An asymmetrical check valve has an inlet end, a transition part and an outlet end, with a pair of vertical lips facing one another. The lips may be flat or they may be curvilinear. The check valve is asymmetric about a horizontal longitudinal plane through the check valve. The bottoms of the transition...http://www.google.de/patents/US5931197?utm_source=gb-gplus-sharePatent US5931197 - Asymmetrical check valve
