I would like to ask about the automation and cascade control of control valves
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May I ask, when putting into service the level control valve for the middle tower in the DCS and the flow control valve for the reboiler pump, which one should be activated first and which one later? Which one operates automatically, and which one operates in a cascade mode? I hope experts can explain this in detail :P
Generally, the auxiliary loop is activated first; after it is set to automatic mode, the main loop is activated.
First manual mode, then automatic mode; after the system parameters operate stably, it switches to automatic mode. For cascade control, the secondary controller is activated first, followed by the primary controller.
Explain the control loop clearly: under normal circumstances, it should be operated manually at first; once the system parameters stabilize, it can be switched to automatic mode. For cascade control, the secondary controller should be activated before the primary controller. If there are no requirements regarding fluctuations, it’s possible to switch directly to automatic mode
From what I see on the current DCS screen, the level control is set to automatic mode, and the flow control is set to cascade mode. Do you mean that the flow is controlled automatically first, then the level is controlled automatically, and finally cascade control is applied to the level? I’m new to this and don’t quite get it.
From what I see on the current DCS screen, the level control is set to automatic mode, and the flow control is set to cascade mode. Do you mean that the flow is controlled automatically first, then the level is controlled automatically, and finally cascade control is applied to the level? I’m new to this and don’t quite get it. The liquid level here should be the main loop, right? Why does cascade control require the secondary loop to be activated before the main loop?
Based on changes in the tank bottom liquid level, the setpoint of the pump outlet flow is automatically adjusted. Who is the main circuit here? Who is the secondary loop?
First, adjust the pump outlet flow to make it stable; then enable the cascade control. After that, use the level controller to ensure stability, and finally switch to automatic mode. That’s all
I’m referring to the requirements at the time of commissioning. The control system shown in your DCS screen is a cascade control system, where the level is the primary controller and the flow rate is the secondary controller; it’s already in an automatic control state.
First, enable automatic flow control to stabilize the liquid level, and then put the cascade control into operation
This post was last edited by hbrrn on 2015-9-25 17:01. Two points must be clarified: 1) The output of the main loop serves as a given channel for the secondary loop； 2) The auxiliary loop output directly controls the actuator ； When the auxiliary flow rate channel is not set to automatic mode, the auxiliary loop operates in single-loop control; the system automatically adjusts the opening of the flow valve based on the flow rate feedback value. After placing the main loop in automatic mode, the external setpoint of the secondary loop can automatically track the output of the main loop, thereby achieving cascade control. Generally, a secondary loop is installed first; since it causes less interference, it is easier to achieve stability through single-loop control ； If stabilizing can be achieved by controlling only the main loop first, then the concept of cascading control wouldn’t arise at all.