Note: Descriptions are shown in the official language in which they were submitted.
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TWO-PART BACK CAP FOR A PLUG VALVE AND PLUG
VALVES INCORPORATING SAME
FIELD OF THE INVENTION
This invention generally relates to fluid flow control
valves and, in particular, to back caps for high-pressure
plug valves.
BACKGROUND OF THE INVENTION
High-pressure plug valves are well known in the art
and used principally in the oil and gas industry. A high-
pressure plug valve is described in Assignee's co-pending
United States Patent Application published under
publication number 2006/0027779 on February 9, 2006, which
is entitled High-Pressure Plug Valve with Replaceable
Inserts and Method of Refurbishing Same.
FIG. 1 shows a side elevational view of a plug valve
10 of this type. The plug valve 10 includes a plug valve
body 12 having a top flange 14 and a stud pad 16. As will
be understood by those skilled in the art, the flange 14
and stud pad 16 could be any of a flange, stud pad or
threaded union. The plug valve 10 has a stem 18 for
rotating a plug (not shown) within the plug valve body 12
to control fluid flow through the valve in a manner well
known in the art. The plug is retained in the plug valve
body 12 by a back cap 20.
FIG. 2 is a schematic cross-sectional view of a back
cap 20 in accordance with the prior art. The back cap 20
has an inner end 22 and an outer end 24. The outer end 24
includes a plurality of radial bores 26 through which a
tool can be inserted to install the back cap 20 in the plug
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valve body 12 or remove it from the plug valve body 12.
The inner end 22 includes a plug support surface 28 which
supports the plug of the plug valve 10. The inner end 22
further includes a seal rim 30 having a peripheral 0-ring
groove 32 that receives an 0-ring 34 for providing a seal
above pin threads 35 that engage box threads (not shown) in
the plug valve body 12 to retain the back cap 20 in the
plug valve body 12. The inner end 22 further includes a
seal bore 36 which receives a fluid seal for sealing around
a stud end of the plug. The stud end of the plug is
received in a guide bore 38 that aligns the plug within a
plug cage (not shown) of the plug valve body 12. The stud
end also extends through a stud bore 40 for fluid pressure
balance with the stem 18 shown in FIG. 1.
A problem with the back cap 20 occurs during
installation of the back cap 20 in the plug valve body 12.
Because the back cap 20 is retained in the plug valve body
12 by the pin threads 35, it must be rotated during
installation. When the 0-ring 34 encounters a sealing
surface against which it seals in an interior of the plug
valve body 12, the 0-ring 34 resists rotation due to
frictional engagement with the sealing surface.
Consequently, the 0-ring 34 can be damaged, or it may roll
partially out of the 0-ring groove 32. If this occurs the
valve will leak around the back cap, even though the leak
may not develop until the valve has seen a certain amount
of high-pressure service.
Even if the 0-ring 34 stays in the 0-ring groove 32
and is not damaged during installation, a leak around the
back cap can still develop due to stress on the 0-ring 34.
As understood by those skilled in the art, when subjected
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to high fluid pressures the plug valve body 12 expands, or
"grows" as it is referred to in the industry. As the plug
valve body 12 expands, a gap develops between an outer
peripheral surface of the seal rim 30 and the seal surface
of the plug valve body 12. Even though an undercut 31 is
provided in the back cap 20 to permit the seal rim 30 to
expand with the plug valve body 12, experimentation as
shown that under very high pressures the seal rim 30 does
not expand at the same rate as the plug valve body 12.
This creates a space between the seal rim 30 and the plug
valve body 12 that puts stress on the 0-ring 34. That
stress eventually causes the 0-ring 34 to fail, which
results in a leak around the back cap 20, requiring
maintenance of the plug valve 10.
There is therefore a need for a back cap for a plug
valve that provides a long service life without
maintenance. There is also need for a plug valve with a
back cap that can be installed without danger of damaging
back cap seals.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide
a back cap for a high-pressure plug valve that is easily
and reliably installed and provides a long service life
without maintenance. It is a further object of the
invention to provide a plug valve with a back cap that
consistently provides a reliable high-pressure seal.
The invention therefore provides 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 0-ring groove that
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accepts an 0-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 invention further provides 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 a line
perpendicular to an axis of the back cap seal body; and the
back cap retainer nut comprises 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.
The invention further provides 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 0-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
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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.
The invention further provides a plug valve,
comprising: a plug valve body; and a two-part back cap
that includes 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 0-ring groove that accepts an 0-ring
for providing a fluid seal between the back cap seal body
and the plug valve body, 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 invention yet further provides a plug valve,
comprising: a plug valve body; and a two-part back cap
comprising a back cap seal body having an inner end for
supporting a plug of the plug valve and a seal rim that
supports at least one 0-ring for sealing against an end cap
seal surface of the plug valve body, an outer end having
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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.
The invention yet further provides a two-part back cap
for 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 a line perpendicular to an axis of the back cap
seal body; and the back cap retainer nut comprises 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.
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BRIEF DESCRIPTION OF THE DRAWINGS
Having thus generally described the nature of the
invention, reference will now be made to the accompanying
drawings, in which:
FIG. 1 is a side elevational view of a plug valve in
accordance the prior art;
FIG. 2 is a cross sectional view of a back cap for the
plug valve shown in FIG. 1;
FIG. 3 is a cross-sectional view of a two-part back
cap for a plug valve in accordance with the invention;
FIG. 4 is a cross-sectional view of another embodiment
of the two-part back cap in accordance with the invention;
FIG. 5 is a detailed cross-sectional view of one side
of the two-part back cap shown in FIG. 3;
FIG. 6 is a detailed cross-sectional view of the side
of the two-part back cap shown in FIG. 5 installed in a
plug valve in accordance with the invention; and
FIG. 7 is a detailed cross-sectional view of the two-
part back cap shown in FIG. 6 when the plug valve is
subjected to very high fluid pressure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention provides a two-part back cap for a plug
valve that provides a reliable long-term seal without
maintenance. The two-part back cap includes a back cap
seal body that can be inserted into a plug valve body
without rotation to ensure that an 0-ring seal between the
back cap seal body and the plug valve body is not damaged
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during installation of the back cap seal body. The two-
part back cap also includes a hollow retainer nut that
securely retains the back cap seal body in position. An
outer end of the back cap seal body is contoured to provide
a gap between an outer edge of the back cap seal body and
an inner end of the back cap retainer nut. That gap
permits the back cap seal body to expand under fluid
pressure along with the plug valve body, ensuring that the
0-ring seal is not stressed when the plug valve is
subjected to high fluid pressures. A long service life of
the two-part back cap is thereby ensured.
FIG. 3 is a cross-sectional view of a two-part back
cap 50 in accordance with the invention. The two-part back
50 includes a back cap seal body 52 and a back cap retainer
nut 54. The back cap retainer nut 54 is a hollow nut that
includes an inner end 58 having an inner surface 60. The
inner surface 60 is inclined at an angle with respect to an
axis of the back cap retainer nut 54, as will be explained
in detail below with reference to FIG. 5. The back cap
retainer nut also includes a pin thread 59 for engaging box
threads of a plug valve body, as will be explained below
with reference to FIGs. 5-7.
The back cap seal body 52 includes an outer end 64
with an inclined surface 66. The inclined surface 66 mates
with the top surface 60 of the back cap retainer nut only
part of the way along the inclined surface 66, as will also
be explained below in detail with reference to FIG. 5. The
back cap seal body 52 further includes an inner end having
a plug support surface 62. An outer edge of the inner end
further includes a seal rim 68. An outer periphery of the
seal rim 68 includes an 0-ring groove 70, which accepts an
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0-ring 72 for providing a fluid seal between the back cap
seal body 52 and a plug valve body of a plug valve in
accordance with the invention. The back cap seal body 52
further includes a seal bore 74, a guide bore 76 and a stud
bore 78.
FIG. 4 is a cross-sectional view of a two-part back
cap 80 in accordance with another embodiment of the
invention. The two-part back cap 80 is identical to the
two-part back cap 50 described above with reference to FIG.
3, except that it includes a seal rim 82 having a first 0-
ring groove 84 and a second 0-ring groove 86. The first 0-
ring groove 84 accepts an 0-ring 88 and the second 0-ring
groove 86 accepts an 0-ring and 90. As noted above, in all
other respects the two-part back cap 80 is identical to the
two-part back cap 50 and the other components will not be
redundantly described.
FIG. 5 is a detailed cross-sectional view of one side
of the two-part back cap shown in FIG. 3. As explained
above the with reference to FIG. 3, the back cap retainer
nut 54 has an inner end 58 with an inner surface 60 that is
inclined toward the seal rim 68 at an angle of inclination
"a" with respect to a line perpendicular to a central
axis of the back cap retainer nut 54. The angle "a"
can be in a range from 20 -40 . In one embodiment of
the invention the angle "a" is 30 . The
back cap seal body 52 has an outer end 64 with an outer
surface 66 that includes a first surface 66a adjacent the
stud bore 78 (see FIG. 3). The first surface begins at a
point 89 and ends at a point 90. The first surface is
inclined toward the seal rim 68 at an angle of inclination
equal to the angle "a". The outer end 64 further includes
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a second surface 66b which is continuous with the first
surface 66a. The second surface 66b extends from the point
90 to an outer periphery of the back cap seal body 52. The
second surface 66b is inclined toward the seal rim 68 at an
angle of inclination "b". The angle "b" may be about 0.5 -
1.5 greater than the angle "a". In one embodiment, the
angle "b" is about 0.5 greater than the angle "a". In that
embodiment, a gap 92 between an outer edge of the back cap
seal body 52 and an outer edge of the back cap retainer nut
54 is about 0.014".
As will be explained below with reference to FIG. 7, a
difference between the angles "a" and "b" is dependent on a
rigidity of the plug valve body 12 into which the two-part
back cap 50, 80 is installed. In general, the more
flexible the plug valve body 12, the greater should be the
difference between the angles "a" and "b". It should be
noted that in one embodiment, the point 90 is aligned with
an inner edge of a radius 67 formed at a bottom of an
undercut 69 between the plug support surface 62 and the
seal rim 68d, adjacent the plug support surface 62.
FIG. 6 is a detailed cross-sectional view of the two-
part back cap 50 installed in a plug valve 11 in accordance
with the invention. As explained above, the pin thread 59
on the back cap retainer nut 54 engages a box thread in a
valve body of the plug valve 11. Only a small part of one
side of the plug valve 11 is shown. The plug valve 11
includes a back cap seal surface 95. The 0-ring 72 received
in the 0-ring groove 70 in the seal rim 68 of the back cap
seal body 52 seals against the seal surface 95. When the
back cap seal body 52 is installed in the plug valve 11,
the back cap seal body is inserted into the plug valve 11
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without rotation. Consequently, the 0-ring 72 is not
damaged. Thereafter, the back cap retainer nut 54 is
installed and tightened against the back cap seal body 52.
When the two-part back cap 50 is installed in the plug
valve 11, a gap 94 exists between the seal rim 68 and the
seal surface 95. Likewise, the gap 92 exists between outer
edges of the inner surface 60 of the back cap retainer nut
54 and the outer surface 64b of the back cap seal body 52.
FIG. 7 is a detailed cross-sectional view of the two-
part back cap 50 shown in FIG. 6 when the plug valve 11 is
under high fluid pressure. As a can be seen, when the plug
valve 11 is under high fluid pressure, the back cap seal
body 52 "grows" with the plug valve body of the plug valve
11 because the seal rim 68 flexes outwardly and downwardly
with the increased fluid pressure. Consequently, the gap
92 and the gap 94 are eliminated and there is metal-to-
metal contact between an outer periphery of the seal rim 68
and the seal surface 95. Likewise, there is metal-to-metal
contact between the surfaces 60 and 66b. This ensures that
in the 0-ring 72 is not stressed at extreme fluid
pressures. As a result, the two-part back cap 50, 80 in
accordance with the invention has a long service life and
rarely requires maintenance, even when repeatedly exposed
to fluid pressures of up to 20,000 psi. As noted above,
the difference between the angles "a" and "b" is dependent
on a flexibility of a plug valve body of the plug valve 11.
In general, the more flexible the plug valve body, the
greater a difference between the two angles should be.
Once installed in the plug valve 11, removal of the
back cap retainer nut 54 will not remove the back cap seal
body 52. Consequently, a plurality of tapped bores 79
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which permit the connection of a puller are provided. Any
one of a number of pullers well known in the art may be
used to extract the back cap seal body 52 from the plug
valve 11.
The invention therefore provides a plug valve 11 that
is more reliable during high pressure fluid service. The
plug valve 11 in accordance with the invention therefore
requires less frequent maintenance.
As will be understood by those skilled in the art, the
embodiments of the invention described above are intended
to be exemplary only. The scope of the invention is
therefore intended to be limited solely by the scope of the
appended claims.
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