Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HIGH-PRESSURE THREADED UNION WITH METAL-TO-
METAL SEAL. AND METAL RING GASKET FOR SAME
RELATED APPLICATIONS
This is a division of application number 2,512,128 filed July 14, 2005.
TECHNICAL FIELD
The present invention relates generally to sealed joints for high-pressure
fluid conduits and, in particular, to a metal ring gasket for threaded unions
for
use in very high fluid pressure applications.
BACKGROUND OF THE INVENTION
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 hammer or a wrench. 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 12") and a variety of pressure ratings (1000 psi to over 20,000 psi).
One substantial disadvantage of most prior-art threaded unions is that
they rely on elastomeric seals for achieving a fluid-tight joint. Elastomeric
seals
are vulnerable to the extreme temperatures generated by fire. In the event
that
a fire erupts around a high-pressure conduit, the elastomeric seal in the
threaded union may leak or fail completely which may exacerbate the fire if
the
leak permits combustible fluids to escape to the atmosphere.
While flanged unions are commonly used in well trees, pipelines and
other high-pressure applications where temperature tolerant seals are
required,
flanged unions are relatively expensive to construct and time-consuming to
assemble in the field. Metal ring gaskets are known for flanged unions, such
as
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the BX ring gasket manufactured in accordance with API 6A. In operation,
however, these BX ring gaskets are deformed beyond their yield strength and
must be discarded after a single load cycle.
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 unions.
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 reliable
high-pressure metal-to-metal seal, use of threaded unions for well tree
components and other high-pressure temperature tolerant applications has not
been endorsed.
Therefore, it is highly desirable to provide an improved threaded union
having a high-pressure metal-to-metal seal.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an improved
threaded union for providing a high-pressure, fluid-tight, metal-to-metal
seal.
The invention therefore provides a threaded union for providing a high-
pressure, fluid-tight, metal-to-metal seal in a fluid conduit, the threaded
union
comprising: first and second subcomponents that are interconnected by a nut,
the first and second subcomponents having respective mating ends with
complementary ring gasket grooves therein that form an annular cavity when
the mating ends abut; and a metal ring gasket received in the annular cavity,
wherein the metal ring gasket has an outer diameter that is slightly larger
than
an outer diameter of the annular cavity and the metal ring gasket is
elastically
deformed in the annular cavity to provide a high-pressure fluid-tight seal
between the first and second subcomponents when the first and second
subcomponents are securely interconnected by the nut.
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The invention further provides a metal ring gasket for providing a high-
pressure, fluid-tight metal-to-metal seal in an annular cavity formed by
annular
grooves at an interface of first and second subcomponents of a threaded union,
the metal ring gasket comprising a generally annular body having beveled outer
comers for receiving compressive loads exerted on the metal ring gasket by
complementary surfaces on an outer diameter of the annular cavity when the
first and second subcomponents are securely interconnected, the metal ring
gasket having an outer diameter that is slightly larger than an outer diameter
of
the annular cavity and the metal ring gasket is elastically deformed in the
annular cavity to provide a high-pressure fluid-tight seal between the first
and
second subcomponents when the first and second subcomponents are securely
interconnected by the nut.
The invention further provides a method of providing a high-pressure
fluid-tight seal between first and second subcomponents of a threaded union,
the method comprising: determining an inner diameter and an outer diameter of
an annular metal ring gasket groove in a mating face of the first and second
subcomponents; and manufacturing a metal ring gasket to be received in an
annular cavity formed by the respective metal ring gasket grooves when the
first
and second subcomponents are securely interconnected by a nut of the
threaded union, the metal ring gasket having outer faces for mating contact
with
complementary faces in the respective metal ring gasket grooves, an outer
diameter that is slightly larger than an outer diameter of the annular ring
gasket
grooves and an inner diameter that is slightly larger than an inner diameter
of
the ring gasket grooves so that the metal ring gasket is elastically deformed
when placed in the annular grooves and the first and second subcomponents
are securely interconnected, but a gap remains between an inner side of the
metal ring gasket and an inner surface of the annular cavity.
The threaded union in accordance with the invention can be used to
construct wellhead components, well tree components, or joints in any fluid
conduit where a reliable high-pressure temperature tolerant fluid seal is
required.
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BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages of the present invention will become
apparent from the following detailed description, taken in combination with
the
appended drawings, in which:
FIG. 1 is a cross-sectional view of a threaded union and a metal ring
gasket in accordance with one embodiment of the invention;
FIG. 2 is an exploded, cross-sectional view of the threaded union shown
in FIG. 1;
FIG. 3 is a cross-sectional view of a threaded union and a metal ring
gasket in accordance with another embodiment of the invention;
FIG. 4 is a cross-sectional view of the metal ring gasket shown in FIGs.
1-3 immediately prior to elastic deformation of the metal ring gasket as the
threaded union is tightened to a sealed condition;
FIG. 5 is a cross-sectional view of the metal ring gasket shown in FIGs.
1-3 schematically illustrating an extent of elastic deformation of the metal
ring
gasket when the threaded union is in the sealed condition;
FIG. 6 is a cross-sectional view of the metal ring gasket shown in FIGs. 1
and 3 when the threaded union is in the sealed condition and under elevated
fluid pressure; and
FIG. 7 is a cross-sectional view of another embodiment of a metal ring
gasket in accordance with the invention in a sealed condition under elevated
fluid pressures.
It should be noted that throughout the appended drawings, like features
are identified by like reference numerals.
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DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The invention provides a threaded union with a metal ring gasket that
provides a high-pressure, temperature tolerant, metal-to-metal fluid seal
between a first subcomponent and a second subcomponent of the threaded
union. The metal ring gasket is made of ductile carbon steel for non-corrosive
fluid service or ductile stainless steel for corrosive fluid service. The
metal ring
gasket has outer beveled corners and is received in a beveled annular groove
in a mating end of the first subcomponent. When compressed between the first
and the second subcomponents, the metal ring gasket deforms elastically to
provide an energized high-pressure fluid seal. The high-pressure seal is
capable of containing fluid pressures of up to at least 30,000 pounds per
square
inch (psi), and is not affected by elevated temperatures below a melting point
of
the ductile steel of the metal ring gasket.
Throughout this specification, the terms "first subcomponent" and
"second subcomponent" are meant to denote any two contiguous components
of a joint in a fluid conduit that are joined together using a threaded nut.
FIG. 1 illustrates a threaded union 10 in accordance with an embodiment
of the invention. The threaded union 10 includes a first subcomponent 12 and a
second subcomponent 14. The first and second subcomponents 12, 14 are
generally annular bodies that are interconnected to define a central fluid
passageway 15 as part of a high-pressure fluid conduit. The first
subcomponent 12 has a mating end 16 that abuts a mating end 18 of the
second subcomponent 14. The first subcomponent 12 has a top surface that
includes an upwardly facing annular groove 20. The upwardly facing annular
groove 20 is dimensioned to receive a metal ring gasket 30 in accordance with
the invention. The second subcomponent 14 has a bottom surface that includes
a downwardly facing annular groove 22. The upwardly facing and downwardly
facing annular grooves 20, 22 mate when the second subcomponent 14 is
connected to the first subcomponent 12 to define a hexagonal annular
cavity 24. However, as will be explained below, the annular cavity 24 need not
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necessarily be hexagonal to provide the energized high-pressure fluid seal in
accordance with the invention.
As shown in FIG. 1, the second subcomponent 14 is secured to the first
subcomponent 12 by a threaded nut 40. The threaded nut 40 has box threads
42 for engaging pin threads 44 formed externally on the first subcomponent 12.
In one embodiment, the threaded nut 40 is a wing nut and includes a plurality
of
lugs 46 that extend radially from a main body 48 of the threaded nut 40. The
lugs 46 have impact surfaces 46a which may be impact-torqued using a
hammer or mallet (not shown) in the usual way in which a hammer union is
"hammered up". In another embodiment, the threaded nut 40 is a "spanner nut"
that includes flats, bores, or the like, that are gripped by a spanner wrench
(not
shown) to permit the threaded nut 40 to be tightened to a required torque. As
will be understood by those skilled in the art, the wrench used to tighten the
nut
may be a torque wrench, which indicates the torque applied to the threaded nut
40 to ensure that it is tightened with a precise amount of torque.
The threaded nut 40 in accordance with this embodiment of this invention
is constructed in three parts so that a main body of the nut 40 can be a
single
piece construction for greater strength. As is understood by those skilled in
the
art, the nuts for hammer unions are commonly cut into two parts that are
welded
together in situ after the nut is positioned above an annular shoulder 14a of
the
second subcomponent 14. However, this compromises the holding strength of
the nut, which is strained when the hammer union is exposed to very high fluid
pressure. The threaded nut 40 in accordance with the invention has an upper
annular shoulder 47 that extends radially inwardly from a top of the main body
48 of the nut 40. The annular shoulder 47 abuts a flange 52 that extends
radially outwardly from an adapter collar 50. The adapter collar 50 is a
generally annular multi-piece body having an inner diameter dimensioned to
slide over an outer surface of the second subcomponent 14 until a bottom
surface 54 of the adapter collar 50 abuts the annular shoulder 14a of the
second subcomponent 14. A bottom surface of the annular shoulder 14a, in
turn, abuts a top surface 16 of the first subcomponent 12. When torque is
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applied to the nut 40, the upper annular shoulder 47 of the nut 40 is forced
downwardly on the flange 52, which in turn exerts a downward force on the
annular shoulder 14a, thereby forcing the bottom surface 18 of the second
component 14 against the top surface 16 of the first subcomponent 12, and thus
forcing the metal ring gasket 30 to a set position in the annular cavity 24.
In one
embodiment, the multi-piece adapter collar 50 is constructed of two
symmetrical
parts.
As further shown in FIG. 1, the adapter collar 50 includes an annular
groove 56 dimensioned to receive an inner edge of a segmented retainer plate
60. The segmented retainer plate 60 is secured to a top of the nut 40 by
threaded fasteners 62, which are received in a plurality of tapped bores 49
distributed in a circular pattern around a top of the nut. In one embodiment,
the
segmented retainer plate 60 is constructed of three wedge-shaped pieces.
In the embodiment shown in FIG. 1, the metal ring gasket 30 provides a
high-pressure metal-to-metal seal between the first and second subcomponents
12, 14. The threaded union 10 also includes a pair of elastomeric backup
seals,
e.g. O-rings, which are seated in annular grooves 70 in the second
subcomponent. Alternatively, the annular grooves 70 could be machined into
the first subcomponent. It will be appreciated that the number of elastomeric
annular sealing elements can be varied from zero to three or more.
FIG. 2 illustrates, in an exploded view, the threaded union 10 shown in
FIG. 1. As shown in FIG. 2, the threaded union 10 includes a pair of O-rings
80,
each having its own backing member 82. The O-rings 80 and backing members
82 are dimensioned to be received in each of the two annular grooves 70 in
order to provide the elastomeric backup seal to the metal-to-metal seal
provided
by the metal ring gasket 30. As is apparent, in this embodiment the first
subcomponent 12 includes a socket 64 and the upwardly facing annular groove
20 is located in the bottom of the socket 64. The second subcomponent 14
includes a pin end 66 and the downwardly facing annular groove is located on a
bottom of the pin end 66. When the first subcomponent 12 and the second
subcomponent 14 are interconnected, the socket 64 receives the pin end 66.
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FIG. 3 illustrates a threaded union 10 in accordance with another
embodiment of the invention. The high-pressure fluid-tight seal between the
first and second subcomponents 12, 14 is provided only by the metal ring
gasket 30. Otherwise, the embodiments shown in FIGs. 2 and 3 are identical.
In testing, the metal ring gasket 30 has maintained a fluid-tight seal up to
a fluid pressure of 30,000 psi. The metal ring gasket is also able to maintain
a
high-pressure seal even if exposed to elevated temperatures due to fire.
As illustrated in FIG. 4, in one embodiment of the invention the metal ring
gasket 30 has beveled corners (or beveled surfaces) and an octagonal cross-
section. In one embodiment, the comers of the metal ring gasket are beveled at
an angle of 23 1 with respect to the side surfaces and 67 1 with respect
to
the top and bottom surfaces. Persons skilled in the art will appreciate that
the
bevel angle may be changed within limits without affecting the efficacy of the
energized seal. The metal ring gasket 30 is preferably made of steel. Plain
carbon steel or stainless steel is selected depending on whether a fluid to be
contained is corrosive or non-corrosive.
For service where corrosion is not generally problematic AISI 1018
nickel-plated cold-drawn steel may be used. The AISI 1018 steel has a carbon
content of 0.18% (although it may vary from 0.14% to 0.20%), a manganese
content of 0.6% to 0.9%, a maximum phosphorus content of 0.04% and a
maximum sulfur content of 0.05%. The AISI 1018 steel exhibits high
machinability (its average machinability rating is 70%), good fracture
toughness,
good surface hardness (126 HB), high tensile strength (440 MPa), high yield
strength (370 MPa), superior ductility (40-50% reduction in cross-sectional
area
at the fracture load) and is relatively inexpensive. Alternatively, other
plain
carbon steels may be substituted, provided they have approximately similar
mechanical properties.
For service where corrosion is problematic the metal ring gasket may be
made using either AISI 316 stainless steel or AISI 304 stainless steel. Not
only
are these stainless steels corrosion-resistant but they also possess desirable
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mechanical properties (in terms of machinability, fracture toughness, surface
hardness, tensile strength and yield strength).
Alternatively, persons skilled in the art will appreciate that, for certain
applications, the metal ring gaskets in accordance with the invention may be
made using metals other than steel (such as aluminum or copper alloys like
brass or bronze, for example), which are more temperature-resistant than
elastomeric gaskets.
As illustrated schematically in FIG. 4, when the threaded nut 40 is
tightened, the nut 40 exerts a force F on the first and second subcomponent
12,14. The forces F elastically deform the metal ring gasket 30 within the
annular cavity 24. The compressive force Fc acting on the outer beveled
surfaces can be expressed by the equation: Fc = F sin 23 , assuming a bevel
angle of 23 .
FIG. 4 shows the undeformed metal ring gasket 30 in substantially
unloaded contact with the inner beveled surfaces of the annular cavity 24, for
example immediately prior to or immediately after torquing of the threaded
nut 40. When the threaded nut 40 has been tightened to the extent shown in
FIG. 4, the annular cavity 24 has a hexagonal cross section with internal
beveled surfaces, or facets, that have angles that correspond to the bevel
angles of the octagonal cross section of the metal ring gasket 30. As shown in
FIG. 4, the top, bottom and inner side surfaces of the metal ring gasket do
not
contact the top, bottom or inner surfaces of the annular cavity 24. In other
words, there remains at all times, even after full torgue, an upper gap Gu
between the top surface of the metal ring gasket 30 and the top (inner)
surface
of the annular cavity 24. Likewise, there remains at all times a lower gap GL
between the bottom surface of the metal ring gasket 30 and the bottom (inner)
surface of the annular cavity 24, a gap G5 between the inner side of the metal
ring gasket 30 and the inner side of the annular cavity 24; and, a gap GB
between the inner beveled comers of the metal ring gasket 30 and the annular
cavity 24.
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When the forces Fc act on each of the outer beveled comers the metal
ring gasket 30 the forces cause the metal ring gasket 30 to be elastically
deformed inwardly to provide a fluid-tight seal between the first and second
subcomponents.
As schematically illustrated in FIG. 5, the metal ring gasket is over sized
to have an outer diameter such that the outer beveled corners must be
displaced by elastic deformation of the metal ring gasket 30 by about 0.003"
when the first subcomponent 12 and the second subcomponent 14 are securely
interconnected. It should be noted that in FIG. 5 the oversizing of the metal
ring
gasket is shown at an exaggerated scale for the purposes of illustration. The
0.003" oversizing is considered optimal for the steels described above because
the metal ring gasket 30 is elastically, and not plastically deformed in the
annular cavity 24. It will be appreciated that for other steels, and/or for
other
sizes of threaded unions, the oversizing may be different to provide an
optimal
seal. As further shown in FIG. 5, an inner diameter of the metal ring gasket
30
is larger than a diameter of the annular cavity 24. In one embodiment the
inner
diameter of the metal seal ring is such that the gap GB is at least 0.001"
after
the metal ring gasket 30 is elastically deformed in the annular cavity 24. In
one
embodiment, the inner diameter of the metal ring gasket 30 is about 0.014"
larger than an inner diameter of the annular cavity 24 before the metal ring
gasket is elastically deformed. However, this difference in the diameters is
not
critical and can be varied considerably, so long as the metal ring gasket 30
can
be elastically deformed without the elastic deformation being inhibited by
contact with an inner face of the annular cavity 24. Consequently, if an inner
diameter of the metal ring gasket is at least 0.003" larger than an inner
diameter
of the annular cavity 24, the metal ring gasket can be elastically deformed as
required.
In operation, the threaded union 10 is torqued or "hammered up" by
tightening the nut 40 until the end surfaces 16,18 of the first and second
subcomponents 12,14 abut. Due to the slight over-sizing (about 0.003" ) of the
metal ring gasket 30, the threaded union cannot be over-torqued, and there is
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no danger of plastic deformation of the metal ring gasket 30. The metal ring
gasket 30 can therefore be repeatedly reused so long as the sealing surfaces
on its beveled faces are not scratched or marred.
FIG. 6 schematically illustrates the threaded union 10 when it is exposed
to elevated fluid pressures. As is understood in the art, high fluid pressures
in
the fluid passage 15 (FIG. 3) force the end surfaces 16,18 of the first and
second subcomponents 12,14 apart due to elastic deformation of the threaded
nut 40. This creates a gap 90 between the first and second subcomponents
12,14. Fluid pressure flows through the gap 90 on the inner side of the metal
ring gasket 30. The fluid pressure induces hoop stress in the metal ring
gasket
30 that forces the sealing surfaces 30a, 30b of the metal ring gasket 30 into
tighter contact with the corresponding surfaces of the annular cavity 24, and
the
seal is "energized". Consequently, the higher the fluid pressure (within the
pressure capacity of the first and second subcomponents 12,14) in the central
fluid passage 15, the more energized and tighter the fluid seal.
FIG. 7 schematically illustrates another embodiment of a metal ring
gasket 32 in accordance with the invention. The metal ring gasket 32 has an
outer diameter and outer beveled corners that are configured the same as
described above with reference to FIGs. 4-6 and the metal ring gasket is
elastically deformed in the annular cavity 24 in the same way. However, the
metal ring gasket 32 is hexagonal in cross-section and has a flat inner side
that
is spaced from an inner surface of the annular cavity 24. The fluid pressure
acts on the flat side to energize the seal as described above. As will be
understood by those skilled in the art, the shape of the inner side, and
consequently, the cross-sectional shape of the metal ring gaskets 30,32 is a
matter of design choice.
The threaded union 10 in accordance with the invention may be used to
construct a high-pressure, fluid-tight seal between a drilling flange,
described in
Applicant's U.S. Patent No. 7,159,652, which issued on January 9, 2007 and a
wellhead on a wellhead assembly, as described and illustrated in Applicant's
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U.S. Patent No. 7,125,055 and is entitled METAL RING GASKET FOR A
THREADED UNION, as well as a fluid conduit for any other application.
The metal ring gasket in accordance with the invention has been
extensively pressure-tested in a number of threaded unions integrated into
different wellhead and well stimulation tool components. It has proven to be
extremely reliable and provides a very high-pressure energized seal that is
easy
to "torque up" using a hammer or a wrench. This permits such components to
be more economically constructed and more quickly assembled. Cost savings
are therefore realized, while worker safety and environmental protection are
ensured.
As will be understood in the art, the metal ring gasket 30,32 for the
threaded union 10 can be used in a variety of applications to reduce cost,
while
ensuring high performance and safety in fluid conduits of all types, including
wellhead assemblies and well stimulation equipment, where very high pressure
and very high temperature resistance are especially important.
The embodiments of the invention described above are therefore
intended to be exemplary only. The scope of the invention is intended to be
limited solely by the scope of the appended claims.
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