lesson13_boiler control.docVIP

Lesson 13 Boiler control There are two basic types of boilers，the drum type and the once-through type．In the drum type，which always operates at subcritical pressures 1ess than 22.1MPa ，the steam is separated from the water in a drum which stores a large volume of steam and water．In the once-through type of boiler for either subcritical or supercritical pressure，there is no drum and the only storage of fluid is in the tube paths．Although the control of the two types differs，the primary functions of boiler control are the same for both the drum and once-through types of boiler．They are： 1 To supply heat to the boiler in accordance with the demand for steam． 2 To properly proportion the fuel and combustion air． 3 To supply the proper amount of feedwater． 4 To maintain the proper steam temperatures． 5 To protect against failures of the boiler and auxiliaries and to initiate tripping as required． The controls utilize measurements of the various boiler parameters to be controlled pressure，temperature，level，etc． and operate control valves，dampers，vanes，and variable-speed drives through various forms of actuating devices．These actuators are driven either pneumatically or electrically． Controls for a drum-type boiler generally include： 1 Combustion pressure control． 2 Feedwater or drum level control． 3 Temperature control． 1、Combustion Control Combustion control controls the heat or fuel input to the boiler to maintain a desired steam pressure．To assure proper combustion，the air flow to the boiler must be coordinated with the fuel input．If the boilers were not subject to wide and rapid fluctuations in load，the fuel and air requirements could be determined by steam pressure alone．For example，when the demand for steam increases，the boiler pressure drops．This pressure drop can be sensed and the fuel and air flows increased to bring the steam pressure back to normal．Power plant boilers are subject to rapid load changes and a two-element control is commonly used．