Air pocket level issue

When the steam flow suddenly increases, the pressure inside the drum decreases, and the saturation temperature of the water in the drum drops; as a result, the water boils and the liquid level in the drum rises. At this point, the level gauge indicates an increase in liquid level, which is known as a false level. Ordinary level gauges cannot overcome false levels. Level control employs three-shot control, which can overcome the effects of false levels.

Thank you. Could you also explain the three impulses in detail? How can feedwater flow rate, steam flow rate, and air vessel liquid level be subjected to cascade and feedforward control?

Principle: Structurally, an impulse control system is a cascade control system with a feedforward signal. The level controller LC and the flow controller FC form a cascade control system. The drum level LIA2104 is the primary variable, while the feedwater flow rate is the secondary variable. The introduction of auxiliary variables enhances the system’s ability to overcome fluctuations in feedwater pressure. Fluctuations in steam flow are the factor that causes changes in the drum level LIA2104; they represent disturbances. When there are fluctuations in steam flow, FC is introduced to cause a corresponding change in the feedwater flow FA2101. Thus, this is control based on disturbance handling, with the steam flow signal being used as a feedforward signal in the control system. Regulation process: In accordance with the principle of selecting the forward and reverse action modes for the primary and secondary controllers in a cascade control system, the water level controller LC is set to reverse action, while the flow controller FC uses forward action; the actuator used is a air-operated valve. When the water level rises due to disturbances, its output decreases as a result of the LC reaction. Upon entering the adder, this causes the FC set value to decrease and the output to increase; consequently, the opening degree of the control valve reduces, the feedwater flow FA2101 decreases, and the water level drops back to the set value ； As the steam flow rate FAQ2102 increases, the FC set value rises while the output decreases; the opening of the control valve increases, resulting in an increase in the feedwater flow rate, which helps maintain steam balance and prevents fluctuations in the water level ； The secondary circuit overcomes the disturbances caused by the feedwater itself, thereby further stabilizing the automatic control of the water level ； As the feedwater flow rate FA2101 increases, the FC output rises; the opening degree of the control valve decreases, resulting in a reduction in the feedwater volume, thereby maintaining steam balance