Issues related to the drain control valve in the Koga accident

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The drain control valve in the Koga accident exhibits surge at values below the specified level. (1) When the manual on-off valve in front of the control valve is closed, the control valve is operated remotely, and it functions properly. (2) When the manual on/off valve in front of the control valve is opened, the control valve is operated remotely, and fluttering begins to occur at an opening degree of 40%. And the amplitude of vibration increases as the opening degree increases. As shown in the figure below: Based on the analysis, 1) there will be no significant fluctuations in the pressure of the high-pressure heater before the valve and the condenser after the valve; 2) the current operating parameters of the process fall within the range specified for the valve during its design (size of the actuator, opening/closing force, control valve thrust, and size of the valve body). 3. Since the condenser is under negative pressure, flash vaporization or a two-phase flow of vapor and liquid occurs behind the control valve. But after a vapor-liquid two-phase flow is formed, shouldn’t the valve’s feedback opening value fluctuate around the specified value? I hope the experts can help me analyze the cause of the fault. Thank you