Three-element control for the drum
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This is the three-pulse control for a drum. Could everyone please explain in detail the principle of this three-pulse control based on this diagram?
The principle of the three impulses for bubble level control can be understood without looking at a diagram; what you have here is just a part of the P&ID diagram, and it’s not possible to determine the conditions for these three impulses from this drawing. Three impulses: steam flow rate, feedwater flow rate, drum level. The above are my views
This diagram shows the steam flow rate, feedwater flow rate, and drum liquid level
The three impulses refer to the liquid level, feedwater flow rate, and steam generation rate. The steam generation rate is the main measurable disturbance variable, which serves as a feedforward compensation signal added to the output of the main controller in order to adjust the feedwater flow rate. When Kn=0, no compensation is applied, and the three impulses become cascade control. The closer Kn is to 1, the greater the feedforward compensation effect, and the lesser the impact of disturbances on the liquid level. When Kn=1, complete compensation is achieved, and in this case a high level of accuracy is required from the steam generation flow meter. The effects of other disturbances on the liquid level are eliminated by cascade control. The advantage of three-impulse control is that it provides feedforward compensation for the main measurable disturbances, enabling the liquid level to stabilize quickly in the presence of such disturbances. But this also adds a measuring instrument, increasing investment and maintenance costs, and introduces the risk of failure of that instrument into the control system.
If PID tuning is done well, three-shot control works fairly well; however, in practical applications, to prevent dry operation and reduce fluctuations, manual control or cascade control based on level and flow rates is usually employed.
The drum water level is the primary impulse (primary variable), while steam and feedwater flow rates are the secondary impulses. When the drum residence time is short and the \"false water level\" is severe, it is necessary to introduce the differential effect of the steam flow signal. This differential signal should be a negative differential action, in order to prevent the boiler from stopping due to the water level deviating too far above or below the set value when the load increases or decreases suddenly.