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Note for Industrial Process Control and Dynamics - IPCD by Hinsermu Alemayehu

  • Industrial Process Control and Dynamics - IPCD
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Process control fundamentals    EEng5510  Chapter – 1  Review of fundamentals of process control.  Introduction to process control   Process engineers are often responsible for the operation of chemical processes. As these processes become larger scale and/or more complex, the role of process automation becomes more and more important. The objective of this course is to teach control engineers how to design and tune feedback controllers for the automated operation of chemical and other processes. 1. Process: It can be defined as the conversion of feed materials to products using chemical and physical operations. In Practice, the term process tends to be used for both the processing operation and the processing Equipment. Or it can also be defined as a system where Single-variable or Multivariable’s may require regulation- also is called ‘Plant’. The primary objective of process control is to maintain a process at the desired operating conditions, safely and efficiently, while satisfying environmental and product quality requirements. The subject of process control is concerned with how to achieve these goals. In large-scale, integrated processing plants such as oil refineries thousands of process variables such as compositions, temperatures, flow, level and pressures are measured and must be controlled. Large numbers of process variables (mainly flow rates) can usually be manipulated for this purpose. Feedback control systems compare measurements with their desired values and then adjust the manipulated variables accordingly. As an introduction to the subject, we consider T- = TEMPERATURE representative process control problems in several industries. P- = PRESSURE REACTANT IN F- = FLOW -V = VALVE -T = TRANSMITTER STEAM IN TV 101 REACTION VESSEL FO 102 REACTANT OUT 3-15 psi -R = -O = -C = -CY = RECEIVER ORIFICE CONTROLLER TRANSDUCER STEAM OUT 101/102 = LOOP DESIGNATION PY 101 FV 102 FT 102 I/P TT 101 4-20mA TC 101 = ORIFICE PLATE = PNEUMATIC SIGNAL FC 102 = ELECTRICAL SIGNAL FR ASTU                                                                                                                                                               2016  102 = PNEUMATICALLY-OPERATED VALVE

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Process control fundamentals    EEng5510  A Q A To Q S T A TS CONTROLVALVE T 1 B Q B Q Q = FLOWRATESINPIPESA&B A B Q = STEAMFLOWRATE S To= INLETFLUIDTEMPERATURE T = AMBIENTTEMPERATURE A T = STEAMTEMPERATURE S OUTPUT CONTROLLER MEASUREMENT Tl = LIQUIDTEMPERATURE(CONTROLLEDVARIABLE) SETPOINT Fig.1 typical temperature control for a process 2. Control strategy development The standard steps that are followed to design a control system for any type of process is based on formulating and identifying the following points. 1) Control objective(s). The first step of developing a control strategy for a process is to formulate the control objectives. The objective may be for example: to control  the temperature  the level  the pressure  the flow rate  The ratio of a unit in chemical or other processes. 2) Input variables—classify these as (a) manipulated or (b) disturbance variables; inputs may change continuously, or at discrete intervals of time. ASTU                                                                                                                                                               2016 

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Process control fundamentals    3) 4) 5) 6) EEng5510  A manipulated input is one that can be adjusted by the control system (or process operator). A disturbance input is a variable that affects the process outputs but that cannot be adjusted by the control system. Inputs may change continuously or at discrete intervals of time. Output variables—classify these as (a) measured or (b) unmeasured variables; measurements may be made continuously or at discrete intervals of time. The output variable is the variable which is being controlled. Constraints—classify these as (a) hard or (b) soft. Any process has certain operating constraints, which are classified as hard or soft. An example of a hard constraint is a minimum or maximum flow rate—a valve operates between the extremes of fully closed or fully open. An example of a soft constraint is a product composition—it may be desirable to specify a composition between certain values to sell a product, but it is possible to violate this specification without posing a safety or environmental hazard. Operating characteristics Operating characteristics are usually classified as continuous, batch, or semi-continuous (semi-batch). Continuous processes operate for long periods of time under relatively constant operating conditions before being “shut down” for cleaning, maintenance, and so forth. For example, some processes in the oil-refining industry operate for 18 months between shutdowns. Batch processes are dynamic in nature—that is, they generally operate for a short period of time and the operating conditions may vary quite a bit during that period of time. Example batch processes include beer or wine fermentation, as well as many specialty chemical processes. For a batch reactor, an initial charge is made to the reactor, and conditions (temperature, pressure) are varied to produce a desired product at the end of the batch time. A typical semi-batch process may have an initial charge to the reactor, but feed components may be added to the reactor during the course of the batch run. Safety, environmental, and economic considerations. In a sense, economics is the ultimate driving force. An unsafe or environmentally hazardous process will ultimately cost more to operate, through fines paid, insurance costs, and so forth. In many industries (petroleum refining, for example), it is important to minimize energy costs while producing products that meet certain specifications. Better process automation and control allows processes to operate closer to “optimum” conditions and to produce products where variability specifications are satisfied. The concept of “fail-safe” is always important in the selection of instrumentation. For example, a control valve needs an energy source to move the valve stem and change the flow; most often this is a pneumatic signal (usually 3–15 psig). If the signal is lost, then the valve stem will go to the 3-psig limit. If the valve is air-to-open, then the loss of instrument air will cause the valve to close; this is known as a fail-closed valve. If, on the ASTU                                                                                                                                                               2016 

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Process control fundamentals    EEng5510  other hand, a valve is air to close, when instrument air is lost the valve will go to its fully open state; this is known as a fail-open valve. 7) Control structure—the controllers can be feedback or feed forward in nature. A feed-forward controller measures the disturbance variable and sends this value to a controller, which adjusts the manipulated variable. A feedback control system measures the output variable, compares that value to the desired output value, and uses this information to adjust the manipulated variable. For the first part of this textbook, we emphasize feedback control of single-input (manipulated) and single-output (measured) systems. Determining the feedback control structure for these systems consists of deciding which manipulated variable will be adjusted to control which measured variable. The desired value of the measured process output is called the set-point. 8) Once the control structure is determined, it is important to decide on the control algorithm. The control algorithm uses measured output variable values (along with desired output values) to change the manipulated input variable. A control algorithm has a number of control parameters, which must be “tuned” (adjusted) to have acceptable performance. Often the tuning is done on a simulation model before implementing the control strategy on the actual process. A significant portion of this textbook is on the use of model-based control, that is, controllers that have a model of the process “built in.” This approach is best illustrated by way of example. Since many important concepts, such as control instrumentation diagrams and control block diagrams, are introduced in the next examples, it is important that you study them thoroughly. ASTU                                                                                                                                                               2016 

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