锅炉三冲量给水控制系统设计.doc

摘 要 工业锅炉是工业中不可缺少的动力设备，而汽包水位是锅炉运行的主要指标，也是一个非常重要的被控变量。维持水位在一定的范围内是保证锅炉安全运行的首要条件。影响汽包水位的因素主要包括给水扰动和蒸汽流量扰动，汽包液位控制系统的目标是尽可能克服这些扰动因素，将汽包液位保持在合适的范围。传统的方法包括单冲量水位控制系统和双冲量水位控制系统，前者仅仅将汽包水位作为控制变量，后者虽然加入了蒸汽流量对扰动进行校正，但依然无法做到静态补偿，也无法对给水系统的扰动直接补偿。因此，在双冲量控制系统基础上加入了给水流量这一控制变量，构成三冲量汽包液位控制系统。这一系统可以很好的克服给水流量和蒸汽流量带来的扰动，同时能够做到静态补偿，当负荷发生变化时，可以保持液位无差。方案中利用外围的A/D模块对变送器传来的冲量信号进行转换，输入PLC中，在前馈串级控制系统中通过PID调节得到输出值，然后进行D/A转换，输送给变频器，通过改变水泵电机的转速来将汽包液位控制在一定的范围内。在以PLC为核心的前馈-串级系统，对控制变量进行PID调节，能够较好的达到预期设计指标。 关键词 锅炉汽包液位控制，三冲量，前馈串级，三菱PLC，PIDABSTRACT Industrial boiler is a significant power equipment, and the drum level is the main control target in boiler operation.Besides, it is also a very important manipulated variable.In order to ensure the boilers safe operation,we need to keep the drum level in a certain range.The main factors which could influence drum level include water-supply disturbance and steam-flow disturbance.The design target of drum level control system is to prevent the factors from disturbing drum level to the possible extent.Traditonal methods includes single-element drum level control system and double-element drum level control system.The former one utilizes drum level as control element, and the latter one adds water-supply as control element,however,it still couldnt complete static compensation and couldnt compensate the water-supply disturbance directly.Consequently,three-element control system is built on basis of double-element control system by adding the third element steam-flow element which plays an important role.Compared to the former ones, this system could prevent water-supply and steam-flow from disturing the drum level,and could perform static compensation.When the load varying, the drum level can be no different to default value.In this project, it uses A/D module to transfer the analog signals which come from transmitter,and inputs them to PLC,then ajust deviation value via PID regulating in feedforward cascade system.Then PLC converts the digital signal to anal