I need urgent advice on cascade control! !
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I have a tower that needs to be fed with steam. First, the steam flow rate at the inlet must be adjusted, but a certain pressure also needs to be maintained inside the tower. Therefore, there are valves at the tower’s outlet for regulating pressure, as well as a pressure control circuit using a steam inlet pressure transmitter. However, this pressure regulation definitely affects the adjustment of the steam flow rate. So, a cascade control system is used: the primary control is for the steam flow rate, while the secondary control is for the pressure. Will this approach allow for stable pressure regulation?
Whether it can be stable depends on the size of the valve, the adjustment accuracy, and the speed at which adjustments are made
In my opinion, this method cannot maintain stable pressure regulation; the flow rate and pressure at the steam inlet are related to the amount of steam consumed. To keep the pressure stable, it is first necessary to ensure stability in steam consumption, so that the pressure can be regulated properly.
Two sets can be made: one with a large steam control valve and another with a smaller one; the smaller one is used for adjustment while the larger one is designed for custom control
Now the main steam flow is being adjusted; the steam serves only a stripping function. Once normal operation is achieved, the steam goes to the condenser, and there is basically no consumption of steam inside the tower. Therefore, by stabilizing the flow rate, the pressure should also remain stable
First, it is necessary to understand the impact of changes in steam flow rate on the process, determine its appropriate range, and establish its relationship with the operating load. Based on what you have described, in general, steam feed is controlled using a constant flow rate. The control objective for the control valve on the outlet line at the top of the tower is to stabilize the pressure before the valve, that is, at the top of the tower; each pipeline is regulated separately. There is no need to form a cascade control loop with the steam flow control valve, as this would lead to conflicts and prevent effective control.
When each system operates independently, it leads to conflicts and instability. Cascade control aims to minimize such interactions as much as possible, with a form of lead control in place
It mainly comes down to whether to ensure pressure or flow – the two cannot be achieved simultaneously.
What impact does pressure have on ensuring flow rate?
Remove this control valve in front of the heat exchanger; one control valve for the heat exchanger will be sufficient. The key lies in selecting the appropriate valve for the rear side, and then establishing a cascade control loop.
There’s absolutely no need for it. In terms of processing, all the various conditions are interrelated; should they all be implemented in a cascade configuration?