Understanding the distortion of operating characteristics of control valves?
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I have a question to ask. When selecting a regulating valve, the books require that the pressure drop of the regulating valve accounts for more than 30% of the total pressure drop of the system to reduce the distortion of the working flow characteristics. The book says that after distortion, the flow rate of the regulating valve is reduced when it is fully opened, and the adjustable ratio becomes smaller. I don’t understand this. The distortion has occurred, the KV value remains unchanged, and the valve pressure drop is still the pressure drop given when I selected the model. (It is impossible to give the entire system pump head to the regulating valve to determine the KV value when selecting the model. I always select the pump based on the given pressure drop and combined with the pipeline resistance.) Why is the flow rate so small? Can anyone explain what is meant by the deterioration of adjustment quality? Shouldn't it be adjusted to normal after distortion?
Let me explain my understanding. For example, when the S value is relatively small under the design pressure drop, the valve opening increases by 1 and the flow rate increases by 1. The proportional gains of the two are determined during control. When the system flow requirement decreases, the valve opening becomes smaller. At this time, the pressure difference between the two ends of the valve becomes larger. The valve opening increases by 10, and the flow rate increases by 1. The proportional gain of the two changes. The control still maintains the original proportional gain, and the flow rate is difficult to adjust. However, designing a larger S value in the initial stage requires a large initial valve pressure drop. When the system flow requirement decreases, the valve opening becomes smaller. At this time, the pressure difference between the two ends of the valve still increases, but the increase ratio is very small, and the impact on the proportional gain is small. Adjustments are still similar to the previous design. As for the adjustable ratio becoming smaller, this is my understanding. When S is small, due to the influence of the pressure difference, it is easier for the linear valve to reach an opening below 20, so the adjustable ratio becomes smaller.