Boilers, drum feedforward control

FIGURE 15.54 Boiler drum level control, (a) Feedback, (b) feedforward, and (c) feedback and feedforward combined. 

Figure 15.54b is a schematic of a feedforward controller applied for steam drum level control. If the flow rate of the makeup feedwater is equal to the steam usage, the drum level remains constant. One is tempted to conclude that the feedforward controller is aU that is needed for this application. Unfortunately, the measurements of the steam usage and the feedwater flow rate are not perfectly accurate. Even small errors in measured flow rates add up over time, leading to one of two undesirable extremes. The drum can till with water and put water into the steam system, or the liquid level can drop, exposing the boiler tubes, which can damage them. As a result, neither feedback nor feedforward are effective by themselves for this case. In general, feedforward-only controllers are susceptible to measurement errors and umneasured disturbances, and, as a result, some type of feedback correction is typically required. 

Feedforward control of a drum boiler (shown in Figure 21.2b) Here the objective is to keep the liquid level in the drum constant. The two disturbances are the steam flow from the boiler, which is dictated by varying demand elsewhere in the plant, and the flow of the feedwater. The last is also the principal manipulated variable. 

Feedforward is commonly applied to level control in a drum boiler. Because of the low time constant of the drum, level control is subject to rapid load changes. In addition, constant turbulence prevents the use of a narrow proportional band, because this would cause unacceptable variations in feedwater flow. The feedforward system simply manipulates feedwater flow to equal the rate of steam being withdrawn, since this rqiresents the load on drum level. The system is shown in Fig. 8.2. 

Feedforward control was not widely used in the process industries until the 1960s (Shinskey, 1996). Since then, it has been applied to a wide variety of processes that include boilers, evaporators, solids dryers, direct-fired heaters, and waste neutralization plants (Shinskey et al., 1995). However, the basic concept is much older and was applied as early as 1925 in the three-element level control system for boiler drums. We will use this control application to illustrate the use of feedforward control. 

Figure 15.3 Feedforward control of the liquid level in a boiler drum.

Figure 15.4 Feedforward-feedback control of the boiler drum level.

In a drum boiler, water is circulated at a rapid rate upward through the furnace tubes, in which it partially vaporizes. Upon reaching the drum, the liquid disengages the vapor and returns through relatively cool downcomers to the bottom of the furnace to begin another pass upward. The most characteristic feature of drum boilers is the difficulty of controlling the level of liquid in the steam drum. A feedforward-feedback system for its control was described briefly in Chap. 8. 

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