Note: Descriptions are shown in the official language in which they were submitted.
HEA~ RESISTANT SEAL
TECHNICAL FIELD.
This invention relates to seals and more particularly
to seals capable of maintaining a flu:id tight seal upon the
~application of high temperatures to the exterior of a connection
as when subjected to high flame temperature.
BACKG~OUND ART.
_ .
In many industrial environments, ~ire is a constant and
almost unavoidable hazard. This is particularly true of the oil
and gas ind~stry. Where highly fla~mable liquids are being
handled under high pressures, it is vitally important ~hat, once
a local outbreak of a fire does occur, all internal packing
elements maintain pressure tight seals lest a local mishap assume
uncontrollable proportions.
A wide variety of packing elements d~signed to maintain
a pressure differential between telescopin~ parts in a connection
are generally known in the art. Such materials are most con~only
selected, however, on the basis of such features or propertie6 as
compressibility, elasticity, chemical resistivity, and lubricity.
`~Som~ of the most common packing materials, such as gasXet paper,
cork composition, or sheet rubber are strictly limited in their
application to low temperature environments. A number of special,
high temperature resistant packing materials are ~nown in the
art, as for example, asbestos which is widely used in the form of
asbestos mats or sheets or as filling material between corrugated
sheet metal, metal jackPts or spirally wound steel strips. Even
though these materials or combinations of materials do not burn,
char, or disintegrate under high temperature conditions, main-
taining a pressure tight seal using such materials under such
conditions still presents a dif icult en~ineering problem In
most cases, a sizable temperature differential exists between the
two surfaces that are to be sealed, especially where high tempera-
tures are lGcally confined and intermittent in nature, e.g.l
,' ~
where a ire breaks out ~lthin or around the telescoping machine
elements containiIlg the packing between them. Ere~uently, the
material dimensions of the elements joined or separated by the
seal are guite different and will therefore result in diff~rent
e~uilibrium temperatures after heating. In addition, even when
temperature eguilibrium is maintained across the interface,
different materials employed side by side will undergo different
thermal expansions and thereby cause leakage or loss of sealing
effect. If, for example, the outer element is both longer in
axial dimerlsion and hotter in temperature, its greater axial
expansion tends to relieve any load which had been supplied by it
to actuate the seal, therèby permitting leakage to result. The
differing radial expansion between the (hotter) outer element and
the (cooler) inner member will also result in a radial gap which
destroys the seal or permits it to extrude into the gap. Since
prior art packings have g~nerally employed packing materials of
thermal expansion coefficients substantially different from those
most commonly used for the stuffing box, e.g., st~el, both types
of sealing breakdowns are experienced under high temperature
diferential conditions. It would, therefore, ~e desirable to
employ a packing material having a thermal expansion coefficient
such that it will expand appxoximately as fast as, or slightly
faster than, the elements to be sealed when the elements are
exposed to a high temperature environment. It would also be
desirable to employ an applicator which prevents extrusion of the
packing material while maintaining a fairly constant sealing load
upon the packing material and which applies a load ~reater than
the fluid pressure being sealed off between the elements.
Graphite has long been known to have some of the prop-
~rties generally desired for high temperature packing ~aterials.
It must, however, frequently be combined with other materia~s,
which) in turn, negat:ively influence the thermal properties
before its mechanical properties can satisfy the design demands
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3~
of hlgh pressure seals. One of the substances not sharing -this
cornmon shor-tcoming is "Grafoil*" described in the publica-tions
"Grafoil - Ribbon-Pack, ~niversal Flexible Graphite Packiny for
Pumps and Valves" by F. W. Russell (Precision Products) Ltd. of
Great Runmow, Essex, England, and "Grafoil Brand Packing" by
Crane Packing Company of Morton Grove, Illinois.
Grafoil is an all-graphite packlng ma-terial manufac-
tured by DuPont containing no resin binders or inorganic fillers.
It has the chemical inertness and lubricity typical oE pure
graphite but~ unlike conventional graphite, has highly direc-
tional values for thermal conductivity, thermal expansion, and
electrical resistivity. Grafoil is a material which will with-
stand extreme temperatures over 2000 F, is self-lubricating,
will not vulcanize or bond to metal surfaces, exhibits no em-
brittlement and possesses high thermal conductivity. Its highly
directional thermal properties can be readily controlled by the
manner in which the material is wrapped. A loose wrap produces
packing having the highest thermal expansion in the a~ial direc-
tion. A tight wrap produces the reverse, i.e., highest thermal
conductivity axially, and highest thermal expansion radially. A
medium wrap will exhibit nearly isotropic thermal conductivity
and expansion.
Grafoil has been used in various applications, many of
which are described in the above Grafoil publications. Grafoil
packing has been used in fire safe controls because the packing
will not deteriorate under high temperatures and sealing is
maintained after cool down. Grafoil has also successfully been
employed in boiler feed pumps, centrifugal, reciprocating and
rotary pumps, and for valve stems. It has, however, been limited
30' in application since Grafoil is susceptible to extrusion.
In the oil and gas industry, where extremely high down-
hole pressures are encountered and must be sealed against, no*Trade Mark
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successful mechanical comblnation of packing materials has been found which
would permit the thermally desirable properties of graphite to be employed
~mder the ncessary high pressure conditions. These deficiencies in the prior
art are overcome by the present invention.
S~M~IARY OF r~lE I_V NTION
According to the broadest aspec-t of the present inventi.on there is
provided an assembly for maintaining a seal between a first member and a
second member housed within tl~e first member, the seal being maintained at
both a low temperature and a high temperature resulting from applying extreme
heat to the exterior of the firs-t member, comprising:
nonmetal seal means for sealingly engaging the first and second
member, said nonmetal seal means having a thermal expansion at least as great
as that of the first member;
metal seal means disposed above and below said nonmetal seal means
for sealingly engaging the first and second members; and
load maintaining means for maintaining a compressive load on said
nonmetal means within a predetermined load range.
According to another aspect of the present invention there is
provided a packing for making up a fire resistant seal between a fi.rst mem-
ber and a second member housed within the first member, comprising:
packing means for sealingly engaging the first and second members,said packing means having a thermal expansion coefficient which is at least
as great as the thermal expansion coefficient of the first and second members;
upper and lower antiextrusion means on both sides of said packing
means for preventing the extrusion of said packing means under compression;
and
load maintaining means for compressing said packing means within a
predetermined load range.
A wellhead assembly includes a wellhead supporting a hanger sus-
pendlng a string of casing i.nto the well. The head has an adapter telescoping
a nipple on the hanger and between the adapter and nipple is employed a pack-
ing to seal the outer surface of the nipple and the inner surface of the
adapter. If the wellhead and adapter are exposed to excessive heat, their
radial thermal expansion will widen the gap between the inner and outer seal-
ing surfaces ancl their axial expansion will relieve the packing pressure nec
essary to maintain sealing contact. The packing of the present invention is
designed to maintain sealing contact between these two surfaces even a-fter the
application of extreme temperature differentials between them, such as com-
monly occurs when a fire breaks out.
More particularly, according to another aspect of the present in-
vention there is provided a fire resistant seal assemb]y housed within a well-
head between a hanger and a head, comprising;
graphite compound seal means around the hanger for sealing with the
hanger and head;
metal-to-metal seal means disposed above and below said graphite
compound means for establishing a metal-to-metal seal between the hanger and
head and for preventing the extrusion of said graphite seal means;
crushable metal means around the hanger for engagement with said
metal-to-metal seals means; and
means for compressing said graphite compound seal means, metal-to-
metal seal means and crushable metal means for actuation.
A fire-resistant seal is composed of three main elements; one or
more packing rings, one or more antiextrusion rings and a crushable follower.
A reduced diameter portion forming the upper part of the nipple defines a
space between the nipple and the inner wall of the adapter wi-thin which the
seal is housed. The main sealing element employed is a packing ring or a
series of packing rings made of a graphite compound and separated by metal
spacer rings. The wrap of the graphite compound is dependent upon the appli-
cation. The looser the wrap, the greater the radial thermal conductivity.
The tighter the wrap9 the greater the radial expansion. The present invention
generally employs a medium wrap.
Wedge assemblies abut each side of the cross-sectional surfaces of
the packing ring to prevent extrusion since a graphite
-4a-
compound has little stre~gth of iks own. One or both of the
wedge asse~blies include back~up rin~s which expand radially
inwardl~ and outwardly upon the application of a downwardly load
on the wedge assembly.
~ One wedge assem~ly has a flat baseline surface against
which a crushable follower maintains a predetermined load. The
crushable follower consists of an elongated cylindrical ring
whose alternating series of inner and outer grooves ~ives it a
meander-like cross-section. The crushable follower eliminates
the need for close vertical tolerances and acts like a one time
spring. Its nonreversible resiliency generally maintains the
applied load without giving way under higher downhole pressures.
IJpon assembly, the fire-resistant seal is compressed
with the crushable follower contractin~ to proper dimenslons with
the load range being chosen such that the groove becomes com-
pressed to approximately one half its preassembled width. Thi~
compression causes the compressed packing to establish a seal
between the outer surface of the nipple and inner surface of the
adapter and actuates the wedge assemblies to cam the back-up
rings into a metal-to-metal sealing engagement between the back-up
rings and the sealing surfaces above and below the packing. ~hen
the wellhead becomes exposed to fire and causes the adapter to
expand axially and radially, the graphite compound, having an
expansion coefficient comparable to the adapter, will also expand
to maintain contact with the surfaces and the back-up~rings. The
back-up rings will maintain metal~to-metal contact with the
sealing surfaces and restrain graphite compound extrusion as the
graphite compound expands to maintain sealing contact. The upper
and lower back-up rings thus prevent extrusion of the graphite
compound ring while at the same time creating metal-to-metal
seals against both upward and downward pressures. Other objects
and advantages of the invention will appear from the following
description.
BRICr D~SCRIP--;011 9F n~ ~1AIIINGS .
~ or a further understandiny of the nature and objects
of khe present invention, reference should be had to the follow-
ing detailed description, taken in conjunction with the accom-
~p~nying drawings, in which like parts are given like referencenumerals and wherein:
Figure 1 is a cross-sectio:nal view of a first embodi-
ment of the packing assembly of the present invention shown
installed in a typical environment such as the top section of a
nipple.
Figure 2 is an enlarged cross~sectional view of th~
packing assembly shown in Fi~ure 1 in the nonengaged condition;
Figure 3 is a cross~sectional view of the packing
assembly shown in Figure 2 in the engaged condition;
Figure 4 is a cross-sectional view of the packing
assembly shown in Figure 3 upon the application of high external
heat;
Figur~ 5 is a cross-sectional view of a second embodi~
ment of the packing assembly of the present invention;
. Figure 6 is a cross~sectional view of a third embodi-
ment of the packing assembly of the present invention; and
Figure 7 is a cross-sectional view of a fourth embodi
ment of the packing assembly of the present invention;
DESCRIPTION OF THE PREFERRED EMBODIMENT.
Referring now to Figure 1, there is shown a typical
installation of the present invention. It should be understood
that the following illustrative envircnment is for descriptive
purposes only and that the invention has other applications not
hereinafter described. The illustrative environment includes a
wellhead 10 having a tapered seat 14 projecting into the wellhead
bore. Nipple 20 includes h central flow passage ~2, a lower
threaded box 24 for suspending a string of tubing or casing 26,
and a downwardly facing frustoconical shoulder 28 for supporting
enga~ement with wellheadlseat 14 upon the lowering of nipple ~0
and string 26 into the well. Upon installat,ion of string 25,
string 26 forms an annulus 12 extending down into the well.
Nipple 20 further includes a reduced diameter portion
~30 at its upper end for telescopically receiving packing assembly
50. Reduced diameter portion 30 includes external threads 32 at
its upper end and forms an upwardly facing shoulder 34 for sup-
porting packing assembly 50.
Upon landing nipple 20 wit:hin wellhea~ 10, an adapter
40 is telescopically receivsd over reduced diameter por'tion 30
projecting from wellhead 10. Adapter 40 has a counterbore 42
forming a downwardly facing annular shoulder 44 which approaches
but does IlOt engage the upper end of nipple 20.
Wellhead 10 has a counterbore 16 above sea~ 14 for
receiving a seal assembly 18 telescopically received over nipple
20 for sealing between nipple 20 and wellhead 10 upon make up.
Wellh~ad 10 and adapter 40 have mating annular partially recessed
and protruding flanges 46, 47, respectively, connected toyether
by clamps 48.
`~ Referring now to Figure 2, packing assembly 50 of the
present invention is shown in greater detail. Packing assembly
50 includes a tubular spacer ring 52, an upper expanslble wedge
assembly 56, a plurality of packing rings 58a, 58b, and 58c
having spacer rings 60 therebetween, a lower expansible wedge
assembly 57, and a crushable follower 54. Starting with crush~
able follower 54, packing assembly 50 is received over the upper
end of reduced diameter portion 30 of nipple 20 with crushable
follower 54 engaging shoulder 34.
Crushable follower 54 consists of an elongated cylin-
drical ring whose radial width is approximately egual to that of
lower wedge assembly 57 and packing rings 58. Along the outer
and inner surfaces of crushable follower 54, however, are cut
generally rectangular grooves 70 on th~ inside and, in alternat-
.. -7-
ing step ~ashion, simila~ ~rooves 72 along the outside wall of
~ollower S4. Grooves 70, 72 permit follower 54 to be compressed
in a vertlcal direction whereby the vertical. ridges 55 formed by
grooves 70, 7~, respectively, bend, at a predetermined compres-
sive load.
Upper expansible wedge assembly 56 includes wedge ring
64 and outer and inner metal rings 66, 68. Lower expansible
wedge assembly 57 includes a wedge ring 65 and outer and inner
metal rings 67, 69. Rings 66, 68 abut the upper annular sur~ace
of packing ring 58a with inner ring 68 adjacent axial wall 38 and
outer ring 66 adJacent cou~terbore 42 of adapter 40 upon assembly~
Rings 66, 68 have generally triangular cross-sections forming a
V-shape upon assembly and have their facing annular corners
truncated so as to form abutting vertical surfaces. Wedge ring
64 has a lower yenerally wedge shaped, trapezoidal cross æection
for matin~ engagement with the upwardly facing surfaces of rings
66, 68, orming the V. Wedge rin~ 65 and outex and inner metal
rings 67, 69 have cross-sPctions comparabl~ to those of wedge
ring 64 and rings 6~, 6~. However, wedge ring 65 and rings 67,
69 form an inverted Y-shape upon assembly. Thus, the lower
annular side of wedge ring 65 engages the upper surace o crush
able follower 54 and the upper annular side formed by wedge ring
65 and rings 67, 69 engages the lower annular surface of packing
ring 58c. These cooperating surfaces of upper and lower wedge
assemblies 56, 57 form cam surfaces for actuation as hereinafter
described.
Each packing 58 is a graphite compound, such as Grafoil,
consisting principally of graphite combined with nonresin binders,
which is commercially available in filament or wrap form. Such
graphite seal means combines the desirable thermal and chemical
characteristics of graphite with the added mechanical character
istics of its nonresin binder. In one size of s~al assembly,
each packi~g ri.ng 58 consists of 15 wraps of 0.75 inch graphite
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t~
compound ribbon, and is i!nltially of generally square cross-
section. Such graphite compound rings are compressed to a
tightly wound ilament bundle in a die prior to assembly by
applying a make-up pressure which is higher than, e.g., approxi-
~mately 20 percent more than, the rated working pressure of thevalve. For a 5,000 psi rated valve the make-up pressure is ~,300
psi which is suficient to compress a 0.75 inch ribbon to 0.31
inch height.
Grafoil is an all graphite packing material manufac-
tured by DuPont containing no resin binders or inorganic fillers.
It has the chemical inertness and lubricity typical of pure
graphite but, unlike conventional graphlte, has highly direc-
tional values for th~rmal conductivity, thermal expansion, and
electrical resistivity. Graoil is a material which will with
stand extreme temperatures over 2000 F, is self-lubriGating,
will not vulcanize or bond to metal surfaces, exhibits no embrit
tlement and possesses high thermal conductivity. Its highly
directional thermal properties can be readily controlled by the
manner in which the material is wrapped. A loose wrap produce~
` packing having the highest thermal expansion in th~ ~xial direc-
tion. A tight wrap produces the reverse, i.e., highest thermal
conductivity axially, and highest thermal expansion radially. A
medium wrap will exhibit nearly isotropic thermal conductivity
and expansion. Since the packing of the present invention must
adapt to possible loss of sealing contact due to axial as well as
radial thermal expansion o wellhead members, a medium wrap o
the graphite compound packing is generally preferred because of
its nearly isotropic expansion properties.
There are three such packing rings 58a, 58b, 58c,
respectively, in this particular embodiment of the present inven-
tion. Packing rings 58a, 58b and packing rings 58b, 58c are
s~parated from one another by metal spacer rings 60 for evenly
distributing the deflection of packing rings 58 under high pres-
sures and temperatures. ISp~cer rings 60 prevent any caking ofadjacent graphite compound rings. As.later de~cribed with respect
to another embodiment used under medium or low load conditions,
the packing of the present inventio~ has also been used wlthout
such spacer rings between abutting packing rings since caking
conditions exist under extreme pressures and temperatures encoun-
tered primarily in the larger diamete~r seals.
Cylindrical spacer ring 52, in combination with crush-
able follower 54, eliminates khe neecl or close tolerance~ along
the vertical direction of the neck formed by r~duced diameter
portion 30. Spacer ring 52 has a co~mterbore 74 forming a stop
shoulder 76. A gland 78 is received by counterbore 7~ for threaded
engagement with threads 32 of reduced diameter portion 30. Gland
78 acts as a retainer ring, restraining the upward movement of
packing assembly 50 by engaging stop shoulder 76 of spacer ring
52. Thus, gland 78 and spacer ring 52 prevent assemb~ly~50 from
slipping off of nipple 20 when nipple 20 i5 unassembled.
- Referring now to Figure 3 showin~ the assembled posi-
tion, the packing assembly 50 of Figure 2 is shown in its sealing
or compressed stage under normal temperatures and working condi~
tions. Adapter 40 is telescoped over nipple 20 and clamped ~nto
wellhead 10 by clamp 48 as shown in Figure 1. The upper terminal
end of spacer ring 52 thereupon engages shouldar 44 of adapter
40, thereby causing a downward movement of spacer ring 52 to
actuate and compress the pacXing assembly 50.
Initially upon actuation, crushable follower 54 engages
shoulder 34 causing lower wedge ring 65 to cam upward along the
inverted V surface formed by rings 67, 69, thereby expanding the
latter into metal~t~-metal engagement with counterbore 42 and
axial wall 38, respectively. Packing rings 58 are compressed
between upper and lower expansible wedge assemblies 56, 57 with
spacer rings 60 therebetween for ~tiffening and radial guidance.
Packing rings 58 are flattened; thereby increasin~ their radial
.
. , , ~
3~3~ 7
thickness and causing se~ling engagement with axial wall 38 on
the inside and counterbore 42 on the outside. Wedge ring 64 is
forced downwardly hetween rings 66, 68 camming outer ring 66 into
metal-to-metal engagement with counterbore 42 and camming inner
ring 68 into metal-to-metal engagement with axial wall 38. Upper
rings 66, 68 and lower rings 67, 69 a:re also antiextrusion rings
and prohibit the extrusion of the graphite compound along ~xial
wall 38 and counter~ore 42.
Upon wedge assembly 56, pac:king rings 58, and wedge
assembly 57 reaching their act.uation hei~ht, spacer ring $2 is
depressed fuxther until almost flush with the upper en~ of ni.pple
20 thereby further depressing. and crushing ollower 54 as show~
in Eigure 3O The load characteristics of crushable follower $4
are such that a constant compressive force is maintained over a
wide deflection range.
Crushable follower 54 i5 made o a relatively ductile
material such as carbon steel or austenitic stainless s~eel. The
mechanical properties o its material and its design with a step
fashion annular groove arrangement combine to produce a load
deflection curve which is n~arly constant over a fairly wide
range of elastic deformation, such as, for example, between 0 o~'l
and o.201' compression. For th~ anvironment of the present inven-
tion~ the make~up load for the follower is chosen such that it
remains well above the pressure load on the packing exerted by
the downhole pressure. Without the effect of the extreme temper-
ature differential, the make-up pressure is designed to perma~
nently close the annular grooves to approximately one-half closure.
For a nominal 5,000 psi rating (cold~, the follower make-up loads
are 50 tons for a 6" seal and 20 tons for the smaller 3" seal,
e~uivalent to a pressure of approximately 8,300 psi. Eor other
ratings throughout the range of commonly encountered rated working
pressures, i.e., 2,000 - 15,000 psi, different values would be
use~, of course, so long as the previously stated conditions are
-11~
met that the make up pre~sure must be hi~her than the rated
working pressure. The radial dimensions of c:rushable follower 54
are chosen such that in its partially crushecl state the radial
dlsplacement caused by the bending of vertical ridges 55 will
result in the midsections of vertical ridges 55 touching but not
scoring sealins surfaces 38 and 42.
Prior to describin~ Figure 4 showing the environment 50
und~r high temperature, reference is made again ko Fi~ure 1,
illustrating the en~ironment of the pr~sent invention. The
telescoping connection of reduced diameter portion 30 of nipple
20 and counterb~re 42 o~ adapter ~0 form~ a cylindrical chamber
41. The axial wall 38 of portlon 30 orms the inner sealing
surface ~f çhamber 41 and counterbore 42 forms the outer sealing
surface. Upwardly facing shoulder 34 of nippl2 20 and ~ownwardly
facin~ shoulder 44 o adapter 40 form the lower and upper ends of
chamber 41.
The radial gap or thickness of chamber 41 is th~ d~-
ference between the outer diameter of reduced diameter portion 30
and the inner diameter of counterbore 42. The axial height or
len~th of chamber 41 is the distance between shoulders 34 and 44.
The radial gap and axial height o chamber 41 will vary with the
temperature of nipple 20 and adapter 40. Packing assembly 50
housed wi thin chamber 41 must maintain a seal across the radial
gap o cha~ber 41 regardless of the temperature of nipple 20 and
adapter 40, especial.ly when adapter 40 îs subjected tG the extreme
heat of a fire around we~llhead 1().
After assembly and actuation as described, packing
assembly 50 provides both a metal~to-metal seal and a ~raphite
seal across the rad.ial gap between axial wall 38 and counterbore
42. Wedge assemblies 55, 57 ha~e a dual role, providing metal to-
metal seals acros~ the radial gap and orming antiextrusion rings
for pa~king rings 5~. Packing rings 58 provide a nonmetal seal
across the radial gap. The assembly and actuation of wedse
assemblies 56, 57 and pa~kiny rings 58 will cause such sealing
engagement across the radial ~ap under normal temperatures an~
conditions.
To maintain this seal across the. radial gap, pacXing
assembly 50 must fill the radial gap and remain in sealing engage-
ment with axial wall 38 and counterbore 4~ even as the dimensions
of chamber 41 change under fire conditions and must particularly
expand as the radial ~ap of chamber 41 enlarges due to the thermal
expansion of adapter 40. At the same time, wedge asse~blies 56,
57 must increase their camming expansion to prevent the extrusion
of khe graphite compound along wall 38 and counterbore 4~ as the
axial height of cha~ber 41 increases due to the thermal expansion
of adapter 40.
As can be seen in Figure 1, the axial distance betwee~
support shoulder 28 and the upper end of nipple 20 is much greater
than the radial wall thickness of adaptar 40. Therefore, if
adapter 40 and wellhead 10 become engulfed by fire, the thermal
expansion of adapter 40 will, even if proportionally uniform,
result, in absolute terms, in a greater axial elonga-tion than
`~radial expansion. Since nipple 20 is not only further removed
and shielded from such fixe but is also cooled by the 10w of
liquid through flow passage 2~, the thermal expansion between
adapter 40 and nipple 20 will tend to move the two apart and,
through their differing elongations, cause the pac~ing pressure
upon packing assembly 50 to be relieved, resulting in loss of
sealing contact with surfaces 38, 42. In addition, as described
above, the differing radial expansions of nipple 20 and adapter
40 will also widen the radial gap between surfaces 38 and 42. It
is these problems w:hich the present invention is designed to
prevent.
To that end, graphite compound is pr~erred as the
nonmetal seal means, having a thermal expansion coeffici2nt which
is substantially the same as, or ~ven directionally greater than,
. -13-
the expansion coefficien~ of the metal most commonly used for the
hanger and nipple of the wellhead eguipment of the present environ-
ment.
Referring now to Figure 4, the high temperature applica-
tion of packing assembly 50 of the present invention is depicted
in a somewhat schematic fashion. A~apter 40, having expanded
more than the relatively cooler nipple 20, has caused shoulder 44
of adapter 40 to become vertically upwardly displaced with respect
to the upper end of nipple 20 thus increasing the axial height of
chamber 41. Packing rings 58 are shown as having partial~y
counterbalanced this expansion diferential by their own axial
expansion. In co~bination therewith, the thermal expansion of
crushable follower 54 remaining well within the constant por.~ion
of the load deflection curve helps to maintain an approximately
constant compression load under expanded conditions sufficient ~o
maintain a sea' r~g~inst the maximum design downhole pressure.
Antiextrusion rings 66, 68 a~d 67, 59 are shown as.
having been displaced vertically upwardly and downwardly, respec-
tively, by the expanding packing rings 58~ and radially outwardly
and inwardly, such movement being facilitated by the triangularly
sloped cam surfaces of wedge rings 64, 65 between them. In this
fashion, agaln, axial extrusion of packing rin~s 58 is prevented
and radial metal-to-metal contact maintained with both counter
bore 42 and axial wall 38 by rings 66, 68 and 67, 69. Thus, both
count~rbore 42 of adapter ao and axial wall 38 of nipple.20
remain sealingly er}~aged by packi.ng rings 58 and by back-up rings
66, 68, and 67, 69 as the thermal expansion of crushable follower
54, and the even greater axial expansion of packing rings 58,
functionally cooperate to maintain the preselected compression
load within a range sufficient to prevent leakage ~nder downhole
pressure.
The functional cooperation between the main elements o
the fire-resistant packing assembly of the present invention has
63~ ~7
so far been described pr~r,larily with respect to one particular
embodiment, having three packing rings 58 disposed between wedge
assemblies 56, 57, each having two antiextrusion rings 66, 68 and
67, 69, respectively, wedge rings 64, 65, ~respectively. The two
~m~in advantages of this embodiment of the invention are that the
crushable follower 54 need not withstand the higher test pressure
which is applied from under the seal when the wellhead is assembled
and that the crushable follower 54 is not exposed to potentially
corrosive well fluid. As can be seen from Fi~ures 5, 6 and 7,
various other embodiments may be employed for difering wellhead
sizes or downhole pressures.
Referring now to Eigure 5, a second embodiment of the
present invention is illustrated and is identical with the irst
embodiment except for a different design in the wedge asse~bly
and the positioning of the crushable follower. Crushable fol-
lower 54 is now positioned directly below spacer ring 52 and
above upper expansible wedge assembly 56, which is the only such
wedge asse~bly used in this embodiment of the present inve~tion.
The lower expansible wedge assembly shown in the first en~odiment
`~is replaced with a lower metal back-up ring 62. Packing rings 58
and metal spacer rings 60 are also identical in design, size and
function to the arrangement previously described in connection
with the description of Figures 1-4. Packing supportlng shoulder
34 formed by the reduced diam~ter portion 30 o nipple 20, however,
is provided with a frustoconical seat 36 at its outer~periphery
for engagement with lower metal ~ack-up ring 62.
Frustoconi.cal seat 36 i5 slanted downwardly and outwardly
from the axial wall 38 of reduced diameter portion 30 starting at
shoulder 35 at an angle of generally 45. Lower back-up ring 62
has a triangular shaped cross-section with its lower bevelled
side resting against frustoconic~l seat 36. Ring 62 both acts as
an antiextrusion device for packing ring 58c above it and estab-
lishes a metal-to-metal sealing contact with the interior surface
of counterbore 42 of ada~ter 40. In the unassembled position,
the upper leg of triangular lower back-llp ring 62 abuts lower
packingtring 58c.
Under load and high temperature conditions, trian~ular
a~tiextrusion ring ~2 will move downwardly under the pressure
exerted by the expanding packing ring 58c. Because of its conical,
triangular cro~s--section, ring 62 is able to slide radially out-
wardly and down frustoconical seat 36. Ring 62 is thus still in
a position to prevent axial extrusion of expanded packing ring
58c and to maintain sealing contact with count rbore 42 of adapter
40.
~ eferring now to Eigure 6, a third emb~diment of the
present invention i9 shown which employs wedge assembly 80 and a
smaller lower antiextrusion ring 62. New wedge assembly 80
includes a wedge ring 82 and an antiextrusion ring 84, which are
larger than the previously shown rin~s 6&, 68 ~etween wedge ring
64 in the first and second en~odiments. New wedge assembly 80
does away with a wedge ri~g betwe~n the antiextrusion rings and
crushable follower 5~. Wedge ring 82 ar~d antiextrusion ring 84
are, in appearance, the upper and lower halves, respectively, o
a s~uare cross-sectional ring cut diagonally in two halves of a
trian~ular cross-section. The legs enclosing the right angle of
wedge ring 82 and antiextrusion ring 84 are equal in length to
the radial depth o packing rings 58, and therefore fill the
entire recessed portion between counterbore 42 of adapter 40 and
axial wall 38 of~nipple 20. The diagonal interface between rings
82, 84 fulfills the function of wedge ring 64 previously shown in
the first embodiment. Along that common interface, antiextrusion
ring 84 is able to slide radially outwardly, while wedge ring 82
slides radially inwardly under load conditions. The axial and
radial displacement caused by thermal expansion maintains the
antiextrusion enclosure of the packing ring and the sealing
contact with axial wall 38 and counter~ore 42 in the manner
9~al7
analogous to the one prevliously described. Frustoconical seat 36
and lower antiextrusion ring 62 are identical in form and func~-
tion to those described in connection with Figure 5 above.
Referring now to Figure 7, a fourth embodiment of the
~p~esent invention is shown which may be said to be characterized
by the absence of metal spacer rings. The particular version of
the fourth embodiment shown in Figure 7, for example, employs
only one packing ring 90. Ring 90 directly abuts antiextrusion
ring 84, which is generally the same as the one described in
connection with Figure 6 abo~e and adapted to cooperate with a
similar wedge ring 82, as there described. The lower metal
back-up ring 92 abutting graphite compound ring 90 at i-ts down-
hole face is different from the previous embodiment, however.
The nipple 94 of Figure 7 has its frustoconical seat 96 tapering
from the base of axial wall 98 rather than at the outer terminus
of shoulder 34 as in the first embodiment. Thus, seat 96 extends
across the entire radial depth of reduced diameter portion 100,
back-up ring 9~ abuts sat ~6 and also covers the entire radial
depth, being of a radial thickness e~ual to that of packing ring
`~ 90 and lower antiextrusion ring 84.
Yet another slightly different modiIication of khe type
of sealiny arrangement shown in Figure 7 has been employed for
3-inch seals, utilizing two graphite compound rings 90 rather
than one, with both graphite compound rings abutting each other
directly, having no metal spacer ring between them.
Because many varying and different embodiments may be
made within the scope of the inventive concept herein taught, and
because many modifications may be made in the embodiments herein
detailed, it should be understood that the details described
herein are to be interpreted as illustrative and not in a limit-
ing sense.
