Threaded unions are used to provide fluid-tight joints in fluid conduits. Threaded unions are held together by a threaded nut that is tightened to a required torque using a wrench or a hammer. In the oil industry, threaded unions are generally constructed using “wing nuts” and are commonly called “hammer unions” or “hammer lug unions”. Hammer unions are designed and manufactured in accordance with the specifications stipulated by the American Petroleum Institute in API 6A entitled “Specification for Wellhead and Christmas Tree Equipment”. Hammer unions are usually available in a variety of sizes (1″ to 10″) and a variety of pressure ratings (1000 psi to over 20,000 psi).
As illustrated in FIG. 1, a typical prior-art hammer union 10 includes a first subcomponent 12 and a second subcomponent 14. The first subcomponent has an inner lateral surface 13 which abuts an outer lateral surface 15 of the second subcomponent. The first subcomponent has at least one annular groove 16 for receiving an elastomeric O-ring 18. The annular groove is located at an interface of the inner and outer lateral surfaces. The second subcomponent is secured to the first subcomponent by a wing nut 20.
The wing nut 20 has box threads 22 that engage pin threads of the first subcomponent. The wing nut 20 further includes a plurality of lugs 24 that extend radially from a main body of the wing nut 20. The lugs have impact surfaces 25 which may be impact-torqued using a hammer or mallet (not shown) to tighten or loosen the wing nut 20.
The wing nut 20 also has an annular top wall 26 which abuts a radial flange 28 of the second subcomponent. When torque is applied to the wing nut, the annular top wall 26 is forced downwardly on the radial flange 28, thereby locking together the second subcomponent and the first subcomponent.
The hammer union 10 is shown with an elastomeric O-ring 18 having a circular cross-section. In the prior art, the O-rings are made of materials such as rubber, nylon, polyurethane and Teflon™.
Depending on the nature and shape of the first and second subcomponents, a different type of elastomeric seal may be used. For example, lip seals and flat gaskets may be used instead of the O-ring shown in FIG. 1. For sour service wells, the lips seals and O-rings are typically made of nitrile rubber (NBR) and fluroelastomers (FPM). Elastomeric seals may also be energized using stainless steel garter springs to prevent the seal from being damaged by extrusion.
FIG. 2 is a schematic cross-sectional view of a prior art hammer union manufactured by FMC/Weco that utilizes a lip seal. The hammer union shown in FIG. 2 is very much like the hammer union shown in FIG. 1, with the exception that the high-pressure seal is provided by the lip seal 17, which includes a peripheral lip 19 that is received in a groove 21 at a bottom of a seal seat 23 in the first subcomponent 12. The lip seal 17 is made of a rubber composition, and the hammer union is rated for up to 15,000 psi of fluid pressure.
One substantial disadvantage of prior-art hammer unions is that their elastomeric seals are vulnerable to the extreme temperatures generated by fire. In the event that a fire erupts in the well or at the wellhead, the elastomeric seal in the hammer union may leak or fail completely. This permits hydrocarbons to escape to the atmosphere, which may exacerbate the fire.
To the best of the applicant's knowledge, a prior art hammer union with a metal-to-metal seal for providing a high-pressure, fluid-tight seal has never been designed or manufactured. While metal seals and metallic gaskets are known in the art (e.g. U.S. Pat. No. 4,832,381 (Boulton) entitled “Seal”, U.S. Pat. No. 5,257,792 (Putch et al.) entitled “Well Head Metal Seal”, U.S. Pat. No. 4,056,272 (Morrill) entitled “Seal”, and U.S. Pat. No. 1,825,962 (Laird) entitled “Gasket”) but each of the above describes a seal for a flanged union. While flanged unions are widely used in well trees, they are relatively expensive to construct and time consuming to assemble in the field.
It is well known in the art that there is increasing pressure on the oil industry to produce hydrocarbons at a lower cost. Consequently, an interest has developed in utilizing wellhead equipment that is less expensive to construct and is more quickly assembled than prior art flanged well tree components. Threaded unions provide a good alternative to flanged unions from a cost standpoint because they are faster to assemble and less expensive to construct. However, due to safety concerns related to the lack of a metal-to-metal seal, use of threaded unions for well tree components has not been endorsed.
Therefore, there exists a need for a threaded union with a metal-to-metal seal.