a. a housing, said housing comprising a throughflow channel for allowing fluid communication from an intake of said channel to an exhaust of said channel; and

b. a sealing flap secured to said housing such that said sealing flap closes said exhaust of said channel and is adapted to flexibly open in response to pressure, said sealing flap comprising a relatively pliable layer oriented toward said intake and a relatively rigid layer dimensioned to be substantially co-extensive with said pliable layer and oriented toward said exhaust, whereby said relatively rigid layer operates to close said sealing flap irrespective of said pressure relief valve's orientation with respect to gravity.