Check valves

1. A check valve comprising:

a flexible tubular valve member extending longitudinally between an inlet and a longitudinally opposite outlet, the flexible tubular valve member having a longitudinal passage for conducting a fluid from the inlet to the outlet, and a valve wall extending longitudinally between the inlet and the outlet and circumferentially around the passage;

the valve wall including a bill extending longitudinally along the valve wall and terminating at a bill end contiguous with the outlet, the bill having a bill wall including laterally opposite lips extending in longitudinal and altitudinal directions adjacent the outlet and having a prescribed altitudinal extent, the bill wall including a transition portion extending longitudinally between the inlet and the lips, and the lips being biased in lateral directions into engagement with one another along a slit extending altitudinally across the bill adjacent the bill end such that the outlet normally is closed against backflow of the fluid into the passage through the outlet by engagement of the lips along the slit; and

at least one fold in the bill wall, the fold extending longitudinally along a corresponding lip and, when in a folded configuration, including overlying legs extending across the bill in an essentially altitudinal direction, essentially parallel to the slit, so as to overlie the corresponding lip, the fold having a prescribed altitudinal extent and a prescribed longitudinal extent such that upon fluid entering the passage at the inlet and exerting fluid forces upon the valve wall at the bill to unfold the at least one fold and move the legs of the fold laterally, the relative dimensions of the altitudinal extent of the legs of the fold and the altitudinal extent of the bill enables unfolding of the at least one fold and lateral movement of the legs of the fold to part the lips in lateral directions and open the outlet to permit the flow of fluid from the inlet for discharge through an opening at the outlet of the valve member in response to minimum fluid forces upon the bill, and for providing the opening with an increased maximum available perimeter.

2. The check valve of claim 1 including at least two folds extending serially along the corresponding lip.

3. The check valve of claim 2 wherein the at least two folds are essentially contiguous with one another along the essentially altitudinal direction.

4. The check valve of claim 1 including a plurality of folds extending along the corresponding lip.

5. The check valve of claim 4 wherein at least two of the plurality of folds overlie one another.

6. The check valve of claim 1 wherein the at least one fold is unitary with the bill wall.

7. The check valve of claim 1 wherein the at least one fold is integral with the bill wall.

8. The check valve of claim 1 wherein the at least one fold is constructed of a material having memory characteristics establishing biasing forces for biasing the fold into the folded configuration.

9. The check valve of claim 1 wherein the valve member includes at least one fold overlying each lip, each fold, when in a folded configuration, including corresponding legs overlying a corresponding lip and extending essentially parallel to the slit so as to overlie the corresponding lip.

10. The check valve of claim 9 including at least two folds extending serially along each lip.

11. The check valve of claim 10 wherein the at least two folds are essentially contiguous with one another along the essentially altitudinal direction.

12. The check valve of claim 9 including a plurality of folds extending along the corresponding lip.

13. The check valve of claim 12 wherein at least two of the plurality of folds overlie one another.

14. The check valve of claim 9 wherein the at least one fold overlying each lip is unitary with the bill wall.

15. The check valve of claim 9 wherein the at least one fold overlying each lip is integral with the bill wall.

16. The check valve of claim 9 wherein the at least one fold overlying each lip is constructed of a material having memory characteristics establishing biasing forces for biasing the fold into the folded configuration.

17. A check valve comprising:

a flexible tubular valve member extending longitudinally between an inlet and a longitudinally opposite outlet, the flexible tubular valve member having a longitudinal passage for conducting a fluid from the inlet to the outlet, and a valve wall extending longitudinally between the inlet and the outlet and circumferentially around the passage;

the valve wall including a bill extending longitudinally along the valve wall and terminating at a bill end contiguous with the outlet, the bill having a bill wall including laterally opposite lips extending in longitudinal and altitudinal directions adjacent the outlet and having a prescribed altitudinal extent, the bill including a transition portion extending longitudinally between the inlet and the lips, and the lips being biased in lateral directions into engagement with one another along a slit extending altitudinally across the bill adjacent the bill end such that the outlet normally is closed against backflow of the fluid into the passage through the outlet by engagement of the lips along the slit;

the bill wall being folded essentially altitudinal upon itself to establish at least one folded portion in the valve wall, the at least one folded portion extending longitudinally along the a corresponding lip and, when in a folded configuration, including overlying legs extending across the bill in an essentially altitudinal direction, essentially parallel to the slit, so as to overlie the corresponding lip, the at least one folded portion having a prescribed altitudinal extent and a prescribed longitudinal extent such that upon fluid entering the passage at the inlet and exerting fluid forces upon the valve wall at the bill to unfold the at least one folded portion and move the legs of the at least one folded portion laterally, the relative dimensions of the altitudinal extent of the legs of the at least one folded portion and the altitudinal extent of the bill enables unfolding of the at least one folded portion and lateral movement of the legs of the at least one folded portion to part the lips in lateral directions and open the outlet to permit the flow of fluid from the inlet for discharge through an opening at the outlet of the valve member in response to minimum fluid forces upon the bill, and for providing the opening with an increased maximum available perimeter.

18. The check valve of claim 17 including at least two folded portions extending serially along the corresponding lip.

19. The check valve of claim 18 wherein the at least two folded portions are essentially contiguous with one another along the essentially altitudinal direction.

20. The check valve of claim 17 including a plurality of folded portions extending along the corresponding lip.

21. The check valve of claim 20 wherein at least two of the plurality of folded portions overlie one another.

22. The check valve of claim 17 wherein the at least one folded portion is unitary with the bill wall.

23. The check valve of claim 17 wherein the at least one folded portion is integral with the bill wall.

24. The check valve of claim 17 wherein the at least one folded portion is constructed of a material having memory characteristics establishing biasing forces for biasing the folded portion into the folded configuration.

25. The check valve of claim 17 wherein the valve member includes at least two folded portions overlying each lip, each folded portion, when in a folded configuration, including corresponding legs overlying a corresponding lip and extending essentially parallel to the slit so as to overlie the corresponding lip.

26. The check valve of claim 25 including at least two folded portions extending serially along each lip.

27. The check valve of claim 26 wherein the at least two folded portions are essentially contiguous with one another along the essentially altitudinal direction.

28. The check valve of claim 25 including a plurality of folded portions extending along the corresponding lip.

29. The check valve of claim 28 wherein at least two of the plurality of folded portions overlie one another.

30. The check valve of claim 25 wherein the at least one folded portion overlying each lip is unitary with the bill wall.

31. The check valve of claim 25 wherein the at least one folded portion overlying each lip is integral with the bill wall.

32. The check valve of claim 25 wherein the at least one folded portion overlying each lip is constructed of a material having memory characteristics establishing biasing forces for biasing the folded portion into the folded configuration.