PID tuning
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As shown in the figure, the equipment is a steam heater, LC3120 is the condensate level control valve, LC3120-PV is the actual liquid level value, LC3120-LMN is the valve position output, and the liquid level is the steam condensate level. When other parameters remain unchanged, why do both the valve position (red line) and liquid level (black line) begin to oscillate, and the amplitude becomes larger and larger. The liquid level is basically unchanged, but the valve position is opening wider and wider? ? ? Please tell me how to adjust it?
As you said, your LC3120 is a liquid level control valve, and LC3120SP and PV are liquid level parameters. Are you sure that the opening of the valve of your steam heater is closed-loop controlled with your liquid level? It's usually a closed loop with temperature, don't get it wrong. From your picture, it seems that your liquid level has nothing to do with the valve opening. The liquid level SP is 30, while the liquid level PV has been 40 from the beginning, without any signs of adjustment.
How should it be changed? Do I need to change the control logic?
Is the temperature of the condensate used to control the valve opening? The green line is the condensate temperature, which is relatively related to the valve opening.
Yes, you can ask your self-control company to help you take a look. It must be related to the temperature. The steam valve and the liquid level should have nothing to do with it. It’s not easy for me to show you this,,,,
Without a control system, P&ID diagrams are difficult to analyze. Steam condensate discharge is related to the opening of the condensate discharge valve and the pressure inside the steam heater. If the steam pressure is low, the steam inlet resistance is large, and the steam condensation efficiency is high, negative pressure may form in the steam heater. At this time, it is useless to open a large condensate valve. If this is the case, the condensate valve will open wider and wider.
The main control parameter of the steam heater must be the temperature of the heated process material. What is the main control loop with the steam flow or condensation pressure? The poor control of the condensate level is just a result. It is estimated that the reason is that the condensation pressure fluctuates during open-loop control.
You can try manually to see if this problem still occurs.