Note: Descriptions are shown in the official language in which they were submitted.
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WO 95/23935 PCT/US95/01617
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LOW FRICTION PACKING
This invention relates to packing contain-
ments for sealing operating members in a housing with
fluids, and in particular to a packing containment
assembly useful in meeting very stringent fluid leakage
restrictions and fire safety conditions and in environ-
ments where the prevention of catastrophic leakage of
for instance valve structures is desired.
Background of the Invention
Packing materials are widely used to prevent
fluid leakage around an operating member in a housing
with fluid, such as a rotary shaft or a sliding stem in
fluid control valves or in a reciprocating pump shaft.
Normally such packing is formed of a resilient member
and is placed under a static load by being bolted into
position within a packing box around the operating
member. In other instances the packing is subjected to
spring loading in what is known as a live loaded pack-
ing configuration. Live loaded packing is particularly
useful in attempting to prevent undesired leakage of
fluids into the environment. Also, at operating
temperature conditions above approximately 450°F
(232°C) (i.e., operating temperature in the packing
area) it is desired to use commonly available graphite
packing rather than packing material formed of
polytetrafluorethylene (PTFE) because PTFE packing
tends to extrude at such elevated temperatures which
could lead to packing failure and fluid leakage. At
temperatures below 450°F (232°C) PTFE packing is
desired because it has substantially lower friction
than graphite packing. Excessive friction contributes
to poor control of fluid processes.
As an example, certain applications of a
fluid control valve require not only that the valve
meet stringent leakage requirements and have low
friction, but that it also meet a stringent packing-
SUBSTITUTE SHEET (RULE 26)
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performance fire test to prevent catastrophic packing
failure. Packing for fluid-controh valves used in
petroleum refineries, and in other chemical processing
applications, is desired to have low friction and
nearly zero leakage (i.e., less than 500 ppm as per EPA
Method 21) of fluid around the top of the valve under
normal operating conditions. In addition, it is
desired that the packing not fail when exposed to high
temperature thereby permitting excessive leakage of
flammable fluids during a fire.
In such applications, graphite packing is
well suited for preventing excessive leakage during a
fire, however, the high packing stress required to
provide nearly zero leakage during normal operating
conditions results in high friction from the graphite
packing, leading to less precise process control when
compared to PTFE packing. Attempts to decrease the
loading on the graphite packing provides a significant
reduction in friction but leads to excessive leakage
(greater than 500 ppm). To reduce the leakage and
friction, it is desired to use PTFE packing material
which can provide a tighter, longer lasting seal than
graphite material. However, the use of PTFE packing at
elevated packing temperatures is normally not
recommended, and particularly where the valve must meet
packing-performance fire tests, as in refinery
applications. The potential extrusion, decomposition,
and vaporization of PTFE packing under high temperature
fire conditions would lead to catastrophic packing
failure and undesired hazardous fluid leakage.
Thus, it is desired to provide packing for
control valves which exhibit low friction, fire safety,
nearly zero leakage, long service life, and reasonable
cost concurrently.
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Summary of the Invention
In accordance with the principles of the
present invention, there is provided a packing
containment system having low friction characteristics
and useful for fire safety and low fugitive emission
sealing of an operating member in a housing with fluid,
such as in a sliding stem valve, rotary shaft valve, or
reciprocating pump shaft. A two-part packing assembly
is provided mounted on the operating member, and
includes a respective plurality of first and second
packing ring sets providing a fluid seal surrounding
the operating member for preventing fluid from leaking
from the housing around the operating member. The
first packing ring set is mounted on the operating
member end closest to the fluid, which in a valve would
be closest to the valve plug end. The first packing
ring set includes at least one flexible graphite ring,
at least two anti-extrusion bushing rings, and a washer
formed of polymeric material with low friction charac-
teristics, such as PTFE, mounted between each of the
aforementioned rings. The first packing ring set
includes a substantial amount of graphite material to
provide fire safety and wiping action, and includes a
sufficient amount of low friction material, such as
PTFE, to prevent graphite adhesion to the operating
member and add lubrication.
A second packing ring set is mounted on the
operating member at the operating member end away from
the fluid, closer to the exterior of the housing, such
as the valve exterior. The second packing ring set
includes V-type PTFE packing rings and an anti-extru-
sion ring at each end. The second packing ring set
provides low fugitive emission sealing of the fluid
around the operating member in the housing and low
friction operation of the operating member.
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The two-part packing assembly is live loaded
with spring loading means, such as disk~springs, to
maintain the packing assembly at a stress level to
provide a low friction, low emission seal for both fire
safety and fugitive emission service capable of meeting
for instance the fire test requirements on the packing
system under specification API-589 and the specifica-
tions for API-607.
In particular, in accordance with principles
of the present invention, the second packing set
closest to the load source developed by the disk
springs, is subjected to a specific maintained load,
resulting in the optimal packing stress for low leakage
during normal operating conditions taking into con-
sideration the stem or shaft diameter, the packing
cross section, and the packing material. The load is
sufficient to cause the PTFE material to deform and
fill voids in the operating member and in the packing
bore thereby providing low friction and low emission
characteristics. Due to the low friction of the second
packing ring set with respect to the housing and the
operating member, the load is effectively transferred
through the second packing ring set to the first
packing ring set which is further from the loading
source. During the initial installation and adjustment
process, a load in excess of the normal operating load
is imposed temporarily on the second packing ring set.
This load is effectively transferred to the first
packing ring set and is sufficient to cause permanent
deformation and thereby effect a lasting seal capable
of providing fire safety as well as provide wiping
action. The wiping action prevents damaging con-
taminants from getting to the second packing ring set
during normal operation, even though the spring load
has been reduced to the optimal load for sealing at the
second packing ring set.
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Accordingly, flexible graphite packing
materials are used in the present invention in
conjunction with PTFE packing materials to provide a
low friction, low fugitive emission, fire safe packing
system. The graphite materials are located closest to
the process fluid to insulate the PTFE material from
the process fluid and provide a fire-safe seal. The
PTFE materials provide a seal on the operating member
to limit fugitive emissions below 500 ppm. Low
friction, as compared to traditional graphite packing
systems used for fugitive emission service, is obtained
by loading the graphite packing to lower stress levels.
The friction of the combined PTFE/graphite system is
significantly less than the friction of a sole graphite
packing system with similar leakage characteristics.
In a constructed embodiment of the invention
for leakage test purposes, a half-inch sliding stem
valve incorporated live loading in the form of disk
springs mounted around the stem adjusted to an initial
applied packing stress of 1783 psi (12.3 MPa). The
first or bottom most packing ring set closest to the
fluid included two flexible graphite rings, three PTFE
washers, and two carbon bushings with the bushings on
the end and a PTFE washer between each of the adjacent
graphite rings and carbon bushings. The second or
upper packing ring set included a virgin or unfilled
PTFE V-ring sandwiched between a carbon filled female
PTFE V-ring and a carbon filled male PTFE V-ring and
with a respective carbon bushing on each end.
The packing assembly was subjected to an EPA
Method 21 leakage test with over 100,000 mechanical
cycles of operation, and a packing leakage of less than
500 ppm was obtained using methane as the test fluid.
During the mechanical cycles, the packing temperature
was subjected to varying thermal cycles repetitively
between the ambient temperature and 450°F (232°C). The
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process pressure was also varied between a=pressure of
750 psi (5.17 MPa) at the lower packing temperature to
a pressure of 450 psi (3.1 MPa) atv°the upper packing
temperature of 450°F (232°C). It was found that during
the mechanical cycles, packing friction was very low so
that the friction of the combined two-part packing
assembly with PTFE and graphite material is
significantly less than the friction of a sole graphite
packing system with similar leakage characteristics.
Rather than the flexible graphite rings in
the first packing ring set, composite graphite rings
could be substituted. Such graphite composite packing
rings are slightly harder than the flexible graphite
ring material and are disclosed for instance in U.S.
Patent No. 4,826,181. Similarly, braided graphite
rings could be used to give different levels of fric-
tion and sealing capability in the packing system.
Also, other non-graphite materials, such as
fluoroelastomers or perfluoroelastomers, may be used to
provide a low friction, low fugitive emission seal.
Brief Description of Drawing's
The features of this invention which are
believed to be novel are set forth with particularity
in the appended claims. The invention may be best
understood by reference to the following description
taken in conjunction with the accompanying drawings, in
which like reference numerals identify like elements in
the several figures and in which:
Figure 1 is a cross-sectional view illustrat-
ing a preferred embodiment of the two-part packing
assembly of the present invention in a sliding stem
valve; and
Figure 2 is an exploded view illustrating the
two-part packing assembly details.
WO 95123935 21 g 4 6 2 p PCT/US95/01617
Detailed Description
As'indicated previously, the principles of
this invention are applicable to sliding stem or
rotating shaft valves, as well as reciprocating pump
shaft units. Figure 1 illustrates for instance a
sliding stem valve incorporating a two-part packing
assembly in accordance with the principles of this
invention.
The illustrated portion of a fluid valve 10
of the sliding stem valve type includes a valve body
with a valve bonnet 14 through which extends an
operating member such as a sliding stem 16. A series
of packing nuts 18 are threadably mounted on respective
packing studs 20 so as to adjust the loading on the
packing within the bonnet and around the valve stem.
Within a packing box formed of a packing bore
22 in valve bonnet 14, there is provided a first
packing ring set 24 mounted around the valve stem 16 at
stem end 26 closest to the process fluid. A second
packing ring set 28 in the two-part packing assembly is
mounted within the packing bore 22 around the valve
stem 16 at the valve stem end 30 further away from the
process fluid than the valve stem end 26.
As can be seen in more detail with respect to
Figure 2, the first packing ring set 24 includes a pair
of flexible graphite rings 32 which are conventional
packing rings of die formed ribbon flexible graphite.
The flexible graphite rings 32 are relatively soft
material each of which readily acts to seal the valve
stem. At each opposite end of the first packing ring
set 24 there is provided a conventional carbon bushing
ring 34 which act as anti-extrusion members to help
prevent transfer of the softer flexible graphite
material on the valve stem 16. Since the carbon
bushings 34 are harder than the flexible graphite rings
32, bushings 34 tend to wipe the sliding stem valve 16
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during operation thereof so as to aid in the prevention
of transfer of flexible graphite material to the stem.
The bushings 34 also aid in maintaining the valve stem
16 centered in the packing box to prevent~deformation
and destruction of the softer packing~anaterial in
packing ring set 24. A packing box~~nd ring 36 takes
up the rest of the space within the packing box.
Three polymeric rings having low friction
characteristics, in the form of thin PTFE disks or
washers 38, each about 0.015 inch (0.381 mm) are
provided so that a respective PTFE disk 38 is inserted
between each of the flexible graphite rings 32 and
between the flexible graphite ring 32 and the carbon
bushings 34. The PTFE disk material is intended to
deform and partially extrude inward against the valve
stem 16 so that the PTFE material will tend to lubri-
cate the sliding of the stem through the packing ring
set 24. Thus, the PTFE disks or washers 38 serve as a
lubricant in the packing ring set 24 as well as a seal-
ing member to aid the graphite packing rings 34 in
their sealing function.
With reference to Figure 2, there is also
illustrated the components of the second or upper
packing ring set 28. Packing set 28 includes three
V-rings including a top female adapter ring 40, a
middle V-ring 42 and a lower male adapter V-ring 44. A
respective carbon bushing 46 similar to carbon bushing
34 is provided at each end of the packing ring set 28.
It is preferred that the V-type packing rings
40, 42, 44 are formed of PTFE material (polytetrafluor-
ethylene - a synthetic resin polymer) and therefore
these rings are known in the trade as a "V-type PTFE
packing". It is preferred that the V-rings 40 and 44
are formed of a carbon filled PTFE material and that
the middle V-ring 42 is formed of a virgin or unfilled
PTFE material.
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A live load packing system is provided so
that the two-part packing assembly consisting of the
lower packing ring set 24 and the upper packing ring
set 28 are maintained at a stress level that is high
enough to readily cause the PTFE material in the
washers 38 and in the rings 40, 42, 44 to deform and
fill voids in the valve stem 16 thereby providing low
friction and low emission characteristics. Because the
upper packing ring set 28 is located closer to the live
load system, the entire loading or stress is applied to
this packing ring set to create a low emission seal.
Because the upper packing ring set 28 has substantially
lower friction and is located closer to the live load
system than the lower packing ring set 24, it can
transfer substantially most of the loading or stress to
the lower packing ring set 24 which is desirable as
this set is required to provide a reliable seal of the
valve stem during a fire test. Also, while the stress
is sufficient to cause deformation of PTFE material
from the washers 38, the friction on packing ring set
24 is significantly less than if the packing set 24
contained only flexible graphite material and did not
contain any of the PTFE washers 38. Thus, desirable low
friction characteristics are attained while still being
able to use the normally higher friction
characteristics of graphite material and thus enabling
the desirable fire safety characteristics of the
graphite material to be utilized in the packing system.
The live load packing includes a packing
follower 48 having a follower base 50 at one end, a
follower guide sleeve 52 at the other end, and a
follower flange 54 there between. Follower 48 includes
a liner 56 formed of carbon filled PTFE or other
suitable material which may be bonded to the inside
surface of the packing follower as disclosed in U.S.
Patent No. 5,129,625, assigned to the same assignee as
WO 95123935 PCT/US95/01617
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herein. A series of disk springs 58.is slidably
mounted on the follower guide sleevev52 with one end of
the disk springs in contact with the flange 54 and the
other end of the disk springs in contact with a packing
flange 60. The packing flange 60 has suitable
apertures 62 through which the packing studs 20 may be
passed and includes a central aperture to allow passage
of the valve stem 16.
A pair of lantern rings 64 are placed around
the valve stem and between the upper packing ring set
28 and the lower packing ring set 24 to take up the
remaining space within the packing bore. As in the
conventional use of lantern rings and valves, the
lantern rings 64 are further adapted in packing bore 22
to enable communication from the valve exterior via an
access hole 66 and a connecting channel 68. This
permits lubricating fluid to be inserted into the
packing bore and adjacent the valve stem 16. A
threaded cap 70 is threadably mounted into the hole 66
and can be removed for access thereto. The lantern
rings 64 also provide sufficient separation of the
packing set 24 from the packing set 28 to prevent
graphite material that has adhered to the stem from
degrading the sealing performance of packing set 28.
After assembly of the valve components as
shown in the drawings, the packing nuts 18 on studs 20
surrounding the valve stem 16 are tightened so that
packing flange 60 transmits the packing stud and nut
load to the disk springs 58. The springs 58 in turn
become compressed with continued tightening of nuts 18
so as to maintain a spring load on the packing ring
sets 24, 28 through the packing follower 48.
The use of the combination of the low
friction characteristics of the upper packing ring set
28 under a maintained high loading stress provides a
reliable seal with desired PTFE material. Also, the
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use of flexible graphite rings 32 in the lower packing
ring set 24 at a valve location away from the load
source enables the beneficial use of graphite material
for fire safety conditions to be obtained.
Furthermore, the PTFE washers 38 react to the
sufficient live load stress and are thin enough to
enable deformation of PTFE material to aid in
lubricating the valve stem during movement through the
packing ring set 24 and reduce adhesion of the graphite
material to the operating member. Accordingly, the
two-part packing assembly of the present invention
incorporates packing ring set 24 for fire safety
protection at elevated temperatures in the packing
area of over 1000°F (538°C) while the upper packing
ring set 28 can be loaded significantly to provide
reliable low fugitive emission sealing and low friction
operation of the operating member at operating
temperatures of up to 450°F (232°C).
As an alternative embodiment, other graphite
based materials, such as composite rings or braided
graphite rings could be used in place of the flexible
graphite rings 32 to give different levels of friction
and sealing capability. Also, other non-graphite
materials, such as other elastomeric materials with low
friction characteristics, fluoroelastomers or
perfluoroelastomers, or other fluorine containing
polymers may be used to provide a low friction, low
fugitive emission seal such as provided by the PTFE
material in the present illustration.
The foregoing detailed description has been
given for clearness of understanding only, and no
unnecessary limitations should be understood therefrom,
as modifications will be obvious to those skilled in
the art.
