Note: Descriptions are shown in the official language in which they were submitted.
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SBALINC3 SYSTB~I
The present invention relates in general to the field
of fluid sealing and more particularly to a sealing system
for use with rotating and reciprocating shafts. The
present invention also relates to a method for forming a
packing ring for use in such a sealing system.
In the field of fluid control valves it is desired to
1o form an effective seal around, for example, a valve
actuator shaft which may need to be moved in a rotating
and/or reciprocing manner. In certain fields of use such
as in the petrochemical industry it is desired to eliminate
leakage in order to avoid cost wastage and also to avoid
is undesirable environmental impact.
It is widely known to use a gland seal, which typically
comprises a sealing chamber having a plurality of packing
rings arranged annularly around the shaft to be sealed.
2o The packing rings are subject to pressure longitudinally
and in response expand laterally to seal against the shaft.
However, a problem arises in that friction between the
packing rings and the shaft significantly inhibits movement
of the shaft. A manual fluid control valve therefore
25 requires significant operator effort, commonly involving
the use of a long control lever. Motorised control valves
suffer significant control problems such as stem judder
which inhibits efficient regulation, and require large
control motors in order to overcome this friction and
30 accurately control movement of the shaft.
Most commonly, prior art packing rings comprise
graphite which is desirably resilient but disadvantageously
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has a relatively high friction co-efficient. It is known
to use materials such as PTFE (polytetrafluoroethylene) as
a packing material in such a gland sealing system. PTFE
has a low friction coefficient and is impermeable to most
fluid compounds and is therefore a desirable sealing
material. However, PTFE is not heat tolerant and deforms
easily, and therefore is not readily suitable for use in a
gland seal. In particular, when exposed to heat and/or
pressure the PTFE material creeps and flows. Further, PTFE
to suffers potential breakdown and vaporisation under high
temperature conditions and is therefore desirably avoided
in situations where fire safety is important.
US 5299812 (Fisher Controls International Inc)
discloses a fluid control valve sealing assembly having
graphite packing rings separated by thin PTFE disks. In
use the PTFE is extruded by longitudinal pressure applied
' to the graphite packing rings to thereby flow into the
interface region between the graphite packing rings and the
2o shaft. However this prior art arrangement uses relatively
large quantities of PTFE which increases manufacturing
costs and reduces heat tolerance.
It is an aim of the present invention to provide a
simple yet effective shaft sealing system, and to provide a
method for manufacturing a packing ring for use in such a
system. Preferred embodiments of the present invention aim
to address the problems of the prior art discussed above.
According to a first aspect of the present invention
there is provided a method for forming a packing ring for
use in a gland seal, comprising the steps of: applying a
layer of secondary material to a predetermined surface area
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of a tape material; wrapping the tape material to form a
_ ring; and compressing the ring in a mould to form a packing
ring of a desired configuration, such that the secondary
material is presented on an externally facing surface of
the packing ring.
Preferably the applying step is performed prior to the
wrapping step. Said secondary material may be applied to
any suitable surface of the tape material. Conveniently
1o the secondary material is applied such that after the
wrapping and compressing steps the secondary material is
presented on an externally facing surface of the
manufactured packing ring. Suitably the external surface
includes a radially inner and/or a radially outer surface.
Suitably, the secondary material layer is applied to at
least one surface of the tape material. In a preferred
embodiment tie tape material has a substantially
rectangular cross section having relatively wide front and
2o rear surfaces and relatively thin side surfaces.
Preferably, the secondary material is applied to the front
surface and/or to the rear surface. Said secondary
material may be provided across substantially the entire
width of the front surface and/or the rear surface,
preferably along a predetermined length thereof.
Alternatively, the secondary material is applied over part
of the width of the front surface and/or the rear surface
along a predetermined length thereof.
3o Preferably, the secondary material comprises a strip.
Preferably, the secondary material is applied to form a
plurality of strips arranged longitudinally along the front
surface and/or along the rear surface of the tape material.
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Preferably, the strips are arranged to lie substantially
parallel to one another. Preferably, the or each the strip
is arranged to lie substantially parallel to at least one
longitudinal edge of the tape material.
In a preferred embodiment the secondary material is
applied to form two substantially parallel strips each
extending part way across the width of one surface of the
tape material. Preferably, each of the strips is arranged
1o adjacent a longitudinal edge of the tape material.
Preferably, the tape material comprises graphite,
preferably flexible graphite.
Preferably, the secondary material is a friction
reducing material. Any suitable material may be employed.
conveniently, the friction reducing material has a lower
co-efficient of friction than the tape material (e. g.
graphite) with respect to moving parts such as an adjacent
2o packing ring and/or the shaft to be sealed. Preferably,
the secondary material is a seal enhancing material. The
seal enhancing material is conveniently readily deformable
against an adjacent surface. Suitably the seal enhancing
material enhances the seal between the packing ring or
rings and the shaft to be sealed, and/or between the
packing ring or rings and the interior surface of a sealing
chamber of a shaft sealing system.
Ideally, the secondary material is selected to have
3o both seal enhancing and friction reducing properties.
Suitably, the secondary material comprises a polymeric
material. In the preferred embodiment the secondary
material is PTFE or an alternative such as PEEK. The PTFE
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is suitably extruded, skived, calendered, stretched,
oriented, or expanded. Most suitably, the secondary
material comprises electric grade PTFE.
5 The secondary material may be applied to the surface of
the tape material in any suitable manner. Preferred
embodiments include mechanical attachment and/or chemical
adhesion. To improve application the surface of the tape
material may be etched or profiled. Preferably, mechanical
1o attachment is achieved by stitching and/or stapling the
secondary material to the tape material. In one preferred
embodiment the secondary material is stitched to the tape
material using a lock stitch. Preferably a joining element
penetrates both the tape material and the secondary
material to provide mechanical attachment. The joining
element may comprise PTFE thread or PEEK thread or
equivalent. The joining element may comprise any suitable
fine textile thread or web. The joining element may be
relatively flexible to perform mechanical attachment by a
2o stitching method, or may be relatively rigid to perform
mechanical attachment by a stapling method. In each case
it is desirable that the joining element is arranged to
maintain the preferred good sealing and/or friction
reducing properties of the secondary layer.
Preferably, the secondary material is adhered to the
tape material. Any suitable bonding agent or adhesive may
be employed. The secondary material is suitably applied in
the form of a tape or film. Preferably the method includes
3o the step of removing a release paper from a surface of the
secondary material tape to expose a layer of adhesive,
prior to application of the secondary material to the
graphite tape material.
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In a preferred embodiment the secondary material is
applied to the graphite tape material to form a layer of
the order of 0.25mm thick. However, any suitable thickness
may be used.
According to a second aspect of the present invention
there is provided a packing ring formed by the method as
set forth in any statement herein.
to
According to a third aspect of the present invention
there is provided a sealing system comprising a packing
ring formed by a method as set forth in any statement
herein.
For a better understanding of the invention, and to
show how embodiments of the same may be carried into
effect, reference will now be made, by way of example, to
the accompanying diagrammatic drawings, in which:
Figure 1 is a cross-sectional side view showing a shaft
and a sealing arrangement;
Figure 2 is a cross-sectional side view showing an
alternate embodiment of the sealing arrangement of Figure
1;
Figure 3 is a side view showing a graphite tape used in
the manufacture of a packing ring; and
Figure 4 is a top view of the graphite tape of Figure
3.
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Referring to Figure 1, a shaft 1 is arranged to move in
a rotating and/or reciprocating manner. As a practical
example, the shaft 1 may be an actuator shaft of a fluid
control valve, the shaft having a first end coupled to a
fluid flow control arrangement and a second end coupled to
an operating arrangement which is commonly exposed to
atmosphere. It is desired to seal the shaft 1 to inhibit
the escape of fluid therealong. For applications such as
the petrochemical industry it is desired to minimise
1o fugitive emissions of liquid and in particular gases along
the shaft 1. Leakage results in costly wastage, has
implications for fire and operator safety, and may have an
undesired environmental impact.
In order to achieve the desired seal, a sealing
arrangement is provided comprising a chamber 3 around the
shaft 1. The chamber 3 is arranged to receive one or more
packing rings 11, 12 & 13 to lie annularly around the shaft
1. Longitudinal pressure applied in the direction of arrows
2o A forces the packing ring or. rings to expand laterally and
seal against the shaft 1 and the interior surface of the
chamber 3. It is particularly desired to provide an
effective seal at the interface between the packing ring or
rings and the shaft 1, and between the packing ring or
rings and the interior surface of the chamber 3. As shown
in Figure 1, the preferred embodiment of the present
invention uses three packing rings 11, 12 & 13 each of
substantially triangular cross-section. This preferred
packing ring arrangement is described in detail in
30~ W098/14724 (Flexitallic Limited) the content of which is
incorporated herein by reference.
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Commonly, the packing rings comprise graphite and it
has been found that a relatively high friction coefficient
exists at the interface between the central packing ring 12
and the shaft I. Therefore, significant force is required
to overcome this friction and move the shaft 1, which tends
to limit the sealing force that may be applied in the
direction of arrows A. That is, the effectiveness of this
sealing arrangement is limited by the need to move the
shaft 1 with a force that may be provided by a human
to operator through a suitable operating arrangement such as a
lever, or by an electric motor. In particular, the
relatively high friction coefficient inhibits the accurate
movement of the shaft and therefore inhibits accurate
control of the valve.
The arrangement shown in Figure 1 includes a PTFE
lubricating and/or seal enhancing layer 121 on the surface
of the central packing ring 12 that faces the shaft 1.
Advantageously the PTFE lubricating layer 121 is arranged
at the interface between the packing ring 12 and shaft 1.
The PTFE layer 121 has a low friction coefficient with
respect to the shaft 1 and therefore movement of the shaft
1 is relatively easy. The PTFE layer readily conforms to
the surface profile of the shaft 1 to form an effective
seal thereagainst. Further, the PTFE layer 121 is
impermeable to a broad range of fluids and therefore
inhibits migration along the shaft 1.
Additionally or alternatively a PTFE layer 131 is
provided on the outwardly facing surface of the packing
ring. In the example of Figure 1 it is convenient to
provide this layer 131 on the outwardly facing surface of
the upper packing ring 13. A similar PTFE layer 111 may be
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provided on the outwardly facing surface of the lower
packing ring il. These layers 111 and/or 131 inhibit
migration of fluid along the interface between the packing
rings 11, 13 and the interior of the sealing chamber 3. In
the embodiment shown in Figure 1, the upper and lower
packing rings 11,13 are not intended to move with respect
to the interior surface of the chamber 3. Therefore, at
the interface between the packing rings and the chamber 3 a
seal enhancing layer is desired, as opposed to a friction
reducing layer. However, the PTFE layers 111,131 on the
upper and lower packing rings 11,13 suitably act as a seal
enhancing layer.
In a variation of this embodiment (not shown) a PTFE
layer may be provided on any convenient surface of any of
the packing rings, including a surface intended to face
toward an adjacent packing ring.
In the first embodiment shown in Figure 1, the PTFE
layer such as layer 121 extends across substantially the
entire width of the relevant surface of the packing ring
12. Referring now to Figure 2 a second example embodiment
is shown wherein the PTFE layer extends only part way
across the relevant surface of the packing ring.
As shown in Figure 2, in this embodiment the upper
packing ring 13 is provided with a PTFE layer 131a. In
this example embodiment the PTFE layer 131a is arranged to
lie adjacent the downward edge of the outwardly facing
3o surface of the packing ring 13 and to extend across
approximately one third of the width of that surface . The
lower packing ring 11 is provided with a PTFE layer llla of
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similar configuration to the PTFE layer 131a of the upper
packing ring 13.
Referring again to Figure 2, the central packing ring
5 12 in this example is provided with two PTFE layers 121a
and 121b. It has been found most effective to position
these PTFE layers 121a and 121b at the outer edges of 'the
inwardly facing surface of the packing ring 12. The upper
PTFE layer 121a is conveniently located adjacent to the
1o uppermost edge of the packing ring and extends across the
inwardly facing surface thereof by a width of the order of
one fifth to one third of the total width. The second PTFE
layer 121b is conveniently symmetrically located adjacent
the lower most edge of the inwardly facing surface of the
packing ring 12 and extends across the surface thereof for
a corresponding width.
Advantageously, the use of a partial layer as shown in
Figure 2 maintains the benefits associated with the PTFE
layer, namely reducing friction and improving sealing,
whilst minimising the amount of PTFE present in the sealing
arrangement in order to minimise creep and improve overall
heat resistance and fire safety.
Although the embodiment of Figure 2 shows the use of
one or two partial PTFE layers, any suitable number of
layers may be used located at any suitable position on the
surface of the packing ring.
3o The total surface area of the PTFE layer or layers is
conveniently selected with reference to the total exposed
surface area of the graphite material of the packing ring
to produce a desired combined friction coefficient.
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Ideally, the surface area of the PTFE is appropriate to
allow movement of the shaft 1 in response to a
predetermined force. Where the shaft is the actuator shaft
of a fluid control valve, periods of movement may be
separated by relatively long periods of inactivity and it
is desired that the shaft 1 should move freely and
predictably at all times. In particular, it is desired
that the shaft should be moveable by even a relatively
small distance with precision, to give accurate control.
A method for forming a packing ring will now be
described with reference to Figures 3 and 4. This method
is suitable for forming the packing rings 11, 12 & 13 shown
in Figures 1 and 2 but the packing rings formed by the
method described below are not restricted to those
configured as shown in Figures 1 and 2.
In the preferred method, a packing ring is formed by
winding a graphite tape 20 onto a cylindrical former prior
2o to moulding in an annular dye to the desired final cross-
section.
Referring to Figure 3, a section of the graphite tape
is shown in side view. The graphite tape 20 is suitably
of rectangular cross-section although tape of any suitable
cross-section may be used. In the preferred method
described herein a layer of PTFE or other suitable
equivalent material is applied to a surface 21 of the
graphite tape 20, suitably at or near one end thereof. The
3o PTFE layer 30 is positioned so that, when the tape 20 is
wound around a former and then compressed in a mould, the
PTFE layer 30 forms an exterior portion of the finished
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packing ring, i.e. is exposed on an exteriorly facing
surface of the packing ring.
Conveniently, the PTFE layer 30 is applied in the form
of a PTFE film or a PTFE tape . Any suitable PTFE material
may be used to form the PTFE layer 30. This includes
axially stretched PTFE tape, biaxially orientated PTFE
tape, expanded PTFE, and most suitably electrical grade
PTFE tape. In one preferred embodiment the PTFE layer is
to adhered to the surface 21 of the graphite tape 20 using a
suitable adhesive layer 31 therebetween. Conveniently, the
adhesive 21 is applied to the PTFE tape or film and is
exposed, for example, by pealing back a release layer, just
prior to application onto the graphite tape 20. This
1s allows for a relatively simple and convenient manufacturing
process of the packing ring and requires minimal adaptation
of existing equipment. The PTFE layer can be applied
accurately at any desired position on the surface 21 of the
graphite tape 20 and more than one PTFE layer 30 may be
2o provided at suitable locations thereon.
In an alternate embodiment (not shown) a mechanical
attachment may be used to apply the PTFE layer 30 to the
graphite tape 20. The surface of the graphite tape 20
2s intended to receive the PTFE layer 30 may be keyed, for
example by being etched or profiled to improve chemical
adhesion and/or mechanical attachment. In one embodiment
the PTFE layer is configured to engage a corresponding
surface profile of the graphite tape 20 to achieve
3o mechanical attachment. However, preferably a joining
element is used to join the PTFE layer 30 and the graphite
tape 20. In a preferred arrangement the PTFE layer 30 is
stitched to the graphite tape 20, such as using a thread
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suitably PTFE or PEEK, or any other suitable fine textile
thread or web. In another preferred embodiment the PTFE
layer 30 is stapled to the graphite tape 20 using a more
rigid joining element, again ideally PTFE or PEEK or
similar.
In the example of Figure 3, where it is desired to
expose the PTFE on a radially inner surface of the packing
ring, the graphite tape 20 is wound around a former
to starting with the PTFE layer 30 adjacent the former.
Similarly, where it is desired to expose the PTFE on a
radially outer surface the graphite tape 20 is wound onto
the former leaving the section having the PTFE layer 30
until last and outwardly presented.
In order to form a PTFE layer 30 on both an inwardly
facing surface and an outwardly facing surface of the
finished packing ring, the PTFE layer 30 may be applied to
both a front surface 21 and to a rear surface 22 of the
2o graphite tape 20.
Although in Figure 3 the PTFE layer 30 is shown raised
above the surface of the graphite tape 20, the PTFE layer
is embedded in to the graphite tape 20 during the moulding
step. The resultant packing ring therefore has a smooth,
contiguous surface without an abrupt stepped transition
from PTFE to exposed graphite.
Referring now to Figure 4 an example embodiment is
shown in plan view having two PTFE tape strips 30a and 30b
applied to an upper surface 21 of the graphite tape 20.
The position of the PTFE strips 30a and 30b is suitable for
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forming a packing ring such as the central packing ring 12
of Figure 2.
As shown in Figure 4, the length "L" of each strip 30a
and 30b is suitably determined according to the desired
position of the PTFE layer on the finished packing ring.
In this example it is desired to provide a complete ring
annularly around the shaft 1. Assuming the shaft has a
diameter d ( see Figure 2 ) the length L of the or each PTFE
1o strip 30a, 30b is determined to be rzd or slightly greater.
Similarly, to form a PTFE layer such as layer 131a of the
upper packing ring 13 shown in Figure 2, a 'strip 30 is
provided having a length equal to or slightly greater than
nD, where D is the internal diameter of the chamber 3
(again see Figure 2).
It will be appreciated that the manufacturing method
described herein minimises the quantity of PTFE required by
concentrating this material where it is most needed, i.e.
2o at the interface between the packing ring and other
components such as the shaft and the interior of the
chamber. The majority of the volume of the finished
.packing ring comprises graphite and therefore maintains
good load carrying abilities. The PTFE layer bears minimal
load and this minimises creep and flow of the PTFE making
the packing rings and the sealing system suitable for use
even at high temperatures. Minimising the use of PTFE
also minimises fire risks and improves longevity, where
otherwise the creep and flow of the PTFE would be excessive
3o and lead to failure of the seal.
Advantageously the PTFE layer isolates the graphite
tape substrate from the sealed fluid. Where such fluid
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includes aggressive chemical such as volatile organic
compounds as are commonly present in the petrochemical
field, the PTFE layer conveniently minimises exposure of
the graphite to such compounds.
5
The reader s attention is directed to all papers and
documents which are filed concurrently with or previous to
this specification in connection with this application and
which are open to public inspection with this
to specification, and the contents of all such papers and
documents are incorporated herein by reference.
All of the features disclosed in this specification
(including any accompanying claims, abstract and drawings),
15 and/or all of the steps of any method or process so
disclosed, may be combined in any combination, except
combinations where at least some of such features and/or
steps are mutually exclusive.
2o Each feature disclosed in this specification (including
any accompanying claims, abstract and drawings), may be
replaced by alternative features serving the same,
equivalent or similar purpose, unless expressly stated
otherwise. Thus, unless expressly stated otherwise, each
feature disclosed is one example only of a generic series
of equivalent or similar features.
The invention is not restricted to the details of the
foregoing embodiment(s). The invention extends to any
novel one, or any novel combination, of the features
disclosed in this specification (including any accompanying
claims, abstract and drawings), or to any novel one, or any
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novel combination, of the steps of any method or process so
disclosed.
