AU1281400A - Sealing system - Google Patents

Description

WO 00/29767 1 PCT/GB99/03739 SEALING SYSTEM The present invention relates in general to the field of fluid sealing and more particularly to a sealing system 5 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 10 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 15 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. 20 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 WO 00/29767 PCT/GB99/03739 2 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 5 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 10 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) 15 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 20 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 25 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. 30 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 WO00/29767 PCT/GB99/03739 3 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 5 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 10 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. 15 Suitably, the secondary material layer is applied to at least one surface of the tape material. In a preferred embodiment the tape material has a substantially rectangular cross section having relatively wide front and 20 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, 25 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. 30 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.

WO00/29767 PCT/GB99/03739 4 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. 5 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 10 adjacent a longitudinal edge of the tape material. Preferably, the tape material comprises graphite, preferably flexible graphite. 15 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 20 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 25 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 30 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 WO00/29767 PCT/GB99/03739 5 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 10 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 15 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 20 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. 25 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 30 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.

WO00/29767 PCT/GB99/03739 6 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 5 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. 10 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. 15 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: 20 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 25 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 30 Figure 4 is a top view of the graphite tape of Figure 3.

WO00/29767 PCT/GB99/03739 7 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 5 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 10 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. 15 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 20 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 25 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 WO98/14724 (Flexitallic Limited) the content of which is incorporated herein by reference.

WO00/29767 PCT/GB99/03739 8 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 1. Therefore, significant force is required 5 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 10 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. 15 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 20 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 25 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 30 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 WO00/29767 PCT/GB99/03739 9 provided on the outwardly facing surface of the lower packing ring 11. 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 5 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 10 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 15 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 20 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. 25 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 30 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 111a of WO00/29767 PCT/GB99/03739 10 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 10 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 15 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 20 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. 25 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. 30 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.

WO00/29767 PCT/GB99/03739 11 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 5 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. 10 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 15 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 20 to moulding in an annular dye to the desired final cross section. Referring to Figure 3, a section of the graphite tape 20 is shown in side view. The graphite tape 20 is suitably 25 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 30 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 WO00/29767 PCT/GB99/03739 12 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 5 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 10 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 15 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 20 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 25 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 30 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 WO00/29767 PCT/GB99/03739 13 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 5 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 10 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. 15 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 20 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 25 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 30 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 WO00/29767 PCT/GB99/03739 14 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 5 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 10 strip 30a, 30b is determined to be nd 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 15 (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. 20 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 25 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 30 and lead to failure of the seal. Advantageously the PTFE layer isolates the graphite tape substrate from the sealed fluid. Where such fluid WO00/29767 PCT/GB99/03739 15 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 10 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. 20 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 25 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 30 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 WO00/29767 PCT/GB99/03739 16 novel combination, of the steps of any method or process so disclosed.

Claims (12)

1. A method for forming a packing ring for use in a gland seal, comprising the steps of: 5 applying a layer of secondary material (30) to a predetermined surface area of a tape material (20); wrapping the tape material (20) to form a ring; and 10 compressing the ring in a mould to form a packing ring (11) of a desired configuration, such that the secondary material is presented on an externally facing surface (111) of the packing ring (11). 15

2. A method as claimed in claim 1, wherein the tape material (20) comprises graphite.

3. A method as claimed in claim 1 or 2, wherein the 20 secondary material (30) is a friction reducing material.

4. A method as claimed in any of claims 1 to 3, wherein the secondary material (30) is a seal enhancing material. 25

5. A method as claimed in any of claims 1 to 4, wherein the secondary material (30) is selected from amongst a group of materials including PTFE and PEEK. 30

6. A method as claimed in any of claims 1 to 5, wherein the secondary material (30) is applied to a front surface (21) and/or a rear surface (22) of the tape material (20) having a generally rectangular cross section. WO00/29767 PCT/GB99/03739 18

7. A method as claimed in claim 6, wherein the secondary material (30) is applied over part of the width of the front surface (21) and/or the rear surface (22) 5 along a predetermined length thereof.

8. A method as claimed in claim 7, wherein the secondary material (30) is applied to form at least one strip (30a,30b) arranged longitudinally along the front 10 surface (21) and/or along the rear surface (22) of the tape material.

9. A method as claimed in claim 8, wherein the or each strip (30a,30b) of secondary material is arranged to lie 15 substantially parallel to at least one longitudinal edge of the tape material (20).

10. A method as claimed in any of claims 1 to 9, wherein the secondary material (30) is adhered and/or 20 mechanically attached to the tape material.

11. A packing ring formed by the method of any of claims 1 to 10. 25

12. A sealing system comprising a packing ring formed by the method of any of claims 1 to 10.