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
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.
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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 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
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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.
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 corners 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:
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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.
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;
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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.
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
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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
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first subcomponent 12 to define a hexagonal annular
cavity 24. However, as will be explained below, the
annular cavity 24 need not 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
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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 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
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threaded union 10 also includes a pair of elastomeric
backup seals, e.g. 0-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 0-rings 80, each having its own
backing member 82. The 0-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.
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
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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 corners of the metal ring gasket are
beveled at an angle of 23 1 . 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
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desirable 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
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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 Gs 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 corners of the
metal ring gasket 30 and the annular cavity 24.
When the forces Fc act on each of the outer beveled
corners 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
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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 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
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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 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
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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 welihead 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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