Origin of flap valve
Examples from the Web for flap valve
At o is a delivery valve, opening outwards, which is often a ball-valve but sometimes a flap-valve.
This blow-hole has a flap-valve, which the noxious blast opens, as often as it passes out.De Re Metallica|Georgius Agricola
A flap-valve nailed over a scupper-hole, serving to keep water from getting in, yet letting it out.The Sailor's Word-Book|William Henry Smyth