Two-part back cap for a plug valve and plug valves incorporating same

1. A two-part back cap for a plug valve, comprising:

a back cap seal body having an inner end and an outer end, the inner end comprising a seal rim that includes at least one O-ring groove that accepts an O-ring for providing a fluid seal between the back cap seal body and a plug valve body of the plug valve; and

a back cap retainer nut having pin threads that threadedly engage box threads in the plug valve body for securely retaining the back cap seal body within the plug valve body, the back cap retainer nut having an inner end that supports only a first part of the outer end of the back cap seal body until the plug valve is subjected to high fluid pressures, the outer end of the back cap seal body comprising first and second surfaces, the first and second surfaces being inclined towards the seal rim at different angles of inclination with respect to a line perpendicular to an axis of the back cap seal body and the back cap retainer nut supports only the first surface until the plug valve is subjected to the high fluid pressures.

2. The two-part back cap as claimed in claim 1 wherein the first surface is inclined toward the seal rim at an angle of inclination between 20°-40°.

3. The two-part back cap as claimed in claim 1 wherein the second surface is inclined toward the seal rim at an angle of inclination that is 0.5°-1.5° greater than the angle of inclination of the first surface.

4. The two-part back cap as claimed in claim 1 wherein the back cap retainer nut comprises an inner end that is inclined toward the seal rim at an angle of inclination equal to the angle of inclination of the first surface of the back cap seal body.

5. The two-part back cap as claimed in claim 2 wherein the angle of inclination of the first surface is about 30°.

6. The two-part back cap as claimed in claim 2 wherein the angle of inclination of the second surface is about 30.5°.

7. The two-part back cap as claimed in claim 1 wherein the back cap seal body further includes an undercut between a plug support surface and the seal rim of the back cap seal body.

8. The two-part back cap as claimed in claim 7 wherein the undercut comprises a radius formed adjacent the plug support surface.

9. The two-part back cap as claimed in claim 8 wherein an end of the first surface and a beginning of the second surface is axially aligned with an inner edge of the radius.

10. The two-part back cap as claimed in claim 1 wherein the back cap seal body further comprises at least two tapped bores for the connection of a puller to remove the back cap seal body from the plug valve body.

11. The two-part back cap as claimed in claim 1 wherein the seal rim comprises first and second O-ring grooves.

12. A plug valve, comprising:

a back cap seal body having an inner end with a plug support surface for supporting a plug of the plug valve and an outer end comprising first and second surfaces inclined at different angles with respect to an axis of the back cap seal body; and

a back cap retainer nut comprising a hollow nut having an inner end for supporting only the first surface of the outer end of the back cap seal body until the plug valve is subjected to high fluid pressure, and pin threads for engaging box threads in a plug valve body of the plug valve to securely retain the back cap seal body within the plug valve body.

13. The plug valve as claimed in claim 12 wherein the back cap seal body comprises a seal rim located along an outer edge of the inner end, the seal rim comprising at least one O-ring groove in an outer periphery thereof.

14. The plug valve as claimed in claim 13 wherein the back cap seal body further comprises an undercut between the seal rim and the plug support surface.

15. The plug valve as claimed in claim 14 wherein the undercut comprises a radius formed adjacent the plug support surface.

16. The plug valve as claimed in claim 15 wherein the first surface is inclined toward the seal rim at a first angle of inclination, and the second surface is inclined toward the seal rim at a second angle of inclination.

17. The plug valve as claimed in claim 16 wherein the inner end of the back cap retainer nut is inclined toward the seal rim at an angle of inclination that is equal to the angle of inclination of the first surface of the back cap seal body.

18. The plug valve as claimed in claim 17 wherein the first surface ends and the second surface begins in axial alignment with an inner edge of the radius.

19. A two-part back cap for a plug valve, comprising:

a back cap seal body comprising an inner end for supporting a plug of the plug valve and a seal rim that supports at least one O-ring for sealing against an end cap seal surface of a plug valve body of the plug valve, and an outer end having first and second surfaces, the first surface being adjacent a stud bore through the back cap seal body and the second surface being continuous with the first surface, the first surface being inclined toward the seal rim at a first angle of inclination and the second surface being inclined toward the seal rim at a second angle of inclination that is greater than the first angle of inclination; and

a back cap retainer nut comprising a hollow nut having an inner end for engaging the outer end of the back cap seal body, the inner end being inclined toward the seal rim at an angle of inclination that is equal to the angle of inclination of the first surface of the back cap seal body, and further having a pin thread for engaging a box thread of the plug valve body to securely retain the back cap seal body within the plug valve body.