Abstract: A check valve has an inlet end, a transition part and an outlet end, with a pair of vertically orientated lips. The transition part is twisted about the axis extending from the inlet to the outlet. As fluid passes through the check valve, the twisted portion of the check valve will impart a swirling motion to the fluid as the fluids leaves the check valve via the outlet end.

Abstract: An asymmetrical check valve has an inlet end, a transition part and an outlet end, with a pair of vertically orientated lips. The check valve is asymmetric about a horizontal longitudinal plane through a center axis through the inlet end. The upper portion of the transition part increases upwardly, from the inlet end to the outlet end. The lower portion of the transition part increases downwardly, but with less of an increase when compared with the upper portion of the transition part. The inlet end includes a flange or a cuff. The vertically orientated lips may have variable rigidity along their height, and a support may be used to maintain the orientation of the upper portion of the transition part.