Pipe joints or couplings – Particular interface – Tapered
Reexamination Certificate
2001-11-19
2003-06-17
Nicholson, Eric K. (Department: 3679)
Pipe joints or couplings
Particular interface
Tapered
C285S390000
Reexamination Certificate
active
06578880
ABSTRACT:
TECHNICAL FIELD
The use of conventional screw threads to connect joints of pipe together so as to convey fluid, is a very old art that has progressed for hundreds of years in an effort to satisfy periodic needs for stronger and better sealing pipe connections. Performance requirements for pipe connections still vary widely today, such as for home piping with less than 80 psi fluid pressure with virtually no mechanical loads, to Oil Well Pipe that may be required to hold over 15,000 psi gas pressure and simultaneously, withstand extreme mechanical loadings and endure wide temperature fluctuations. The use of conventional screw threads to connect structural members is also an old art.
Due to the historical weakness of threaded pipe connections and their tendency to loosen, leak, and/or break, their use in industrial plants and refineries has been limited by Industrial Codes to very small pipe sizes and low pressures. However, because there is no reasonable alternative pipe connection for use within the very limited hole sizes drilled for Oil & Gas Wells, threaded pipe connections are still used today, so most research on and development of pipe connections has been directed toward such use. Structural use of conventional screw threads has been limited by their weakness and their tendency to loosen while in service.
In 1939 API adopted the 8Rd thread connection to connect joints of API tubing and casing, which is still used to connect about 80% of well pipe today. My U.S. Pat. No. 5,427,418 filed in 1992 explained why API 8Rd Connections loosen and leak, and “API Item 2239 Work Group” discovered that fact in 1995 and recently adopted some principles of '418 into API “SR17 Supplemental Requirements for API LTC Connections with Specified Performance”. In an effort to provide pipe connections that sealed better than API 8Rd Connections, special “Premium pipe connections” were developed by numerous other parties who adopted thin annular sealing lips at the end of their pin threads as depicted in my U.S. Pat. No. 2,766,998 that introduced proven reliable pressure-aided, high-pressure, high temperature metal seals to the nuclear and space industries as well as the oil and gas industry. However, such seals when used in oil-well pipe connections are fraught with several serious problems such as: leakage due to lip damage; excessive costs; and loss of connection efficiency when used in flush connections because the pipe wall thickness for supporting axial loads is reduced to form the lip and it's mating sealing surface. Today, most experts in the field believe that a lip seal is required “because pipe threads can't seal gas pressures above 10,000 psi” according to API SR17. There has been considerable confusion in the industry as to what constitutes a reliable qualification test for threaded pipe connections, which has resulted in too many sales claims reflecting hopes more than facts. New standard ISO-13679 gives promise to end that problem in that it allows one to choose the % efficiency ratings relative to the pipe ratings that a connection is to be tested and qualified for, under combinations of: internal pressure; external pressure; tension; compression; bending; temperature; and the choice of water or gas as the pressurizing fluid. It also specifies test procedures that accurately measure performance capability. Therefore, it is expected that the number of new connections offered for sale will decline in face of such stringent standards, but that real progress should accelerate because users can for the first time, begin immediate use of a new ISO qualified connection with confidence. Application for the instant invention is made with that realization in mind. For purposes of the present invention, the following definitions will apply.
Flank angle=The acute angle in a plane coinciding with the pipe axis, measured between a thread flank and a plane positioned 90 degrees to the axis, the angle being plus if the flank faces toward the crest, the angle being minus if the flank faces toward the root.
Included Angle=The algebraic sum of the stab flank angle and the load flank angle.
Thread Turn=A 360 degree portion of a screw thread.
Pin=A male threaded end, the thread turn nearest the pin face being the first thread turn.
Box=A female threaded pipe end formed to mate with pin threads, the thread nearest the box face, being the first thread turn.
Wedgethread=A screw thread form having a crest, root, stab flank and load flank, the load flank being formed on a greater helical angle than is the stab flank such that the axial length of the crest is least at the beginning of the first thread turn, the length gradually increasing to a maximum axial length at the end of the last thread turn, such that the box and pin may be screwed together to a desired position of full makeup at which, both stab flanks and load flanks contact and wedge against their respective mating flanks and thereby prevent further makeup of the connection.
Trapped Thread=A thread form having a negative included angle for at least a portion of the radial width of its flanks.
Open thread=A thread form having no negative included angle over the entire radial width of its flanks.
Metal-to-metal seal=A non-threaded surface formed completely around a portion of a box or pin that is in continuous contact with a mating surface of the other, so as to effect a seal against fluid within or outside of the connection.
Pin wall thickness=A dimension measured radially at mid-length of the engaged threads, extending from the pin thread pitch diameter to the pin bore.
Box wall thickness=a dimension measured radially at mid-length of the engaged threads, extending from the box thread pitch diameter to the box outer diameter.
Stab pitch=axial length between stab flanks, one thread turn apart.
Load pitch=axial length between load flanks, one thread turn apart.
Full-strength connection=a pipe connection that will seal and not rupture under any combination of loads at which, the VME yield stress of the box or pin wall is not exceeded.
Bridge thickness dimension=the maximum width of a gap formed between assembled mating threads that the thread dope used, will seal.
Angle of Friction=The arctangent of the co-efficient of friction, that acts between two solid bodies which slide against one another.
BACKGROUND ART
Moore U.S. Pat. No. 1,474,375 discloses an early form of a trapped thread, but not a wedgethread, that is sealed after assembly by compressing a malleable member between mating threads. Stone U.S. Pat. No. 2,006,520, discloses a square non-wedgethread form and seals elsewhere as at 39, 20 and 28, for a flush joint connection efficiency that “may be 53%”, per Col 8 line 10. He claims no thread seal as made evident in Col 3 ln 5-9, Col 4 lines 3-9 and Col 6 lines 44-47. Rotation is stopped by contact of shoulders 20 and 34, not by the wedging of threads. Col 4 ln 49-53 indicate that sealing “face 39 which is complimentary in angularity to and adapted to seat in fluid-tight relationship with face 18″, which precludes it's ability to form a reliable seal because rotation of the lower end of nose 39′ will cause it to contact seat 18 only at the top of surface 39 whereupon, the lower end of surface face 39 will rotate and no longer be in contact with face 18. Had surface 39 been formed at an angle less than face 18 as taught by '998, then a surface seal may have been formed.
Blose Re. 30,647 discloses Trapped Wedgethreads that suggests a thread seal in Col 2 ln 7-11 but does not teach how to accomplish a thread seal and in fact, cites clearances between roots and crests in Col 3 ln 40-43. Conversely in Col 1 beginning at ln 64, he cites, “Since back flank is intended to always be negative, thread strain reactions against this surface will cause the box member to be pulled radially inward and the pin member to be pulled radially outward.” The force of such pulling away of the box and pin threads fro
Nicholson Eric K.
Ramos, Beverly Watts
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