Is it necessary to perform calculations and select appropriate valves for process instrumentation bypasses?

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Dear sea friends: Generally, the bypasses for control valves used in our manufacturing processes are designed to be stop valves or something similar, right? Are ball valves generally used as the valves before and after the control valve? Is it necessary to conduct a calculation of the flow control capacity of the bypass valve of the control valve before selecting it? Are there any specific regulations for this? Still the same old *habit? Generally, when selecting and designing control valves for use in instruments, the flow capacity CV value is calculated very carefully; the necking section is treated accordingly, and it is also necessary to carry out relevant calculations regarding whether bypass valves are needed. Recently, a control valve that had been in use for over a decade suddenly stopped allowing flow – it became clogged. The production process couldn’t be stopped, and since the valves before and after it hadn’t been operated for a long time, it was not possible to bypass the issue; therefore, manual operation through the bypass was used. It was found that even a small amount of flow would cause the system to exceed its limits, and it was impossible to use a bypass for control. Later, it was checked that the control valve consisted of a main pipe with a diameter of DN40 that narrowed to DN25; upon checking the valve’s nameplate, it was found that its diameter was φ6. This neck shrinkage is way too small! Bypass for control valve, DN40. How do I adjust the control?