Process Control Fundamentals and How to read P&IDs

Types of Process Control, Feedback control, feed-forward control loops, ratio control loop, split range control. How to read Piping and Instrumentation Diagram for Process Engineers

1. Process Control Fundamentals Eng. Ahmed deyab [email protected]

2. Introduction to Process Control Chemical plants are never truly at steady state . Feed and environmental disturbances , heat exchanger fouling , and catalytic degradation continuously upset the conditions of a smooth-running process Process dynamics refer to an unsteady-state or transient behavior. Steady-state vs. unsteady-state behavior Steady state: variables do not change with time

3. Small changes in a process can have a large impact on the end result.

4. Control To maintain desired conditions in a physical system by adjusting selected variables in the system .

5. What does a control system do? Imagine you are sitting in a cabin in front of a small fire on a cold winter evening. You feel uncomfortably cold , so you throw another log on the fire . This is an example of a control loop . In the control loop, a variable ( temperature ) fell below the setpoint ( your comfort level ), and you took action to bring the process back into the desired condition by adding fuel to the fire . The control loop will now remain static until the temperature again rises above or falls below your comfort level.

6. Why is Control necessary? Control is necessary because during its operation, a chemical plant must satisfy several requirements imposed by its designers and the general technical, economic, and social conditions in the presence of ever-changing external influences (disturbances).

7. Importance of Process Control for the Chemical Process Industries Manufacturers control the production process for three reasons: Reduce variability Increase efficiency Ensure safety Process Control has a major impact on the profitability of a company in the CPI.

8. Safety and Reliability

9. Safety The safe operation of a chemical process is a primary requirement for the well-being of the people in the plant and for its continued contribution to the economic development. Thus the operating pressures, temperatures, concentration of chemicals and so on should always be within allowable limits.

10. Production specifications A plant should produce the desired amounts and quality of the final products. For example, we may require the production of 2 million pounds of ethylene per day, of 99.5% purity. Therefore, a control system is needed to ensure that the production level and the purity specifications are satisfied.

11. Environmental regulations

12. Maximizing the Profit of a Plant Many times involves controlling against constraints. The closer that you are able to operate to these constraints, the more profit you can make. For example, maximizing the product production rate usually involving controlling the process against one or more process constraints.

13. Constraint Control Example

14. Types of Controllers

15. Temperature Control – Heat Exchanger

16. How is control done Control is accomplished through a rational arrangement of equipment (measuring devices, valves, controllers, computers) and human intervention (plant designers, plant operators), which together constitute a control system.

17. THREE TASKS Control loops in the process control industry work in the same way, requiring three tasks to occur: Measurement Comparison Adjustment

18. Loop Components

19. Important terms Controlled variable: it is the variable that needs to be maintained or controlled at some desired value or range. Sometimes also referred to as process variable. Set Point: it is the desired value of the controlled variable. Thus the job of a control system is to maintain the controlled variable at its set point. Manipulated variable is the variable used to maintain the controlled variable at its set point. Disturbance : any variable that causes the controlled variable to deviate from its set point. Also referred to as upset.

20. Heat Exchanger Control Controlled variable - Outlet temperature of product stream Manipulated variable - Steam flow Actuator - Control valve on steam line Sensor - Thermocouple on product stream Disturbance - Changes in the inlet feed temperature

21. Logic Flow Diagram for a Feedback Control Loop

22. Feedback vs. Feedforward Feedback Control action after an error exists Feedforward Reacting to the disturbance before the error occurs

23. Feedback Control The key feature of all feedback control loops is that the measured value of the controlled variable is compared with the set point and this difference is used to determine the control action taken.

24. Feed forward Control: 25 Distinguishing feature: measure a disturbance variable

25. Manual Feedforward Control

26. Feed forward Model (Boiler Steam Drum)

27. Typical Process Supply and Demand

28. Temperature Control Loop

29. Simple Feedback Loop

30. Feedback Control Loop

31. Level control The inlet flow comes from an upstream process, and may change with time The level in the tank must be kept constant in spite of these changes

32. Level controller The level controller (LC) looks at the level (monitoring) If the level starts to increase, the LC sends a signal to the output valve to vary the output flow (change) This is the essence of feedback control

33. Feedback control It is the most important and widely used control strategy It is a closed-loop control strategy Block diagram process transmitter controller disturbance comparator manipulated variable controlled variable + – error set-point y sp y

34. Back to level control LT LC SP Flow in Flow out desired value (set-point) transmitter controller controlled variable (measurement) manipulated variable disturbance process

35. Components of Control Loops • Primary element/sensor • Converter • Transducer • Transmitter • Indicator • Recorder • Controller • Correcting element/final control element • Actuator

36. PRIMARY ELEMENTS/SENSORS In all cases, some kind of instrument is measuring changes in the process and reporting a process variable measurement . Some of the greatest ingenuity in the process control field is apparent in sensing devices. Because sensing devices are the first element in the control loop to measure the process variable, they are also called primary elements

37. Examples of primary elements

38. Magnetic flow tubes Primary elements are devices that cause some change in their property with changes in process fluid conditions that can then be measured. For example, when a conductive fluid passes through the magnetic field in a magnetic flow tube, the fluid generates a voltage that is directly proportional to the velocity of the process fluid. The primary element (magnetic flow tube) outputs a voltage that can be measured and used to calculate the fluid’s flow rate.

39. RTD (Resistance temperature detectors ) With an RTD, as the temperature of a process fluid surrounding the RTD rises or falls, the electrical resistance of the RTD increases or decreases a proportional amount. The resistance is measured, and from this measurement, temperature is determined.

40. TRANSMITTERS A transmitter is a device that converts a reading from a sensor into a standard signal and transmits that signal to a monitor or controller. Transmitter types include: - Pressure transmitters - Flow transmitters - Temperature transmitters - Level transmitters

41. SIGNALS There are three kinds of signals that exist for the process industry to transmit the process variable measurement from the instrument to a centralized control system. 1. Pneumatic signal 2. Analog signal 3. Digital signal

42. Pneumatic Signals Pneumatic signals are signals produced by changing the air pressure in a signal pipe in proportion to the measured change in a process variable. The common industry standard pneumatic signal range is 3–15 psig. The 3 corresponds to the lower range value (LRV) and the 15 corresponds to the upper range value (URV). Pneumatic signaling is still common. However, since the advent of electronic instruments in the 1960s, the lower costs involved in running electrical signal wire through a plant as opposed to running pressurized air tubes has made pneumatic signal technology less attractive.

43. Analog Signals The most common standard electrical signal is the 4–20 mA current signal. With this signal, a transmitter sends a small current through a set of wires. The current signal is a kind of gauge in which 4 mA represents the lowest possible measurement, or zero, and 20 mA represents the highest possible measurement. For example, imagine a process that must be maintained at 100 °C. An RTD temperature sensor and transmitter are installed in the process vessel, and the transmitter is set to produce a 4 mA signal when the process temperature is at 95 °C and a 20 mA signal when the process temperature is at 105 °C. The transmitter will transmit a 12 mA signal when the temperature is at the 100 °C setpoint. As the sensor’s resistance property changes in response to changes in temperature, the transmitter outputs a 4–20 mA signal that is proportionate to the temperature changes. This signal can be converted to a temperature reading or an input to a control device, such as a burner fuel valve. Other common standard electrical signals include the 1–5 V (volts) signal and the pulse output.

44. Digital Signals Digital signals are the most recent addition to process control signal technology. Digital signals are discrete levels or values that are combined in specific ways to represent process variables and also carry other information, such as diagnostic information. The methodology used to combine the digital signals is referred to as protocol. Manufacturers may use either an open or a proprietary digital protocol. Open protocols are those that anyone who is developing a control device can use. Proprietary protocols are owned by specific companies and may be used only with their permission. Open digital protocols include the HART® (highway addressable remotetransducer ) protocol, FOUNDATION™ Fieldbus, Profibus , DeviceNet , and the Modbus® protocol.

45. Transducers & Converters A transducer is a device that translates a mechanical signal into an electrical signal. For example, inside a capacitance pressure device, a transducer converts changes in pressure into a proportional change in capacitance.

46. CONVERTERS A converter is a device that converts one type of signal into another type of signal. For example, a converter may convert current into voltage or an analog signal into a digital signal. In process control, a converter used to convert a 4–20 mA current signal into a 3–15 psig pneumatic signal (commonly used by valve actuators) is called a current-to-pressure converter.

47. INDICATORS While most instruments are connected to a control system, operators sometimes need to check a measurement on the factory floor at the measurement point. An indictor makes this reading possible. An indicator is a human-readable device that displays information about the process . Indicators may be as simple as a pressure or temperature gauge or more complex, such as a digital read-out device. Some indicators simply display the measured variable , while others have control buttons that enable operators to change settings in the field.

48. RECORDERS A recorder is a device that records the output of a measurement devices. Many process manufacturers are required by law to provide a process history to regulatory agencies, and manufacturers use recorders to help meet these regulatory requirements. In addition, manufacturers often use recorders to gather data for trend analyses . By recording the readings of critical measurement points and comparing those readings over time with the results of the process , the process can be improved .

49. Trend Display

50. Trend Display

51. CONTROLLERS A controller is a device that receives data from a measurement instrument, compares that data to a programmed setpoint, and, if necessary, signals a control element to take corrective action.

52. PLC vs DSC Controllers may perform complex mathematical functions to compare a set of data to setpoint or they may perform simple addition or subtraction functions to make comparisons. Controllers always have an ability to receive input, to perform a mathematical function with the input, and to produce an output signal. Common examples of controllers include: Programmable logic controllers (PLCs) —PLCs are usually computers connected to a set of input/output (I/O) devices. The computers are programmed to respond to inputs by sending outputs to maintain all processes at setpoint. Distributed control systems (DCSs) —DCSs are controllers that, in addition to performing control functions, provide readings of the status of the process, maintain databases and advanced man-machine-interface

53. PLC is used in the Situations were the SPEED Of OPERATION is an important factor. DCS is used to control the HUGE PLANT with certain Speed but it can handle more complex loops and handle large Inputs and Outputs. As the Pictorial Reprentation of the Entire plant is provided it is considered as the Advanced version of the Industrial Control System.

54. Difference between SCADA, DCS and PLC Systems. SCADA A SCADA (or supervisory control and data acquisition) system. It consists of many remote terminals units for collection of data (field),that is being connected with master station through any communication system, having main task of collection of accurate data and controlling of process for smooth operation. DCS It stands for distributed Control System, controlling is performed by embedded system (Microcontroller based or Microprocessor based controlling unit for device or instruments from which data is to be collect. It provides very intelligent analog control capability. It is very sensitive for HMI (Human machine Interface) for easy and smooth control of process. PLC It stands for Programmable Logic controller, having get this name from the fact that it replace the relay logic at the initial stage then it get the capability for analog channels also for display then it get the ability for close loop control and after some time it has the ability for redundant operation, and also its HMI having the ability for Indicatiion,controlling,data logging ,Alarming and backup data facility. It is also defined as below: “A digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions for implementing specific functions, such as logic, sequencing, timing, counting and arithmetic, to control through digital or analog input/output, various types of machines or process.”

55. DCS System Consoles

56. Digital Controller Display

57. Distributed and Logic Control Analog Control System

58. Distributed Control System with Data Highway

59. DCS System

60. Graphic Display

61. Digital Controller Display

62. Simplified SCADA System (Long Distance)

63. CORRECTING ELEMENTS/FINAL CONTROL ELEMENTS The correcting or final control element is the part of the control system that acts to physically change the manipulated variable. In most cases, the final control element is a valve used to restrict or cut off fluid flow, but pump motors, louvers (typically used to regulate air flow), solenoids, and other devices can also be final control elements.

64. Final Control Element

65. Actuators An actuator is the part of a final control device that causes a physical change in the final control device when signaled to do so. The most common example of an actuator is a valve actuator, which opens or closes a valve in response to control signals from a controller. Actuators are often powered pneumatically, hydraulically, or electrically. Diaphragms, bellows, springs, gears, hydraulic pilot valves, pistons, or electric motors are often parts of an actuator system

66. A disadvantage of feedback control In conventional feedback control the corrective action for disturbances does not begin until after the controlled variable deviates from the set point If either the cold oil flow rate or the cold oil temperature change, the controller may do a good job in keeping the hot oil temperature at the setpoint What if the pressure or flow of the fuel gas changes?

67. Cascade control # 1 Two control loops are nested within each other: the master controller and the slave controller the output signal of the master (primary) controller serves as the set point of the slave (secondary) controller The performance can be improved because the fuel control valve will be adjusted as soon as the change in supply pressure is detected slave loop master loop set point

68. Simple Cascade Control Loop

69. CASCADE EXAMPLE

70. CASCADE EFFECTS FOR THIS EXAMPLE THE FEEDS TO THE REACTOR ARE SET BY ONE MASTER FLOW WITH THE SECOND FLOW FED BY RATIO.

71. Cascade control # 2 The TC may reject satisfactorily disturbances such as reactant feed T and composition If the T of the cooling water increases, it slowly increases the reactor T The TC action may be delayed by dynamic lags in the jacket and in the reactor

72. Tuning a cascade loop Begin with both the master and the slave controllers in manual Tune the slave (inner) loop for set-point tracking first (the tuning guidelines presented before can be used) Close the slave loop, and adjust the tuning online to ensure good performance Leaving the inner loop closed , tune the master loop for disturbance rejection (the tuning guidelines presented before can be used) Close the master loop, and adjust the tuning online to ensure good performance A P-only controller is often sufficient for the slave loop

73. Summary on cascade control  It is used to improve the dynamic response of the process to load disturbances  It is particularly useful when the disturbances are associated with the manipulated variable or when the final control element exhibits nonlinear behavior  The disturbances to be rejected must be within the inner loop  The inner loop must respond much more quickly than the outer loop  Two controllers must be tuned

74. Relative Loop Performance to Supply Upsets

75. Typical Split-Range Control Loop

76. Direct and Reverse Acting Valves

77. Multiple use of an End Device

78. SAFETY & ALARM SYSTEMS OPERATOR ALARMS AND INTERLOCK ALARMS (LO, LOLO, HI, HIHI) SHOULD BE ON LOOPS THAT ARE INDEPENDENT FROM CONTROL LOOPS RELIEF SYSTEMS NEED TO BE DIRECTED TO FACILITIES TO SAFELY PROCESS THE RELEASE SAFETY SYSTEMS SHOULD BE INTERLOCKED TO SHORT- OR LONG-TERM SHUTDOWN LOOPS, AS APPROPRIATE.

79. Piping and Instrument Diagram(P&ID) Contains: plant construction information (piping, process, instrumentation, and other diagrams)

80. ISA Symbology The Instrumentation, Systems, and Automation Society (ISA) is one of the leading process control trade and standards organizations. The ISA has developed a set of symbols for use in engineering drawings and designs of control loops

81. First Letter First letter Parameters controlled A Analysis C Conductivity D Density E Voltage F Flow Rate I Current L Level M Moisture(Humidity) P Pressure or Vacuum T Temperature V Viscosity

82. Next Letter Next letter Control equipment type A Alarm C Controller I Indicator T Transmitter V Control Valve E Element IC Indicator Controller FC Ratio Controller R Recorder HS Hand Switch HV Hand Valve Q Totalizer IQ Indicating Totalizer XV Solenoid Valve Y Calculation FY Ratio Calculation SL Switch Low SH Switch High AL Alarm Low ALL Alarm Low Low AH Alarm High AHH Alarm High High

83. Legend

84. Abbreviations

85. Valves

86. Valves

87. Valves "V" - D# - SQ Where; HV or V - A literal and required part of all hand valve tags D# - last two digits of P&ID drawing number SQ - Sequence Number (01 to 99) V0001 - The first hand valve on P&ID D100 V1205 - The fifth hand valve on P&ID D112

88. Piping & Connection

89. Piping

90. Lines

91. Lines

92. Lines

93. Lines

94. Instruments

95. Instruments