Regarding the flow restriction issue with the manual valve in front of the control valve?
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When the control valve fails to regulate flow stably, the front hand valve is often used for control~ Why not use the back hand valve? Why?
Personally, I think it’s like this: the primary valve is used for primary control (handling a relatively large flow rate), while the control valve is used for secondary control (adjusting the flow rate within a relatively small range that has already been controlled by the primary valve). If the flow rate is controlled by adjusting the opening degree of the rear hand valve, it is actually the control valve that regulates the relatively large flow rate; even a slight change in the control valve results in significant fluctuations in the flow rate, and it becomes difficult to maintain a stable flow rate using the rear hand valve in such cases.
Limiting the upstream valve is mainly due to people’s* habit of performing pressure reduction operations. For gases, when the pressure is reduced, the gas expands; as a result, the volumetric flow rate through the control valve increases, which causes the opening degree of the control valve to increase accordingly, thereby improving its capability to regulate low flow rates. For liquids, this means operating the control valve under lower pressure conditions; by restricting the backflow valve, the entire control valve remains in a high-pressure environment.
In my understanding, the primary valve is first exposed to the fluid; it is used to make rough adjustments to fluids with large fluctuations, while the control valve enables more precise adjustments, resulting in a smoother regulation. If a rear-hand valve is used, the control valve has to deal with fluid with large fluctuations on its own, resulting in unstable regulation.