Both duckbill and inline valves perform well when fully submerged, preventing backflow effectively. In partially submerged conditions, however, the inline valve is at a disadvantage: its higher cracking pressure and slower response time work against it.
The key factor is submerged surface area. A duckbill valve presents roughly 4 times more effective area underwater than an inline valve, which translates directly into more force available to open it. This gap widens further when the inline valve's circular face isn't fully submerged, since only the wetted portion contributes to opening force.
As a result, a duckbill valve typically responds within a single wave period, while an inline valve lags behind.
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