Feedwater Level Control Systems：汽包水位控制系统.docVIP

Feedwater Level Control Systems In drum-type boilers, the flow of feedwater to the boiler drum is normally controlled in order to hold the level of the water in the steam drum as close as possible to the normal water level (NWL) set point. A typical level control loop would measure level with a level sensor, process this measurement in a proportional or proportional-plus-integral controller, and regulate the flow with a control valve. This typical level control loop usually is inadequate for boiler drum level control. This inadequacy results from the shrink and swell characteristics of the boiler, which produce level changes during boiler load changes in a direction opposite that to which level would be expected to change with the particular boiler load change. For this reason, control from level alone will produce an incorrect control action any time the boiler load changes. If, however, the boiler is small, has a relatively large water storage, and the load changes slowly, the simple single element level control may produce control performance that can be tolerated. With larger boilers, relatively less water storage, and faster load changes, the effect of shrink and swell tend to make the simpler systems inadequate. 13-1 Measurement and Indication of Boiler Drum Level The basic indication of the drum water level is that shown in a sight gage glass connected to the boiler drum. The typical arrangement is shown in Figure 13-1. Since the configuration of the boiler and the distance of the boiler drum from the operator may not provide a useful line-of-sight indication, the gage glass image can be projected with a periscope arrangement of mirrors so that the operator may easily view it. In many installations, the use of mirrors to project the water level image to a desired location for viewing may be mechanically complex or practically impossible, and other methods may be necessary. One such method is to use closed-circuit television; yet anoth