressure.
eBook PSP | Process Control & Instrumentation 29 Question 3. Figure 2.17 shows a control system for a pressurized tank. What are the variables involved in stages A, B, C, and D? Figure 2.17 A. A = Feedback signal, B = Error, C = Manipulated Variable, D = Controlled Variable. B. A = Input, B = Error, C = Feedback Signal, D= Controlled Variable. C. A = Input, B = Feedback Signal, C = Manipulated Variable, D = Controlled Variable. D. A = Feedback Signal, B = Setpoint, C = Manipulated Variable, D = Controlled Variable. Question 4. Which component sends the feedback signal to the controller in a closed-loop control system? A. Comparator. B. Transmitter. C. Actuator. D. Signal Amplifier. Question 5. In a closed loop-controlled system, the variable that is regularly measured at the output is the _______________________________________. A. Set Point. B. Process Variable. C. Sensor Amplification. D. Manipulated Variable.
30 eBook PSP | Process Control & Instrumentation ANSWER: 1. C 2. C 3. C 4. B 5. B Question 6. Draw the closed loop block diagrams for the following processes. Explain the control actions based on the error reduction for each case. a) Figure 2.18 b) Figure 2.19
eBook PSP | Process Control & Instrumentation 31 6 a) If the flowrate in the pipe exceeds the setpoint flowrate, the differential pressure cell (DP Cell) sends a feedback signal (the measured pressure corresponds to the flow rate of the liquid in the pipe) to the controller. The comparator algorithm in the controller produces an error value. The controller in turn outputs a manipulated variable signal to reduce the opening of the control valve. This is to enable the flow rate to revert to the setpoint value. On the other hand, if the flow rate in the pipe falls below the setpoint flow rate, the controller increases the opening of the control valve to increase the flow rate to the setpoint value. 6 b) If the temperature of the liquid in the tank exceeds the setpoint temperature, the thermocouple sends a feedback signal to the controller. The controller will calculate the appropriate manipulated variable signal, based on the error produced, to reduce the opening of the control valve. This action decreases the amount of heating fluid in the tank. As a result, the liquid temperature in the tank is reduced to the setpoint temperature. On the other hand, If the temperature of the liquid in the tank is lower than the setpoint temperature, the controller will increase the opening of the control valve to increase the amount of heating fluid. This is done to increase the liquid temperature to the set point temperature.
03 COMPONENTS OF CONTROL LOOPS & ISA SYMBOLOGY This chapter describes the instruments and technologies used to develop and maintain process control loops. In addition, this chapter describes how the process control equipment is represented by the ISA symbols.
34 eBook PSP | Process Control & Instrumentation Sensors Sensors are sensing devices that measure changes in process variable measurements. Sensors are the first elements in the control loop that measure the process variable, They are also called primary elements. Examples of sensors are pressure-sensing diaphragms, resistance temperature detectors (RTDs), thermocouples, orifice plates, pitot tubes, venturi tubes, magnetic flow tubes, ultrasonic transmitters, and receivers. Primary elements cause changes in their properties with changes in process fluid conditions that can be measured. An example of how sensors measure process variables can be shown by RTDs. RTDs are used to measure the temperature of fluids. As the temperature of the fluid surrounding the RTD changes, the electrical resistance of the RTD changes accordingly in a proportional amount. The resistance of the RTD is measured and from the magnitude of this resistance, the fluid temperature is determined. Figure 3.1 shows the physical structure of the RTD. Figure 3.1 Resistance Temperature Detector
eBook PSP | Process Control & Instrumentation 35 Table 3.1 lists some of the types of sensors used for the measurement of various process variables. In Chapter 4, the operating principles of the common sensors used in process control will be explained in detail. Table 3.1 Common Sensors Used in Process Control Type of Sensor Process Variable Thermocouples Temperature Resistance Temperature Detector (RTD) Bourdon Tube Pressure Gauge Pressure Bellow Pressure Gauge Strain Pressure Gauge Capacitive Pressure Gauge Piezoelectric Pressure Gauge Orifice Flow Rate Venturi Tube Vortex Flowmeter Rotameter Ultrasound Flowmeter Float Level Gauge Level Float Switch Magnetic Level Gauge Bubble Tube Ultrasonic Sensor Transducers & Converters A transducer is a device that translates mechanical signals into electrical signals. For instance, inside a pressure capacitance device, a transducer converts a change in pressure to a proportional change in capacitance. A converter is a device that converts one type of signal into another type of signal. For example, a converter may convert a current signal into a voltage signal. An ADC (analogue to digital converter) converts an analogue signal into a digital signal. In process control, a current-to-pressure converter is used to convert a 4-20mA current signal range to a 3-15 psi pneumatic signal range. A transmitter is a device that converts a reading from a sensor or transducer into a standard signal. The signal is then transmitted to a controller or monitor. Transmitter types
36 eBook PSP | Process Control & Instrumentation include level transmitters, temperature transmitters, flow transmitters, pressure transmitters, and analytic transmitters. Signals In process control, there are 3 kinds of signals, representing the measured process variables. that are transmitted from the instrument to the centralized control system. These signals are pneumatic, analogue, and digital signals. Pneumatic signals are produced by changes in air pressure in proportion to changes in the measured process variables. The standard pneumatic signal range is from 3 to 15 psig.3 psig corresponds to the lower range value and the 15 psig corresponds to the upper range value. The most common analogue signal is the 4-20mA current signal. The 4mA represents the lowest possible measurement and the 20mA represents the highest possible measurement corresponding to a measured process variable. Figure 3.2 illustrates this correspondence. The RTD produces a signal of 4mA corresponding to the lowest temperature of 90oC in the vessel. The 20mA signal corresponds to 110oC. The transmitter transmits a signal of 12mA when the setpoint temperature of 100oC is reached. Therefore, the current signals can be converted into their corresponding temperature readings. The signals can also be used to modulate the opening of a valve. Digital signals are the most recent addition to process control signals. Digital signals are discrete values that are combined in specific ways to represent process variables. The methodology used to combine the digital signals is known as a protocol. There are two types of protocol viz: - the open and proprietary protocol. Open protocols can be used by anyone to develop control devices. Proprietary protocols are owned by specific companies and can be used only with the owner’s consent.
eBook PSP | Process Control & Instrumentation 37 Figure 3.2 Use of Analogue Signals Indicators Indicators are devices that display information about the process in operation. Indicators can take the form of simple designs such as analogue temperature or pressure gauges and more complex designs such as digital gauges. The indicators display measured process variables such as temperature, pressure, flow, and level. Figures 3.3 and 3.4 show the analogue and digital gauges respectively. Figure 3.3 Analogue Gauge Figure 3.4 Digital Gauge
38 eBook PSP | Process Control & Instrumentation Recorders A recorder is a device that records the output of measurement devices. Many process manufacturers are required by law to record and provide process history to regulatory agencies in their respective countries. The recorded data is used for analysis. For instance, recording the readings of critical measurement points enables comparison with the data of a normal or desired process. This ensures that the process can be improved in the future. Different recorders display the recorded data differently. Some recorders list the readings at certain time intervals. Others create charts or graphs of readings. Recorders that create charts or graphs are called chart recorders. Figure 3.5 shows a typical chart recorder used in process control. Figure 3.5 Chart Recorder Controllers A controller is a device that receives data from a device measurement or sensor. The data is compared to a programmed setpoint. If there is a difference between these two values an error is considered to have occurred. The controller then sends a signal to a control element or actuator to perform the corrective actions. The controllers used are usually one of three types viz: -pneumatic, electronic, and programmable.
eBook PSP | Process Control & Instrumentation 39 Controllers perform complex mathematical functions in which the measured process variables are compared to the setpoints. In process control, controllers are important because they receive feedback from sensors and send signals to the final control elements to carry out corrective actions. Examples of controllers used in process control systems are programmable logic controllers (PLC) and distributed control systems (DCS). PLCs are usually connected to input and output devices. The PLCs are programmed to respond to inputs by sending outputs to maintain all processes at setpoints. (See Figure 2.13) DCS are controllers that provide readings and status of a process, maintain databases, and implement man-machine interfaces. DCSs can control large-scale processes that are interconnected. Final Control Elements Figure 3.6 Temperature Control Loop The final control element or the correcting element is controlled by the manipulated variable from the controller. In most cases, the final control elements are valves that restrict or cut off fluid flow. Other examples of final control elements are levers, pumps, and solenoids. In process control, final control elements are used to increase or decrease fluid flow. Figure 3.6 shows a temperature control loop in which the final control element is the fuel gas valve. The valve regulates the flow of fuel to the burner.
40 eBook PSP | Process Control & Instrumentation Actuators Figure 3.7 Actuation of Valves An actuator is part of the final control element that actuates the final control element itself when it has received a signal from a controller. For example, the actuator opens or closes the stem of a control valve to control the flow of fluids. Figure 3.7 shows the actuation of a valve to control the flow of fluids. The actuator can be powered pneumatically, electrically, or hydraulically. ISA Symbology The Instrumentation, Systems, and Automation (ISA) Society is one of the leading process control organizations. The ISA has developed a set of symbols for use in engineering drawings and the design of process control loops. Process control engineers and technicians should be familiar with the ISA symbols to correctly label instrument symbols, their locations, and the types of signals used. The following ISA symbols shown are most commonly used in piping and instrumentation diagrams representing control loops.
eBook PSP | Process Control & Instrumentation 41 Table 3.2 ISA symbols Symbol Instrument Field Mounted Instrument Instrument Located in a Primary Location Instrument Located in an Auxiliary Location Display and Control Inaccessible Instrument Flow Readings and Square Root Calculation Computerized Controller Table 3.3 ISA Symbols for Computerized Controller Symbol Instrument Field Mounted Primary Location
42 eBook PSP | Process Control & Instrumentation Auxiliary Location Inaccessible Programmable Logic Controller (PLC) Table 3.4 ISA symbols for Programmable Logic Controller (PLC) Symbol Instrument Field Mounted Primary Location Auxiliary Location Inaccessible
eBook PSP | Process Control & Instrumentation 43 Pump Table 3.5 ISA symbols for Pump Symbol Equipment Pump Valves and Actuators Table 3.6 ISA symbols for Valve and Actuators Symbol Instrument Electrical Valve Pneumatic Valve Manual Valve