Regarding pressure control systems
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For pressure control in a container, a split-control system is used; it consists of two paths – one with a pressure-controlled valve and the other with a flow-controlled valve – which lead to different downstream components. When the pressure is high and the output is between 0-50%, the pressure controller adjusts the pressure-controlled valve to open wider; once the controller’s output reaches 50%, it compares this value with that of the flow controller, and the higher value determines how the flow-controlled valve should be regulated. My question is: 1. Where does this flow control signal come from? Is it a fixed value set by the system earlier? For example, the fixed signal value set by the flow control unit is 70%; when the signal from the pressure control controller is around 50%, and that value isn’t 70%, is it then the flow control unit that controls the flow control valve? And once the output of the pressure-controlled controller reaches 70%, is it this pressure-controlled controller that is used to control the flow control valve? Or what about something else? 2. What is the difference, or rather the advantage, of this approach over conventional step-by-step control? The usual zoning control here refers to the type where the pressure is entirely controlled by a single controller within different signal ranges.
The pressure control of the container is achieved through proportional control; the pressure control valve is entirely regulated by the container pressure, with a 0–50 range on the pressure signal corresponding to a 0–100 range in the opening degree of the pressure control valve； However, the flow control valve is controlled by two signals: one is a pressure signal and the other is a flow rate signal. These two signals pass through a priority selector, with the signal of higher value controlling the flow control valve. This control is necessary not only to regulate the pressure within the container but also to manage the flow rate to the rear path, as an excessive flow rate can have an adverse effect on that path. As for where the flow signal comes from, it should be a signal generated by comparing the flow rate you set with the actual flow rate. I hope the answer is helpful to you
What do you mean by the actual signal? Does it refer to the actual pressure signal? Is it by comparing the actual pressure signal with the set flow signal and then selecting the higher one?
Pressure control includes not only proportional control but also cascade control or single-loop control, depending mainly on the process requirements. It’s either good or not. However, step-by-step control is generally required when the product has high pressure demands, and its own flash pressure sometimes isn’t sufficient; in such cases, an external medium is needed to supplement the pressure
What I mean is, why is a high-selectivity controller used here, rather than a regular controller that controls two valves in a proportional manner?
The pressure controller outputs a 0–100 electrical signal based on the comparison between the actual pressure and the set pressure; it only controls the pressure-controlled valve within the 0–50 range； When the output signal is between 50 and 100, it enters the high selector, where it is compared with the flow signal; the valve controlling flow is activated when the signal is higher. The flow signal is an electrical signal generated by comparing the actual flow rate with the set flow rate. For example: if the pressure in the container is high and the pressure control valve is fully open, with the output signal at 60%, then to reduce the pressure it is necessary to use the flow control valve. But if the flow rate at this point is already much higher than the set value and the output electrical signal is 70%, then the flow control valve will be controlled by this flow signal and will close to reduce the flow rate.
I’m sorry, but I’m still not quite understanding. When the pressure increases, the pressure-controlled valve opens fully, and the 60% you mentioned should be the output of the pressure controller, right? So is this 60% meant to be compared with the set value of the flow controller? If this 60% is greater than the set value of the flow controller, then the pressure controller will control the flow control valve, right? If what I’m saying is correct, then what should be the set value for this flow controller? And what determines this value?
This 60% is not compared to the set flow value, but rather to the signal output by the flow controller. Suppose this signal is denoted as A; signal A is generated by the flow controller as a result of comparing the actual flow rate with the set flow rate. As for what the set value should be, it needs to be determined based on the process requirements – in other words, it should be a value that allows the subsequent processes to function properly. I believe that, as long as the pressure in the container is not too high, it is generally this flow controller that regulates the flow valve, causing the flow rate to fluctuate around the set value. If the container pressure is slightly high, the pressure is first controlled via a pressure control valve; only when the pressure is very high and the signal exceeds 50% will a higher-level selection be made between this signal and Signal A to determine which one will control the flow control valve. Put simply, as long as the increase in my flow rate exceeds the set value by more than the increase in your pressure exceeds that set value, the container will explode. The flow control valve is also controlled by the flow controller; once automatic mode is activated, the system only takes into account the calculated results, without considering the actual conditions.