Origin of flap valve
First recorded in 1865–70
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
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.
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
This blow-hole has a flap-valve, which the noxious blast opens, as often as it passes out.De Re Metallica