Abstract: An elongated, flexible unitary sheet having two longitudinal borders, said sheet being folded lengthwise, in thirds, to define a first elongated fold of said sheet sandwiched between second and third elongated folds; wherein each of said two longitudinal borders is opposite a face of one of said folds and is sealed thereto so that said second and third folds together define the outer wall of a substantially enclosed conduit and so that said first fold provides a diaphragm dividing said conduit into first and second channels; a pipe detachably connected to the upstream end of the conduit, said pipe including flow control means to stop the flow of fluid to either one of said channels while still permitting flow to the other of said channels. The flow control means comprises a longitudinal membrane in the pipe which is connected at its downstream end to the first fold, and which includes a flat elongated spring at its upstream end to move the upstream end from one inner pipe surface to an opposite inner surface.