Abstract: A valve apparatus comprises at least one valve device having a chamber, an inlet and an outlet. A moveable member in the chamber is arranged to displace a membrane such that the membrane is rolled against the wall of the chamber between a closing position at which the membrane covers the opening of the outlet and an opening position at which the opening of the outlet is uncovered. A flow passage is formed for a fluid to flow from the inlet to the outlet. The moveable member is provided with an entrainment member arranged to extend into and be displaced in the flow passage during at least a substantial part of the displacement of the membrane between closing and opening positions.