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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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  • Electrical Engineering
  • B.Tech
  • 4 Topics
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Hinsermu Alemayehu
Hinsermu Alemayehu
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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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Process control fundamentals    EEng5510  Representative Process Control Problems  Example: surge tank Surge tanks are often used as intermediate storage for fluid streams being transferred between process units. Consider the process flow diagram shown in Figure below, where a fluid stream from process 1 is fed to the surge tank; the discharge from the surge tank is sent to process 2. Let us analyze this system using a step-by-step procedure. 1. Control objective: The control objective is to maintain the height within certain bounds. If it is too high it will overflow and if it is too low there may be problems with the flow to process 2. Usually, a specific desired height will be selected. This desired height is known as the set-point. 2. Input variables: The input variables are the flow from process 1 and the flow to process 2. Notice that an outlet flow rate is considered an input to this problem. The question is which input is manipulated and which is a disturbance? That depends. We discuss this problem further in a moment. 3. Output variables: The most important output variable is the liquid level. We assume that it is measured. 4. Constraints: There are a number of constraints in this problem. There is a maximum liquid level; if this is exceeded, the tank will overflow. There are minimum and maximum flow rates through the inlet and outlet valves. 5. Operating characteristics: We assume that this is a continuous process, that is, that there is a continuous flow in and out of the tank. It would be a semi-continuous process if, for example, there was an inlet flow with no outlet flow (if the tank was simply being filled). 6. Safety, environmental, and economic considerations: These aspects depend somewhat on the fluid characteristics. If it is a hazardous chemical, then there is a ASTU                                                                                                                                                               2016 

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