Why can't double sealing valves be used as shut-off valves?
The advantage of the two-seat valve core is force balance structure, allowing a large pressure difference, and its prominent disadvantage is that the two sealing surfaces can not be in good contact at the same time, resulting in large leakage. If it is used artificially and compulsively in cutting off occasion, obviously the effect is not good, even if many improvements have been made for it (such as double-sealed sleeve valve), it is not advisable.
Why is the control valve easy to oscillate when the double-seat valve is working at a small opening?
For single core, when the medium is open, the stability of the valve is good; when the medium is closed, the stability of the valve is poor. Double-seat valve has two spools, the lower spool is in the flow closure, the upper spool is in the flow opening. In this way, when working at a small opening, the flow-closed spool will easily cause the vibration of the valve, which is the reason why the double-seat valve can not be used at a small opening.