Drum Level and Feedwater Control
Feedwater is fed into the drum in a typical subcritical pulverized coal – fired drum boiler via either a series of valves in parallel with a series of constant-pressure feedwater pumps or a battery of variable-speed feedwater pumps. If the feedwater level in the drum goes too high, water can become entrained in the steam going to the turbine and can cause catastrophic results. If the drum feedwater level goes too low, the drum itself can become overheated, possibly resulting in catastrophe.
Feedwater (and drum level) control has two modes of automatic operation: single- and three-element control. The drum level set point for both modes is set by the operator. In single-element control the difference between the drum level and the drum level set point provides the error signal that is used by the single-element controller to control the rate of water entering the drum by modulating the feedwater flow control valve. Three-element control governs the three variables, or elements, that are used in this control scheme: drum level, steam flow, and feedwater flow.
Drum level control uses a cascaded controller scheme consisting of an outer and an inner controller. Steam flow is an indication of the rate at which water is being removed from the drum. A function of steam flow is used as a feedforward to the outer controller. The drum level error is then operated on by the outer controller. The output of this controller is the feedwater flow set point. The difference between this set point and the feedwater flow is then operated on by the inner controller. The output from this controller is then used to modulate the feedwater flow control valve.
Three-element control is much more stable and robust than single-element control. The reason that we use single-element control at all has to do with the nature of the instrumentation. Typically, feedwater flow, and occasionally steam flow, is developed by using a flow-measuring device like an orifice plate or a flow nozzle, where flow rate is proportional to differential pressure. However, a problem occurs at low flow rates (low boiler load), where differential pressures are not as solidly proportional as we would like and therefore untrustworthy for boiler control. Consequently, single-element control is used at low loads.
A well-tuned drum level control can be placed in automatic as soon as a pump is started. By the time steam flow has passed 25% of the total range, we can consider steam flow signals to be reliable. That is a good point at which to switch to three-element control.
There really is not much in the way of manual interlocks or control tracking when it comes to the drum level loop. If the drum level signal or the feedwater flow valve control output goes out of range, or no pump is running, this station is normally locked to manual mode. That’s about it.
Normally, tuning for the single-element controller consists of big proportional and very small integral gain settings. Tuning for the three-element controller has some additional requirements. As in any cascaded loop, it is absolutely crucial that the inner controller be tuned as tightly as time will allow. The inner controller, the feedwater controller in this case, must have an integral action that is faster than that of the outer, or drum level, controller (Figure 4). This is true for all cascade loops.
4. Rapid responder. A typical coal-fire boiler with a properly tuned drum level control will respond very quickly to a substantial load increase (top) or load decrease (bottom). The dynamic response of other key variables in boiler drum level control system is also illustrated. Source: Tim Leopold
You may notice that as the load decreases, the drum level sags downward, and as the load increases, the drum level is slightly elevated. This means that the steam flow feedforward is just a tad too strong. A minute adjustment to the feedforward signal can add stability to the control loop (Figure 5).
5. Small is big. A small increase in the feedforward signal added more stability to the drum level controls. Only very small incremental changes in feedforward should be made when tuning drum level controls. Source: Tim Leopold
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