Note: Descriptions are shown in the official language in which they were submitted.
1 ~72~67
The valve in which the stem sealing means of the present
invention is used is of the high pressure, high temperature,
balanced rising stem gate valve type similar to those disclosed
in our Canadian Patents Nos. 1,113,914, issued December 8, 1981
and 1,125,261, issued June 8, 1982.
The stem ~ealing means of the present inven-tion is related
to the stem sealing means disclosed in the above identified
Patent No. 1,125,261.
B~CKGROUND OF THE INVENTION
-- .
In th.e oïl and gas industry, wells are being drilled with
increasingly higher downhole temperatures and pressures, some-
times in "sour" gas fields where the well fluids contain
relatively high percentages of H2S. Seals for oilfield valves
and wellheads have been developed recently which are relatively
resistant to deterioration from the combined effects of higher
pressure and temperature, and from caustic well fluids such as
H S
2 -
In United States Patent No. 4,056,272, issued Movember 1,
1977 to Morrill, and assigned to the owner of the present
application, there is disclosed a static seal between a well-
head and a pipe hanger supported therein, the seal comprising
a pair of frusto-conical elastic metal ring gaskets of sub-
stantially rectangular cross section, which are flattened by a
lockscrew and compression ring and stressed until the edges
of the metal ring gaskets flow plastically or "coin" into
metal-to-metal sealing engagement with the parallel cylindrical
walls of the wellhead and pipe hanger. In this static seal, a
deformable ring having an initial generally trape~oidal cross
`~ section and made of, for example, Teflon~ polyurethane or
30 rubber, is disposed between the metal ring gaskets and com-
pressed upon flattening of the metal ring gaskets into a
~ ~7`~J~
~.
~ ~ 72~6~
rectangular cross section configuration and into engagement with
the walls of the wellhead and pipe hanger. The deformable
ring in this static seal acts as a backup seal for the metal
ring gaskets in case of less than perfect sealing by the
metal ring gaskets, for example, due to scratches or machine
marks in the wellhead or on the pipe hanger. The deformable
ring will flow into and seal any of such scratches or machine
marks.
One seal used for dynamic as well as static sealing
between the stem af a valve and the valve body or bonnet employs
frusto-conical elastic metal ring gaskets of rectangular cross
section disposed in a stuffing box around the stem, with
sandwich rings of a more compliant material disposed between
the elastic metal ring gaskets. The reasons for the metal ring
gaskets being eLastic, similar to ~elleville springs although
not made of spring steel but a softer material, include (1)
the gaskets have a larger inner diameter and a smaller outer
diameter when unstressed, so that the seal unit can be easily
inserted and withdrawn from the stuffing box without undue
frictional engagement with the valve stem or stuffing box;
(2) the gaskets will maintain their stressed engagement
with the stem and stuffing box during use of the valve despite
slight changes in the valve dimensions due to, for example,
temperature and pressure variations, which would change
slightly the deformation or strain on the gaskets created
initially by tightening of the packing retainer. In other
words, the gaskets must be elastic in order to maintain their
preload.
~he above referred to valve stem seal is disclosed in
the aforementioned Patent No. 1,125,261 and in an ASME paper
! 1 726~ ~
entitled "Seals for Valve Stems and Wellheads in High Pressure
- F~igh Temperature Service," by C. D. Morrill and C. W. Meyer,
prepared for presentation at a conference in Mexico City,
Mexico, on September 19~24, 1976. The seal as disclosed
therein ïs compressed by a threaded packing retainer such
that the metal ring gaskets are flattened, and their inner
and outer peripheral edges are deformed or "coined" into
metal-to-metal sealing engagement with both the outer surface
of the valve stem and the ~Jalls of the stuffing box. The
sandwich rings ~etween the metal ring gaskets are also deformed
upon compression of the seal to conform to the shapes of the
metal ring gaskets and to engage the stem and stuffing box.
The sandwich rings should therefore be elastic to some degree.
The initial deformation will usually cause the sandwich rings
to take on an initial permanent set, but they will remain
elastic to some degree. Materials used for the sandwich
rings in such seals have included fluoroplastics, e.g. tetra-
fluoroethylene polymer, and graphite materials. Such materials
have also included tetrafluoroethylene polymer filled with up
to about 15% molybdenum disulfide.
The sandwich rings of the valve stem seal just described
are dynamic seals, acting primarily during stem motion to seal
between the metal ring gaskets and the stem when the metal
ring gaskets are disturbed by stem drag while the stem is in
; motion. Such sandwich rings also tend to lubricate the areas
of contact between the stem and metal ring gaskets by rubbing
off onto the stem to
~: -3-
! 172667
.
- .
some degree to reduce friction between the stem and the metal
ring gaskets. The sandwich rings are also lubricious so as to
reduce friction between the stem and the sandwich rings, and
~etween the sandwich rings and the metal ring gaskets. The
sandwich rings further act as backup seals for the metal ring
gasXets, l~ke the deformable ring backup seals as described above
for the wellhead-pipe hanger s~al of U.S. patent no. 4,056,272,
to flow into and seal any scratches or machine marks on the stem.
The invention herein disclosed is an improvement over the
valve stem seal described above.
One embodiment of the stem seal descrihed above includes
three metal ring gaskets and two sandwich rings disposed therebe~
tween, and is known as an "SMT" type seal. The SMT type seal has
been found to provide satisfactory stem sealing for valves having
working pressures up to about 25,000 p.s.i., at temperatures from
-20~F. to 300~F.
For valves having working pressures up to 30,000 p.s.i. or
greateE, however, the SMT type seal will not always provide a
satisfactory seal or the valve stems undèr conditiGns expected
to be encountered during service. Graphite materials alone, for
example, are not desirable for SMT type stem seals for valves in
the 30,000 p.s.i. class because such materials tend to extrude
past the metal ring gaskets upon flattening the gaskets to ener-
gize the seal, since the graphite begins to extrude before the
gaskets become flattened sufficiently to form a seal with the
valve stem and stuffing box. Moreover, graphite materials tend
to be worn or eroded away by movement of the valve stem as the
graphite is deposited onto the stem and carried by the stem past
the gaskets and out of the stuffing box. Such extrusion and
wearing away of the graphite material under 30,000 p.s.i. service
conditions may lead to leakage of well fluids past the seal and
the need to replace the graphite ring.
i 1 7~667
.
Likewise, tetrafluoroethylene polymer ("TFE") materials
alone, or such materials including MoS2 as an additive, are not
suitable for SMT type stem seals for valves in the 30,G00 p.s.i.
.. . . .
class because such materials are not capable of always maintain-
- ~ng a tight seal against such pressures after the valve is put
-- through temperature cycling. A valve in oilfield;service through
wh~ch high pressure well fluids are 1Owing from deep wells may
be heated by the well fluids to a tempeFature of about 300F.
When the well fluids stop flowing through the valve, for example
if the well is shut in, the valve may cool down to ambient tempe-
rature, for example 70F., and if the well is reopened and the
well fluids begin flowing again, the valve wlll heat back up to
300F. Thus, the valve's temperature is cycled between about
io~F. and about 300F. A valve stem seal must be capable of
remaining tight at all times, at all temperatures and through all
temperature cycles to which the valve will be subjected in ser-
,~ vice. An SMT type seal usiny TFE rings, or MoS2-filled TFE
rings, will seal against well fluids at pressures of about 30,000
p.s.i. at ambient temperatures, e.g. 70~F., and again at elevated
temperatures, e.g. 300F., but when the valve is put through a
thermal cycle from ambient temperature to 300F. and back to
ambient temperature, the SMT type seal will not always remain
tight; sometimes the seal will exhibit minor leakage, either
during valve stem movement or when the valve stem is stationary,
or both. Although the leakage can be stopped by further tighten-
ing of the packing retainer, leakage will reoccur upon further
-temperature cycling.
It is an object of this invention to overcome the problems
described above by providing a reliable valve stem seal suitable
for valves havin~ working pressures of the order of 30,000 p.s.i.
or more and subject to temperature cycling, between temperatures
in the range of -20F. to 300F. It is another object of this
_nven~ion to provide such a seal that will remain tig~t and will
! 1~266~
not leak when subjected to well fluids having pressures of
30,000 p.s.i. or more and when the temperature of the valve is
cycled from ambient temperature to about 300F. and back to
ambient temperature, without the need to retighten the packing
retainer. It is also an object of this invention to provide
such a seal that is simple, compact and economical, and easy to
manufacture, install and service. It is also an object of this
invention to provide such a seal that is low friction, durable
and relatively resistant to deterioration due to fluctuations
and extremes in temperature, high pressure and chemical activity
of the well fluids sealed against.
SUMMARY OF THE INVENTION
.
According to the invention, there is provided
apparatus for sealing in high pressure, temperature cycled
applications hetween the walls of an axially movable valve stem
and a stuffing box around the valve stem, the stuffing box
having a port in its bottom and the valve s-tem extending
: through the port and out of the stuffing box, comprising:
,
: first and second elastic metal ring gaskets disposed in such
stuffing box around such valve stem, and annular compliant seal
means dispos.ed in such stuffing box around such valve stem and
sandwiched between said gaskets, said seal means including first
and second annular portions of tough, compliant material adjacent
said first and second gaskets~ respectively, and an annular core
portion disposed between said first and second por-tions and
composed of a material having a lower coefficient of thermal
expansion than said material of said first and second annular
portions.
! 172~ ~
BR:[EF DESCRIP~ION OF T~IE DRA~INGS
For a detailed description of a pre:~erred embodiment
of the invention, reference will now ~e made to the accompany-
ing drawings wherein:
" FIGURE 1 is a vertical section through a valve
embodying the invention; and
FIGURES 2A and 2B are fragmentary sectional views taken
in the same plane as Figure 1 ~ut to a larger scale~ and showing
one embodiment of the actuator stem sealing means of the
present invention in which the frustoconical metal ring gaskets
point away from the pressure ~eing sealed against inside the
valve. Figure 2B illustrates the stem sealing means of the
present
,
~ ,~
~ ?
72667
invention as installed in the valve but prior to its being ener-
gized by tightening the packing retainer. Figure 2A illustrates
the stem sealing means as energized.
... .
D~TAILED DESCRIP~ION OF PREFERRED EM30DIMENT
Referring to Figure 1, there is shown a valve including a
hollow body having a chamber part 21 and a bonnet part 23 secured
.hereto by studs 25 and nuts 27. The bonnet part 23 is sealed to
the chamber part 21 by a suitable pressure energized gasket 29.
Seats 31, 33 mounted at the inner ends of passages 35, 37 cooper-
ate with a pair of ported gates 39, 41 whicX control flow of
fluid, e.g. water, oil or gas, through the valve. Sealant material
is stored in reservoirs 43, 45, and sealant is supplied automatic-
ally through the sealant distribution passages 47, 49 to the
interfaces between the gates and seats and between the seat necks
and valve body to effect sealing at such interfaces;
Gates 39, 41 are reciprocated by a generally cylindrical
actuating~stem Sl between a closed position, shown in Figure 1,
where flow of fluids through passages 35, 37 is prevented, and an
open position permitting such flow in which gate ports 53, 55 are
in register with ports 57, 59 in seats 31, 33. The upper end of
actuating stem 51 extends out of valve chamber part 2l and through
port 61 in bonnet 23. Seal means 63 according to the invention
seals between stem 51 and a generally cylindrical stuffing box 65
in bonnet 23. Seal mean 63 is compressed in stuffing box 65 by a
packing retainer 67 which is screwed into internally threaded
neck 69 on the top of bonnet 23. A bleeder port 71 is used to
checX for leakage of fluids from between stem 51 and port 61
after backseating the stem.
The exterior of bonnet neck 69 is also threaded and receives
2 bearing cap 73 screwed thereon. An actuator nut 75 is screwed
onto the threaded upper portion 77 of actuator stem 51 and has a
-iange 79 thereon disposed between upper and lower axial thr~st
~ 1 72667
bearings 81, 83. The lower thrust bearing 83 engages the top of
a washer 85 disposed on top of packing retainer 67. The upper
thrust bearing 81 engages the upper inside end of bearing cap 73.
.
A hand wheel 87 includes a,noncircular aperture 89 fitted over a
-`~orrelativel-y shaped porti'on'91 of actuator nut 75. ~and wheel
~ -87 is he'ld in place by a retainer nut 93 screwed onto t~e upper
en~ of actuator nut 75. A bleed port 9~ allows fluid to escape
'' from between threaded portion 77 of s*e~ 51 and actuat~r nut 7~.
A generally cylindrical balance stem 97 is connected to the
lower ends o~ the gates and extends out of the valve chamber
through port 99. Seal means 101 according to the ,i,nvention seals
; between balance stem 97 and a generally cylindrical stufing box
103 in the valve body. Seal means 101 is compressed in stuffing
box 103 by lower packing retainer 105 which is screwed into a
threaded socket 107 coaxial with port 99 and stuffing box 103. A
bleeder port 109 is used to check for leakage of fluids from
. between balance stem 97 and port 99 when the enlarged portion on
the upper end of the balance stem is fully seated in the annular
seat in the valve body around port 99. A lower cap 111 is secured
to the valve body over lower packing retainer 105 and the lower
end of balance stem 97, by screws 113.
When hand wheel 87 is turned, actuator nut 75 turns and
causes actuator stem 51 to raise or lower gates 39, 41 to which
the stem is connected by hub 115. This in turn causes balance
stem 97, connected to the gates by hub 117, to move up or down.
~hus, there is relative axial motion between each o stems 51, 97
-and the respective stem seal means 63, 101. Seal means 63, 101
must remain tight before, during and after such relative axial
motion.
Seal means 63, 101 are alike, so only one need be described
in further detail. Referring to Figures 2A and 2B, seal means 63
includes an upper seal set 119 and a lower seal set 121. A base
adap.er rlng 123 is disposed in the botto~ o ~nnular stuffing
~ 172~67
-box 65 below lower seal set 121. The bottom of base adapter ring
123 engages flush with and is correlative to the bottom of the
stuffing box. A follower adapter ring 125 is disposed between
--~he lower face 127 of packing retainer 67 and upper seal set 119.
- -~ ~e upper end of follower--adapter ring I25 engages flush with and
. ._
----is correlative to lower face 127 of packing retainer 67. An
.= .
intermediate adapter ring 129 i5 disposed between seal sets 119,
121. The upper face of base adapter ring 123, the lower face of
follower adapter ring 125, and both upper and lower faces of in-
termediate adapter ring 129 are frustoconical in configuration,
having the desired cone angles for the shapes of the seal sets
119, 121 in their final assembled positions. In this regard, it
should be noted that Figure 2B-illustrates the seal means of the
present invention as installed in the valve but prior to its
being energized by tightening the packing retainer; Figure 2A
illustrates the seal means 2S energized, with the packing re-
tainer screwed farther into its threaded receptacle and the seal
sets a~d adapter rings in their final assembled positions.- As is
explained in more detail hereinafter, the metal ring gaskets of
the seal sets are more sharply conical, that is, they have smaller
cone angles, in their relaxed states shown in Eigure 2B than in
their final energized states shown in Figure 2A. If desired,
either or both of adapter rings 123, 125 may be omitted, the
bottom of the stuffing box and/or the lower end of retainer 67
being provided instead with a frustoconical surface of the de-
sired cone angle and area. If used, adapter rings 123, 125, and
adapter ring 129, should be made of fairly hard material such as,
for example, 4140 steel. The valve body and bonnet need only be
made of any steel conventionally used for high pressure valves.
Seal sets 119, 121~are alike, so only one need be àescribed
in further detail. Seal set 119 includes a pair of frustoconical
metal ring gaskets 131 between which are disposed two identical
make-up rings 133. Make-up rings 133 are made of tough, compliant,
.
~ i~
- . ~ 1 7 2 ~ ~
.
soiid lubricious material having a lower elastic modulus than
metal ring gaskets 131. One make-up ring is disposed adjacent to
one of the metal ring gaskets, and the other make-up ring is
.. . .
disposed adjacent to the other of the metal ring gaskets.
- ~ ~ A core-ring 135 is disposed between make-up rings 133 Core
~~ ring 135 has a lower coefficient of volumetric thermal expansion
th'an make-up rings 133. The inside diameter of core ring 135 is
' greater ~han the inside diameter of make-up rings 133. Around
the inner periphery of core ring 135, between it and valve actuat-
ing stem 51,- is disposed a bearing ring 137 of tough, compliant,
solid lubricious material which may be of the same. type'used for
make-up rings 133. Bearing ring 137 makes a sliding fit within
the aperture in core ring 135.
The metal ring gaskets 131 in their relaxed state have a
generally rectangular cross-section which lies at about a 30
angle to the horizontal.~ Thus, the cone angle for the gaskets
131 in relaxed condition is about 120. Gaskets 131 in such
relaxed` condi'tion have a radial clearance with both stem 51 and
stuffing box 65 so that neither the stem nor the stuffing box
' will be damaged upon installing gaskets 131 in the valve. Gas-
kets 131 are just dropped into place. Upon ener~izing the seal
means by tightening the packing retainer, the gasket-s are flat-
tened such that the inner diameter of each gasket is reduced and
the outer diameter of each gasket is increased sufficiently that
the inner peripheral edge on the concave side of each gasket and
the outer peripheral edge on the convex side of each gasket are
-deformed or "coined," i.e. they flow plastically, into metal-tc-
metal sealing engagement with the stem and stufing box, respec-
tively. In order not to gall or mar the stems, the metal ring
gaskets should be made of softer metal than the valve stems. The
valve stems may be made of K Monel, for example, but a comparable
steel would be suitable. It is preferred that the stem have a
hard coating on it, such as a tungsten carbide coating of 3 to 5
! 172667
.
mils thickness, to increase the hardness of and enhance the
durability of the stem. Such a coating would raise the hardness
of the stem from about 30 to about 60 Rockwell hardness. The
metal ring gaskets should have sufficient plasticity to 2110w
o~ning of the peripheral edges through high stress to effect the
-metal-to-metal seal, and should have sufficient strength to
wlthstand the high preload and well fluid pressures found in
service. They may be made, for exam~le, of annealed (austenitic)
stainless steel, such as No. 316 stainless steel, or of other
metals such-as carbon or alloy steel. To urther reduce the
possibility of galling or marring the stems, the inner peripheral
edge on the concave side of each gasket is rounded, preferably
with a radius e~ual to about one-half the gasket thickness, e.g.
a radius of 0.02 inches for a gasket that is 0.04 inches thick.
In flattened condition, the metal ring gaskets 131 make an angle
of approximately 15~ to the horizontal, corresponding to a cone
angle of about 150~, conforming to the ~rustoconical faces of
adapter rings 123, 125 and 129 which are also disposed at an
angle of about 15 to the horizontal. It will be understood, of
course, that while the metal ring gaskets are preferably frusto-
conical, shapes for the metal ring gaskets other than frusto-
conical may be employed.
The material used for make-up rings 133 and the bearing ring
137 should have a low coefficient of friction, that is, it should
be a highly lu~riciou~ material. The material for make-up rings
133 and bearing ring 137 should also have sufficient strength and
toughness to remain integral under high pressure, and should be
resistant to chemical activity of the fluid being controlled by
the valve, and should be able to withstand temperatures through-
out the range expected to be encountered during service, e.g.
300~F. down to -20~F. Such material should also be sufficiently
compliant or elastic to flow into any minute gaps which may be
left between the metal ring gaskets and the s~em and stuf f ing DOX
! 1 7 2 6 6 7
due to scratches or machine marks on the stem or stuffing box, or
between the metal ring gaskets and the newly adjacent stem sur-
ace during and after movement of the stem. Gaps of the latter
type might occur, for example, because during and after movement
~f-the stem, the inner pe'ripheries of the metal ring gaskets will
not~instantaneously, and perhaps might not ever, 1OW further
pl-astically to conform to the newly adjacent stem surface. The
make-up and bearing ring materials wi11 flow into and seal any of
these minute gaps. Therefore the make-up and bearing rings may
also be considered to be sealing rings. Materials suitable for
the make-up rings 133 and bearing ring 137 include! for example,
tetrafluoroethylene polymer, such as that sold under the trade-
mark "Teflon," and "Moly-Teflon," which is like Teflon but in-
ciudes up to about 15 percent molybdenum disulfide, MoS2. A
particular material found to be suitable is one having 5% by
weight MoS2 and 95% by weight TFE, and sold by Allied Chemical
Company under the designation'"No. 2021."
Th-e'material used for core ring 135 should, lîke the m'ate-
.
rial used for rings 133, 137, have sufficient strength to with-
s~and the high stress re~uired for preloading the seal, and
should also be resistant to chemical activity o the well fluids
being sealed against and able to withstand temperatures through-
out the range expected to be encountered during service, e.g.
300F. down to as low as -75F.
Further properties of the core ring 135 may be best'appreci-
ated from applicants' discovery of what they believe to be the
cause of the leakage of the SMT type seal at high pressures upon
extreme temperature cycling, although it is to be understood that
the herein described seal solves the leakage problem regardless
of the theory of its operation.
The leakage exhibited by the SMT type stem seal using TFE or
~oS2-filled TFE rings at pressures in the 30,000 p~s.i. range
af.er temperature cycling apparently results from partial loss of
the preload on the seal. Compare the loss of preload in pure
~ 172~67
.
elastomer packings disclosed in the ASME article referred to
above. When the SMT type seal is first made up or energized,
mechanical compression is placed upon the seal through tightening
...
the packing retainer such that the seal is stressed to a greater
-~ ~egree than it would be''stressed by the high pressure well fluids.
. . ~
When a v'alve including the SMT type stem seal-hea~s up from
- -~ambient temperature to 300F., the TFE rings try to expand, but
cannot do so to any great extent, ince they are substantially
confined on all sides by metals which expand less rapidly .han
the TFE rings. This causes the preload stress on the seal to
increase from what it was originally, that is, bef.ore heating.
When the valve cools down to ambient temperature, the preload
stress not only diminishes from its elevated level attained when
the valve was heated, it becomes lower than it was originally.
This is apparently due to one or the ot'her, or perhaps both, of
the following effects. When the valve is heated to about 300F.,
the elevated stressj over and above the already high preload
stress', on-the seal and surrounding metal containment structure
caused by thermal expansion of the TFE rings might cause slight
permanent deformation or set, that is, yielding, of such surround-
ing metal containment structure so as to increase the volume
occupied by the seal when the seal cools back down to ambient
temperature, thereby reducing the stress on the seal belo~ its
preload. Alternatively, or perhaps cumulatively, when the TFE
rings are stressed at the elevated level due to thermal expansion
upon heating the valve to 300~F., the TFE rings could undergo a
permanent deformation beyond that caused by the preload stress,
that is, they might take on a further compressive set, which
remains after the valve cools down to ambient temperature and
results in a stress relief in the TFE rings which offsets somewhat
the effect of the packing retainer's compressing of the seal.
Thus, after temperature cyclin~, in order to regain the proper
preload, the packing retainer would have to be retightened.
1 1726~7
:
.
Without such retightening, the valve would be unable to seal
against pressures of the order of 30,000 p.s.i. without slight
leakage.
According to the present invention, some of the preferred
TFE-or MoS2-filled TFE material between the metal ring gaskets of
the SMT type stem seal is replaced by a'mate'ria~ wh'ich exhibits
relatively little expansion when heated as compared to the TFE or
MoS2-filled TFE material. I~ the preferred embodiment of this
invention, this low thermal expansion material lS comprised in
core- ring 135, which is clad on its three sides adjacent the
metal ring gaskets and stem by ~ake-up rings 133 and bearing ring
137. Thus, core ring 13S must have a coeficient of volumetric
thermal expansion that is lower than that for rings 133, 137.
One such material suitable for core ring 135 is compacted graphite,
such as lS sold under the trademark "Grafoil.'i See U.S. patent
no. 3,404,061. It is believed that the coefficient of volumetric
thermal expansion for TFE, for example, is substantially greater
_;f '
~ than that for Grafoil, indeed several times greater', although
precise values are not known to applicants. Published data
' ~ ' indicate, however, that Grafoil, which is manufactured in such
:,
forms as thin flexible layered sheets or ribbon, has a coeffic-
ient of linear expansion of about -0.02 x lQ S in/inF. in direc-
tions parallel to the graphite layers, that is, along their
length and width, over the range of 70~F. to 2,000F., and of
about 1.5 x 10 5 in/in~F. in directions normal to the layers,
that is, through their thickness, over the range of 70F. to
a~ooo~F~ TFE has a coefficient of linear expansion of about
7.0 - 10.0 x 10 in/in~F. over the range of 78F. to 500~F. For
solids, the coefficient of volumetric thermal expansion is approxi-
mately three times the coefficient of linear thermal expansion.
See The Handbook of Chemistry and Physics, 48th Edition, at page
F-90 (Chemical Rubber Co. 1967). Assuming, then, that Grafoil
were to expand volu~etrically upon heating according to three
- `~ 1 726~7
.
times the larger of the values given above for linear expansion,
this would still be less than one-fourth the volumetric expansion
upon heating expected to be exhibited by TFE for heating of the
valve from temperatures in the ranges of about 78~F. to about
-_300~F.
_- The stack of rings sandwiched between the metal ring gas-
kets, referred to hereinafter as the sandwich stac~, consisting
of the core ring 135 with bearing ring~137 disposed around its
inner periphery and make-up rings 133 disposed adjacent the upper
and lower faces of rings 135, 137, is deformed to conform to the
shape of, and fills substantially all of the space between, the
metal ring gaskets 131 as flattened in their energized condition.
The rings of the sandwich stack typically are originally of
rectangular cross-section, but may be deformed in a press prior
to installation in the valve to assume the frustoconical shapes
~; shown on the right hand side of Figure 2, which are correlative
to the frustoconical shaped surfaces of adapter rings 123, 125,
129 against which the metal ring gaskets are flattened. Alterna-
tively, the rings of the sandwich stack may be installed ~n their
original rectangular cross-section configuration, and deformed
into frustoconical shape upon energizing the seal.
The material of which stress relief ring 135 is made may
have the form of Grafoil ribbon wrapped tightly and compressed
into a solid, endless ring. Such ribbon may be of the type sold
by Union Carbide Corporation under the name Grafoil Ribbon-Pack
and described in Union Carbide Corporation's Technical Informa-
- tion Bulletin No. 52~-204, a copy of which is attached hereto as
Appendix I. A ring of tightly wrapped Grafoil Ribbon-Pack mate-
rial that has been compressed exhibits its highest thermal expan-
sion in the radial direction when such a ring is installed in the
stuffing box around the stem, that is, toward the stem and stuff-
ing box. Alternatively, ring 135 may be cut from a sheet of
\
} 17266~
Grafoil, or sheets of Grafoil stacked together. The Grafoil
available commercially may be only about 70 percent compressed or
compacted, but can be fully, i.e. 100 percent, compressed or
compacted either in a press prior to installation, as described
_~~beve, or when in place in the valve upon energizing the seal.
- ~ Make up rings 133 are thinner axially than~ring 135 and ring
-
13~, which have -su~stantially the same axial thickness when ring
135 is fully compacted, but are wide~ radially than elther of
rings 135,137. The combined radial widths of rings 135 and 137
is substanti-ally the same as the radial width of rings 133.
Rings 133 and 137 should make a close sliding fit with the stem
upon installation of the sandwich stack in the valve.~ When the
sandwich stack is compressed upon energizing thP seal, the inner
peripheries of rings 133, 137 are urged more tightly against stem
51, and the outer peripheries of rings 133 and 135 are urged into
tight engagement with the walls of stuffing box 65.
p Although two seal sets 119, 121 are shown in the drawings,
; appllcants~have found that one seal set alone is sufficient for
providing a satisfactory seal. The second seal set, that is, the
one farthest from the fluids being sealed against, is provided
for emergency or back-up use, in case the first seal set fails.
As another emergency or back-up sealing feature, a passage 139 is
provided in bonnet 23, which passage is in register and fluid
communication with a passage 141 through intermediate adapter
ring 129 when the seal is energized. Passage 139 is also in
fluid communication with an injection fitting 140 disposed in a
threaded socket in the exterior of bonnet ~3. An annular groove
143 in fluid communication with passage 139 is provided around
the walls of stuffing box 65, and an annular groove 145 in fluid
communication with passage 141 is provided around the inner
periphery of intermediate adapter ring 129. If seal sets 119,
121 were to fail, then sealant material can be injected through
injection fitting 140 and passages 139, 141 to grooves 1~3, 145
! 1 72667
.
. .
to provide an emergency or back-up seal around the stuffing box
and stem, respectively.
The seal means of the present invention is preloaded by
tightening the packing retainer to a stress substantially higher
-'th-an the stress expected t'o'be caused by fluid pressure when the
. _ . . . .
~valve is in service. Typical preload stress for the s~em seals
for a valve such as depicted in Figure 1, which has a rated
working pressure of 30,000 p.s.i., is 37,500 p.s.i.
When a valve incorporating the stem sealing means of the
present invention is heated from ambient temperature, e.g. 70F.,
to about 300F., for example by the fluids flowing.through the
valve, and is then cooled back down to ambient temperature, no
further tightening of the packing retainer is necessary. Thus,
the original preload on the valve is mzintained upon temperature
cycling. This is due apparently to the'fact that the stress over
and above the original preload stress caused by attempted thermal
expansion of the material between the metal ring gaskets 131 is
,
not grèat en~ugh to cause the permanent deformation or set of the
metal containment structure of the valve parts adjacent to the
se~l due to the lower thermal expansion of the core ring 135 as
compared to the make-up rings 133 and the bearing ring 137. Eor
the same reason, the additional stress due to heating is appar-
ently low enough to avoid the rings' 133, 137 taking on of a
urther permanent deformation or compressive set, which in turn
avoids loss of preload stress when rings 133, 137 relax upon
cooling.
Applicants have found ~that the stem sealing means of the
present invention works well in sealing 30,000 p.s.i. fluids over
temperature cycles from -20F. to 300~E. without additional
tightening of the packing retainer, when make-up rings 133 and
bearing ring 137 are made of 5% MoS2-filled T~E, core ring 135 is
made of Grafoil, ana when the volume of the stress relief ring
135 is appro~imately one-third the tot~ olume of t~- sandwich
t 1 7266~
.
stack. That is, in the preferred embodiment, the combined volume
of make-up rings 133 and bearing ring 137 is approximately twice
the volume of core ring 135. The material used for the make-up
rings 133 will not extrude past the metal ring gaskets 131 upon
-`~nergizing the seal, and'wil'l confine the Grafoil core ring 135
~ between'them during energizing the seal to prevënt lts extruding
; past the gaskets 131 before the gaskets form the metal-to-metal
'' seals with the stem and stuffing box, Also, the 5% MoS2-filled
TFE bearlng ring 137 around the inner periphery of the Grafoil
core ring 135 reduces the friction between the stem and~the stem
seal means and prevents undue wear on the core ring. Thus, a
Grafoil core ring with its low thermal expansion, clad on its
three sides adjacent the stem and metal gaskets by 5% MoS2-fiIl~d
TFE rings and wherein the Grafoil ring has about one-half the
combined volume of the 5% MoS2-filled TFE rings, is preferred for
the sandwich stack of the present invention. Of course, other
volume relationships for the rings of the sandwich stack may also
work we-ll,~'as may other materials for the rings comprising the
sandwich stack. '~
- Although Figures 1 and 2A and 2B show the cones of the metal
ring gaskets and the rings of the sandwich stack pointing away
from the pressure being sealed against, when sealing between
parallel surfaces the inner and outer peripheries of the seal are
similarly engaged and therefore the seal is reversible. Thus,
the cones of the metal ring gaskets and the rings of the'sandwich
stack alternatively may be arranged to point toward the pressure
being sealed against. Also, it will be understood that rings
133, 137 need not be separate rings. They may instead take the
form of, for example, an integral cylindrical ring with a groove
around the middle of its outer periphery, that is, a ring with a
U-shaped cross-section. The Grafoil ring can then be snapped
into place in the ~roove prior to installation in the valve.
) 1 726~
.
.
While preferred embodiments of the invention have been shown
and described, many modifications thereof can be made by one
skilled in the art without departing from the spirit of the
. . .
invention. Therefore, it should be understood that the details
- se~ forth herein are for illustration only, and are not intended
:to limit the scope of the invention as set forth in the foilowing
claims.
What is claimed as invention ist-
