Feedwater Level Control SystemsIn 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 LevelThe 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 another is the use of a remote level indicator based on fiber optics.While the gage glass is the basic measurement, the indication it provides usually is in error to some degree and is not as correct as a properly calibrated level-measuring instrument. The basis for the error can be recognized from Figure 13-1. Condensate from cooling boiler steam circulates through the gage glass. This cooling of the steam and its condensate results in cooler water in the gage glass than in the boiler drum. The greater density of the cool water in the gage glass then shows a lower height water column to balance the column of water in the boiler drum.Assuming a typical industrial boiler, the gage glass reading often reads I to 3 inches of water below the actual level in the boiler drum. The deviation depends on the boiler pressure and the ambient temperature, plus piping and insulation between the boiler drum and the gage glass. For large high pressure electric utility boilers, the difference may be 5 to 7 inches. Some of the newer types of remote drum level-measuring instruments tend to compensate for the difference in readings described above. This may be in the mechanical design or the potential for moving the gage column closer to the drum.When this fact is sufficiently understood, most of the error can be eliminated by physically lowering the gage glass. This potential for error must be well understood by anyone dealing with boiler drum level measurement and control, or much unproductive work may be performed in trying to make a gage glass and measuring instrument agree.These types of drum measurement errors should not be confused with drum level measurement differences between the two ends of the drum. Because of water circulation-induced lateral flows inside the boiler drum, such differences are common. On large electric utilityunits, differences of six or more inches may be observed. In some cases the higher measure- ment may be at one end of the drum at low boiler loads and at the other end at high flows.A typical arrangement of a drum level-measuring transmitter is shown in Figure 13-2. The transmitter is a differential pressure device in which the output signal increases as the differ- ential pressure decreases. Typically, the differential pressure range is approximately 30 inches with a zero suppression of several inches.To determine the measuring instrument calibration, the necessary design data are the lo- cation of the upper and lower pressure taps into the boiler drum with respect to the normal water level, the operating pressure of the boiler drum, and the ambient temperature around the external piping. With these data and the desired range span of the tansmitter, the exact calibration can be calculated by using the standard thermodynamic properties of steam and water.On the high-pressure side of the measuring device, the effective pressure equals boiler drum pressure plus the weight of a water column at a