A few minor process issues
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Why is the drain valve always set at 3/4 open? Why are the stop valves before and after the control valve usually fully open?
Are there stop valves on both the upstream and downstream sides of the control valve? It’s very rare. If the valves before and after the control valve are not opened, no fluid flows through the control valve; then how can it perform its regulating function?
Drainage generally refers to draining residual liquid from the lowest point; it should be fine to keep the valve fully open during this process. The manual valves before and after the control valve do not serve a regulating function; they only act as on/off valves. Therefore, they are kept fully open during normal operation. When there is a problem with the control valve, the fluid flows through the bypass route, with the manual valves before and after being closed.
Why is the opening degree of the drain valves always three-quarters? Is it because the diameter of the drain valves is also three-quarters?
The pipe used for stealing bells is of 3/4 size, that is, DN20. The use of MS material is just a convention; there’s no particular reason for it – it helps save material. Pipes with DN20 size have poor strength in case of light rain, and they can be bent even by hand. Gate valves are generally used before and after the control valve; of course, ball valves or butterfly valves can also be used, but gate valves offer a moderate size along with strong closing performance
Having it fully open is not conducive to the control valve regulating flow; moreover, fluid erosion can easily damage the valve
You mean the three-quarters mark next to the drain pipe indicates a diameter of 20?
Guide drainage at 3/4; it seems to be based on experience*habit, with no particularly strict restrictions. The manual valves before and after the control valve do not serve a regulating function; they only act as on/off valves. It is preferable to let the control valve regulate the flow rate. If the range of regulation provided by the control valve is limited, the manual valve in front of it can be closed slightly. Stuck hand valves can cause damage to the valves.