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1、Chapter 5 Process Control Systems,Unit 22 Importance of Process Control Unit 23 Process Control Loops Unit 24 Advanced Control Loops Unit 25 Computers in Control Unit 26 Introduction to Sensors,Unit 22 Importance of Process Control,Control in process industries refers to the regulation of all aspect
2、s of the process. Precise control of level, temperature, pressure and flow is important in many process applications. Refining, combining, handling, and otherwise manipulating fluids to profitably produce end products can be a precise, demanding, and potentially hazardous process. Small changes in a
3、 process can have a large impact on the end result. Variations in proportions, temperature, flow, turbulence, and many other factors must be carefully and consistently controlled to produce the desired end product with a minimum of raw materials and energy.,下一页,返回,Unit 22 Importance of Process Contr
4、ol,Process control technology is the tool that enables manufacturers to keep their operations running within specified limits and to set more precise limits to maximize profitability, ensure quality and safety. Process Process as used in the terms process control and process industry, refers to the
5、methods of changing or refining raw materials to create end products. The raw materials, which either pass through or remain in a liquid, gaseous, or slurry (a mix of solids and liquids) state during the process, are transferred, measured, mixed, heated or cooled, filtered, stored, or handled in som
6、e other ways to produce the end product.,上一页,下一页,返回,Unit 22 Importance of Process Control,Process control Process control refers to the methods that are used to control process variables when we manufacture a product. For example, factors such as the proportion of one ingredient to another, the temp
7、erature of the materials, how well the ingredients are mixed, and the pressure under which the materials are held can significantly impact the quality of an end product. Increase efficiency Some processes need to be maintained at a specific point to maximize efficiency. For example, a control point
8、might be the temperature at which a chemical reaction takes place.,上一页,下一页,返回,Unit 22 Importance of Process Control,Accurate control of temperature ensures process efficiency. Manufacturers save money by minimizing the resources required to produce the end product. Ensure safety A run-away process,
9、such as an out-of-control nuclear or chemical reaction, may result if manufacturers do not maintain precise control of all of the process variables. The consequences of a run-away process can be catastrophic. Precise process control may also be required to ensure safety.,上一页,下一页,返回,Unit 22 Importanc
10、e of Process Control,In process systems a commonly used control device called a programmable logic controller, or a PLC, is used to read a set of digital and analog inputs, apply a set of logic statements, and generate a set of analog and digital outputs. For example, the room temperature would be a
11、n input to the PLC. The logical statements would compare the setpoint with the input temperature and determine whether more or less heating was necessary to keep the temperature constant. A PLC output would then either open or close the hot water valve, an incremental amount, depending on whether mo
12、re or less hot water was needed. Larger more complex systems can be controlled by a Distributed Control System or SCADA system.,上一页,返回,Unit 23 Process Control Loops,Control loops can be divided into two categories: Single variable loops and multi-variable loops. Single variable loops Feedback contro
13、l loops A feedback loop measures a process variable and sends the measurement to a controller for comparison with the setpoint. If the process variable is not at setpoint, control action is taken to return the process variable to the setpoint. Fig. 5.23.1 illustrates a feedback loop in which a trans
14、mitter measures the temperature of a fluid and, if necessary, opens or closes a hot steam valve to adjust the fluids temperature.,下一页,返回,Unit 23 Process Control Loops,Pressure control loops Pressure control loops vary in speedthat is, they can respond to changes in load or to control action slowly o
15、r quickly. The speed required in a pressure control loop may be dictated by the volume of the process fluid. High-volume systems (e.g., large natural gas storage facilities) tend to change more slowly than low-volume systems (Fig. 5.23.2). Flow control loops Generally, flow control loops are regarde
16、d as fast loops that respond to changes quickly. Therefore, flow control equipment must have fast sampling and response times.,上一页,下一页,返回,Unit 23 Process Control Loops,To compensate for noise, many flow transmitters have a damping function that filters out noise. Sometimes, filters are added between the transmitter and the control system. Because the temperature of the process fluid affects its density, temperature measurements are often taken with flow measurements and compensation for temper
