Positive seal coupling system

1. A hollow coupling for joining together two pipe ends and having a center, two coupling ends, and an inner profile, the coupling having a longitudinal axis, the inner profile comprising:

a serrated section having a plurality of threads extending circumferentially around the inner diameter of the coupling;

a curved section proximate the serrated section, and having a surface that curves radially outward relative to the longitudinal axis from the serrated section toward one of the two coupling ends;

a smooth section adjacent an end of the coupling; and

a transition section between the curved section and the smooth section, the transition section curving radially inward toward the longitudinal axis from the curved section to the smooth section, and having a smooth, rounded profile that minimizes stress concentrations.

2. The hollow coupling of claim 1 , the inner profile further comprising:

a center section at the center of the coupling proximate the serrated section and having a smooth profile surface.

3. The hollow coupling of claim 1 , wherein the threads are inwardly directed toward the longitudinal axis of the coupling.

4. The hollow coupling of claim 1 , wherein the threads are discrete circumferential threads that act as redundant metal to metal seals against a pipe end inserted into the coupling.

5. The hollow coupling of claim 4 , wherein at least a portion of the threads are helical threads for penetrating the outer surface of a pipe end inserted into the coupling to create mechanical resistance against separation of the coupling and the pipe.

6. The hollow coupling of claim 1 , wherein the inner profile further comprises:

a pipe end receiving section between the end of the coupling and the smooth section, the diameter of the pipe end receiving section tapered from a relatively larger diameter at a coupling end to a relatively smaller diameter at the smooth section.

7. A hollow coupling for joining together two pipe ends and having a longitudinal axis and an inner profile, the inner profile comprising:

a plurality of serrations extending over at least a portion of the inner profile and bounded by a first smooth end surface and a second smooth central surface of the coupling;

a first end serration bounding the plurality of serrations at a first end, and providing a transition from the first smooth end surface of the inner profile to the serrations; and

a second end serration bounding the plurality of serrations at a second end, and providing a transition from the serrations to the second smooth central surface of the inner profile;

the first end serration having a first radius and a second radius, the first radius originating at the first smooth end surface and curving radially inward relative to the longitudinal axis of the coupling from the first smooth end surface toward the plurality of serrations, and the second radius originating from the first radius, and curving radially outward relative to the longitudinal axis of the coupling from the first radius toward the plurality of serrations; and

the second end serration having a third radius and a fourth radius, the third radius originating at the plurality of serrations and curving radially outward relative to the longitudinal axis of the coupling from the plurality of serrations toward the second smooth central surface, and the fourth radius originating from the third radius and curving radially inward relative to the longitudinal axis of the coupling from the third radius toward the second smooth central surface of the profile;

the first and second radii of the first end serration, and third and fourth radii of the second end serration having rounded profiles that reduce stress concentrations in the coupling.

8. The hollow coupling of claim 7 , wherein the plurality of serrations are inwardly facing toward a center of the coupling.

9. The hollow coupling of claim 7 , wherein a portion of the serrations are helical serrations, and a portion of the serrations are circumferential serrations.

10. The hollow coupling of claim 7 , wherein all of the serrations are circumferential serrations.

11. The hollow coupling of claim 7 , wherein the plurality of serrations each has a tip that extends inwardly toward a center of the coupling a sufficient distance to engage the pipe ends and create mechanical resistance against the separation of the pipe ends from the coupling.

12. The hollow coupling of claim 7 , wherein the first and second radii of the first end serration are about 0.75 inches and about 0.25 inches respectively.

13. The hollow coupling of claim 7 , wherein the third and fourth radii of the second end serration are about 1.5 and about 0.5 inches respectively.

14. A coupling for joining together pipe ends and having a first side with a first end and a second side with a second end, the coupling having an inner profile comprising:

first and second serrated sections proximate the center section on the first and second sides of the coupling respectively, the first and second serrated sections having a plurality of threads extending radially around the inner diameter of the coupling;

first and second curved sections proximate the first and second serrated sections respectively, the first and second curved sections each having a surface that curves radially outward relative the longitudinal axis from the first and second serrated sections, respectively, toward the first and second ends of the coupling;

first and second smooth sections adjacent first and second ends of the coupling; and

first and second transition sections between the first and second curved sections and the first and second smooth section respectively, each of the first and second transition section having a surface that curves radially inward relative to the longitudinal axis from the first and second curved sections, respectively, to the first and second sections, the first and second transition sections having a smooth, rounded profile that minimizes stress concentrations.

15. The hollow coupling of claim 14 , wherein the center section has a smooth profile surface.

16. The hollow coupling of claim 14 , wherein the threads of the first and second serrated sections are discrete circumferential threads that act as redundant metal to metal seals against the pipe ends inserted into the coupling.

17. The hollow coupling of claim 16 , wherein at least a portion of the threads of the first and second serrated sections are helical, and penetrate the outer surface of the pipe end inserted into the coupling to create mechanical resistance against separation of the coupling and the pipe.

18. The hollow coupling of claim 14 , wherein the threads of the first and second serrated sections are inwardly facing toward a center of the coupling.

19. The hollow coupling of claim 15 , further comprising:

an annular gasket positioned at the center of the coupling for preventing direct contact between the pipe ends when inserted into the coupling, and for forming a redundant seal between the coupling and the pipe ends.

20. The hollow coupling of claim 14 , wherein the inner profile further comprises:

first and second pipe end receiving sections between the first and second ends of the coupling and the first and second smooth sections respectively, the diameter of the first and second pipe end receiving sections tapered from a diameter larger than the outer diameter of the pipe ends at the first and second ends of the coupling to the relatively larger diameter at the first and second ends of the coupling to a relatively smaller diameter at the first and second smooth sections.