Visual inspection of valves
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Valves such as gate valves, globe valves, throttle valves, control valves, butterfly valves, and foot valves should be in the fully closed position； The closing elements of plug valves and ball valves should be in the fully open position ； The diaphragm valve should be in the closed position, but it should not be closed too tightly to prevent damage to the diaphragm ； The valve disc of the check valve should be closed and secured. The visual inspection of the valve is as described above; could any expert explain the specific reasons for this?
I think it’s to protect the sealing surface. The positions of the sealing surfaces vary among different types of valves; in some cases, they prevent foreign objects from coming into contact when the valve is closed, while in others, this protection is achieved only when the valve is open. Think about it: if a ball valve is closed, are the sealing surfaces exposed?
Gate valves, stop valves, throttle valves, etc. – they are closed to prevent debris from entering and causing damage to the sealing surfaces when they are closed next time. Ball valves and plug valves have seals that can be replaced; however, if the ball surface is damaged, it cannot be replaced or the cost of replacement would be too high, so those valves remain open. Diaphragm valves are kept open to avoid damage to the diaphragm caused by vibrations. Check valves are designed for swing-type and lift-type valves, and there should be no major issues with them. I asked others about this, and these are their opinions
The main concern is to prevent dirt from getting in, especially during pipeline welding, as the regulations have clear requirements in this regard