flap gate - seating and unseating head

huangyhg

flap gate - seating and unseating head
i am trying to get a clarification on this two terms (seating head and unseating head) as it relates to the design of a flap gate.
appreciate your help. thanks.
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as i understand it, the seating head is the pressure head required to close the flap valve and maintain the seal.  so for head levels less than this the valve is not water tight and allows water in.  think of a flap valve on the end of an outfall pipe in a tidal region.  as the tide rises the flap valve is there to prevent water entering the pipe.
the unseating head is the pressure head required to lift the weight of the flap and break a seal.  think when you have a flap valve on a very shallow pipe.  if the depth of water required to open the valve is too high then the line may back up and cause problems upstrema.