Regarding cascade control
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In cascade control, what is the main loop and what is the secondary loop? How are they put into operation?
The liquid level is the primary loop, while flow rate is the secondary loop; when the flow rate and liquid level are stable, cascade control is activated
Is it the main circuit that needs to be controlled? Should the auxiliary system be put into use first, followed by the main one?
Set flow control to automatic first, and set the liquid level to manual. Stabilize the liquid level by continuously changing the flow setting value. After the liquid level stabilizes for a while, cascade control is applied to the flow rate, while automatic control is used for the liquid level.
I’m quite curious about what controls FIC36024 – is it temperature? Is it the reflux tank level that controls the reflux flow, and the top of the tower or the extraction temperature that controls the production flow?
This control system is a cascade control system for level (LIC-36002) and flow rate (FIC-36026). The controlled variable of FIC-36026 is the discharge flow rate of pump P3605. The discharge flow rate of P3605 is divided into two streams (FI-36025 and FI-36024); therefore, the input signal for FIC-36026 is the sum of FI-36025 and FI-36024. Why not directly measure the flow rate in the P3605 pump discharge main? It is because there is another control system above FI-36024, namely the FIC-36024 flow control system. Assuming the speed of pump P3605 remains constant, its outlet flow rate will also remain constant; an increase in the flow rate of FI-36024 will inevitably lead to a decrease in the flow rate of FI-36025. In reality, when the outlet flow rate of pump P3605 changes, its output head also changes. To enable better dynamic adjustment, the values from the two branch pipelines are added together and then sent to FIC-36026.
Operation process of the cascade control system: First, set LIC-36002 and FIC-36026 to manual mode, and manually control the two control valves to bring the liquid level and flow rate close to the process design parameters. Then adjust the manual output of LIC-36002 so that the deviation of FIC-36026 is zero, and switch FIC-36026 to automatic mode. Manually operate the LIC-36002 output to bring the liquid level to the process design parameters, and switch FIC-36026 to automatic mode.
24 is controlled by the sensor plate temperature; the flow rate is obtained externally based on the sensor plate temperature, while the liquid level in the return tank is determined externally as the sum of the return flow rate and the externally obtained value
Can it be understood in this way that when everything is connected in series, if the liquid level setpoint is adjusted and the value is sent to 36026, since the setpoint of the sensitivity plate remains unchanged, 36024 stays the same, with only 36025 changing?
Yes. Furthermore, when fluctuations occur in 36024 due to disturbances (such as fluctuations in pump outlet pressure), 36025 also adjusts to compensate for the changes in 36024.