Corrugated pipe

1. A corrugation comprising:

a radially inner wall circumferentially surrounding an axis and defining an inner cavity configured to conduct a liquid axially through said inner cavity; and

a radially outer wall overlying said inner wall, said outer wall adjoining said inner wall so as to form an outer cavity bounded by said inner and outer walls and separated from said inner cavity by said inner wall and closed to the outside by said outer wall;

said inner wall having first and second openings providing fluid communication between said inner and outer cavities such that said corrugation provides a reservoir for the liquid conducted through said inner cavity;

said corrugation having a predetermined installed orientation defined by a designated bottom end of said corrugation;

said second opening being located higher than said first opening in said installed orientation of said corrugation; and

said corrugation being centered on said axis and, in said installed orientation of said corrugation, said first opening is located at an angle of about 35 to about 55 degrees below said axis, and said second opening is located at an angle of about 35 to about 55 degrees above said axis.

2. The corrugation of claim 1 wherein said inner wall is cylindrical and centered on said axis.

3. The corrugation of claim 1 wherein said outer wall is annular and centered on said axis.

4. The corrugation of claim 1 wherein said outer cavity fully surrounds said inner cavity.

5. The corrugation of claim 1 wherein said corrugation is a spigot corrugation.

6. The corrugation of claim 1 wherein said first and second openings are two of multiple such openings spaced symmetrically about said axis.

7. The corrugation of claim 1 further comprising a another such opening located horizontally opposing and level with said first opening in said installed orientation of said corrugation.

8. The corrugation of claim 1 configured for a pressurized air pocket to be formed in said outer cavity by the liquid rising above said second opening.

9. A pipe comprising an axially extending series of corrugations as defined in claim 1 .

10. The pipe of claim 9 , wherein said first openings are on a common lower horizontal plane and said second openings are on a common upper horizontal plane.

11. A pipe comprising:

a wall circumferentially surrounding an axis and defining an axial-flow cavity for conducting a liquid axially through said axial-flow cavity;

a reservoir structure defining a reservoir cavity separated from said axial-flow cavity by said wall and closed to the outside;

a lower opening in said wall providing fluid communication between maid axial-flow cavity and said reservoir cavity, such that a radially outward flow of the liquid from said axial-flow cavity to said reservoir cavity through said lower opening occurs by the liquid in said axial-flow cavity rising above said lower opening; and

an upper opening in said wall located above said lower opening in a predetermined installed orientation of said pipe, said upper opening providing fluid communication between said axial-flow cavity and said reservoir cavity, such that a pressurized air pocket is formed in said reservoir cavity by the liquid in said axial-flow cavity rising above said upper opening, said pressurized air pocket opposing and slowing said radially outward flow to said reservoir cavity.

12. The pipe of claim 11 wherein said wall is cylindrical and centered on said axis.

13. The pipe of claim 11 wherein said reservoir structure is one of an axially extending series of such reservoir structures of said pipe.

14. The pipe of claim 13 further comprising, for each reservoir cavity, a second such upper opening facing said first upper opening and a second such lower opening facing said first lower opening, wherein said upper openings are all located on a common upper horizontal plane and said lower openings are all located on a common lower horizontal plane.

15. The pipe of claim 11 wherein said reservoir structure is a corrugation of a corrugated pipe and includes said wall.

16. The pipe of claim 11 wherein said wall is centered on said axis and, in said installed orientation of said pipe, said lower opening is located at an angle of about 35 to about 55 degrees below said axis, and said upper opening is located at an angle of about 35 to about 55 degrees above said axis.

17. A method of using the pipe of claim 11 , comprising:

installing said pipe, for conduction of liquid through said pipe, in an orientation in which a pressurized air pocket will be formed in said reservoir cavity by the liquid in said axial-flow cavity rising above said upper opening and said pressurized air pocket will oppose and slow said radially outward flow to said reservoir cavity.

18. A spigot corrugation comprising:

a radially inner wall circumferentially surrounding an axis and defining an inner cavity configured to conduct a liquid axially through said inner cavity; and

a radially outer wall overlying said inner wall, said outer wall adjoining said inner wall to form a spigot corrugation of a corrugated pipe with an outer cavity bounded by said inner and outer walls and separated from said inner cavity by said inner wall and closed to the inside by said outer wall;

said inner wall having an opening providing fluid communication between said inner and outer cavities such that said spigot corrugation provides a reservoir for the liquid conducted through said inner cavity.