FIELD OF THE INVENTION

[0001] This invention relates to a seal for sealing a shaft. In the following explanation the invention will by way of example be primarily explained by referring to a valve. However, in practice the invention may be utilized also in other devices, such as pumps.

DESCRIPTION OF PRIOR ART

[0002] The requirements for a seal vary highly depending on the intended use. In pipelines containing flammable or toxic material, the requirements are high. In order to meet the requirements set by standards a gland seal of a valve, for instance, must be sufficiently tight to allow only a minor Helium emission leakage during test within a specific time period. Additionally, the seal must be able to prevent leakage also at very high temperatures and to minimize damage in case of a fire. Finally, the need of maintenance should be minimal and, thus, the seal should be able to handle a substantial number of work cycles before it needs to be renewed.

[0003] The above-mentioned requirements are, in practice, very demanding. One problem is that the high temperature requirements necessitate the use of materials that are, during use, unable to achieve the required tightness, for instance.

[0004] Seals previously known have not been able to meet the requirements in a satisfactory way. Therefore, there exists a need for a new seal with improved properties.

SUMMARY OF THE INVENTION

[0005] An object of the present invention is to solve the above-mentioned drawback and to provide a seal with improved properties. This object is achieved with a seal according to independent claim 1 .

[0006] The use of a seal with a seal structure comprising fire retardant particles makes it possible to obtain a seal where the fire retardant particles ensure sufficient properties at high temperatures. When the seal is additionally provided with a diffusion barrier of a heat resistant fluid tight material such as metal a seal that ensures minimal leakage during the use of the seal can be obtained.

[0007] Preferred embodiments of the invention are disclosed in the dependent claims. BRIEF DESCRIPTION OF DRAWINGS

[0008] In the following the present invention will be described in closer detail by way of example and with reference to the attached drawings, in which

[0009] Figure 1 illustrates a valve with a seal,

[0010] Figures 2 and 3 illustrate a first embodiment of a seal,

[0011] Figures 4 and 5 illustrate a second embodiment of a seal,

[0012] Figure 6 illustrates a third embodiment of a seal,

[0013] Figures 7a to 7f illustrate a method for manufacturing a seal,

[0014] Figures 8 and 9 illustrate a fourth embodiment of a seal,

[0015] Figure 10 illustrates a fifth embodiment of a seal, and

[0016] Figures 1 1 to 14 illustrate a sixth embodiment of a seal.

DESCRIPTION OF AT LEAST ONE EMBODIMENT

[0017] Figure 1 illustrates a valve 1 with a shaft 2 for adjusting the position of a closing member 3 in the valve. In the illustrated example, the position of the closing member 3 can be changed by rotating the shaft 2 or by linear motion of the shaft 2 and the closing member 3 such that the flow path for fluid through the valve 1 is opened, closed and possibly adjusted to a suitable level (in case of a control valve).

[0018] In order to prevent fluid from leaking out of the valve along the shaft 2, a gland seal in the illustrated stuffing box includes three seals 4 shaped as rings (in the illustrated example) that are arranged to surround the shaft 2 and to tighten the annular gap between the shaft 2 and the surrounding parts (such as the body) of the valve 1 .

[0019] In Figure 1 a partial enlargement of the valve 1 and the shaft 2 is illustrated in circle 5. In this enlargement, the most usual ways for fluid to leak in prior art solutions is illustrated by arrows 6, 7 and 8. Usually, leakage occurs at the interface between the seals 4 and the surrounding part of the valve 1 , as illustrated by arrow 6, as diffusion through the seals 4 as illustrated by arrow 7, and at the interface between the seals 4, and the shaft 2, as illustrated by arrow 8. [0020] One way to minimize the leakage at arrows 6 and 8 is to add force that presses one or more compressible seals into the annular space between the valve 1 and the shaft. In Figure 1 , the gland follower 20 of the valve 1 is tightened by bolts or threads, for instance, downwards in Figure 1 with a force F. Due to this force, the shape of the seals 4 is changed as the seals are compressed in the vertical direction and attempt to expand in the horizontal direction towards the surfaces of the valve 1 and the shaft 2.

[0021] Figures 2 and 3 illustrate a first embodiment of a seal. The seal 4 of Figures 2 and 3 may be utilized in the embodiment of Figure 1 , for instance. In Figure 2 a part of a seal 4 shaped into a half ring is illustrated, and in Figure 3 a part of the seal 4 is illustrated (in more detail than in Figure 2) before shaping it into a ring.

[0022] The seal 4 comprises seal structure including a braid 9 with a plurality of threads or filaments which intermesh with each other such that the filaments of the braid 9 are arranged along outer surfaces of the braid 9 to delimit an empty space in the center part of the braid. In Figures 2 and 3 the seal 4 is illustrated in a situation where this empty space in the center part of the braid is filled with fire retardant particles 10 along the entire length of the seal 4. The fiber retardant particles may be of a suitable mineral, for instance, which has the required heat resistance and which can be utilized in a compressible seal. Suitable materials include glass fiber, ceramic fibers, graphite and vermiculite, for instance. The fire retardant particles may be provided in various forms including flakes, powder, granulate, balls or bands. For simplicity, all of these forms are referred to as 'particles' unless otherwise indicated.

[0023] The fibers included in the braid 9 may be selected from suitable materials that resist the required high temperatures, such as aramid fibers or carbon fibers or metal alloys (such as Inconel, which is a trademark of Special Metals Corporation), for instance. In case of a fire, these fibers may be destroyed, however, leaving the fire retardant particles 10 alone. In that case some leakage may occur, but it is, however, minimized by the fact that most of the cross-sectional area of the seal 4 (the fire retardant particles) will remain intact while only material of a minor part of the cross-sectional area of the seal 4 is potentially destroyed (the braid 9). Reasonable tightness is therefore maintained. [0024] In the figures and in the above explanations, it is by way of example, assumed that the fire retardant particles are located inside the braid. However, in other embodiments it is also possible to attach fire retardant particles to the outer surface of the braid. Accordingly, it is assumed that diffusion barrier and braid coating material are metal, but they can be other materials, such as ceramics or composites, depending on ambient environment and target application.

[0025] In order to prevent fluid, such as gas, from passing through the seal 4, as illustrated by arrow 7 in Figure 1 , a diffusion barrier 1 1 of a metallic fluid tight material has been grown on a first surface 12 extending between a shaft contact surface 13 and an opposite contact surface 14. In this connection, a shaft contact surface 13 refers to a surface that is intended to contact a surface of a shaft 2 (or axle), and an opposite contact surface 14 refers to a surface that is intended to contact a surface of a valve 1 (or a pump) once the seal 4 is shaped into a ring and arranged in an annular space between a valve (or pump) and a shaft (or an axle), as illustrated in Figure 1 . Growing of a diffusion barrier 1 1 on the first surface 12 refers to a process where particles of the barrier material are, in a suitable process, transferred to the surface of the braid such that a very tight interface is obtained between the diffusion barrier 1 1 and the braid 9, where the surface of the diffusion barrier exactly matches the shape of the braid that it contacts. In practice, if the surface of the braid is not smooth (which is usually the case), the grown diffusion barrier layer tightly follows the surface irregularities of the braid. Such growing may, in practice, be implemented electrochemically, by a PVD (Physical Vapour Deposition) process or by a CVD (Chemical Vapour Deposition) process, for instance. An advantage obtained with a diffusion barrier that has been grown on a braid as compared to a separate metal plate use as a diffusion barrier, is that the surface of the diffusion barrier exactly matches the shape of the braid that it contacts. Leakage between the diffusion barrier and the braid can thereby be minimized. The grown diffusion barrier 1 1 layer prevents fluid from passing (by diffusion, for instance) through the seal at the location illustrated by arrow 7 in Figure 1 . It may be impossible to totally stop such a leakage, however, the diffusion barrier efficiently minimizes the leakage to the extent possible. The metallic material used in the diffusion barrier may include silver, nickel, gold, platinum or steel alloys including a lot of nickel or chrome, for instance. The diffusion barrier 1 1 may also consist of a plurality of layers arranged on top of each other such that the different layers are made of different materials, such as a layer of nickel closest to the braid and a layer of silver on top of the nickel layer. Preferably the material of the diffusion barrier 1 1 layer is softer than the material of the shaft 2 in order to ensure that a possible contact between the barrier 1 1 and the shaft 2 does not scratch and damage the surface of the shaft.

[0026] As possible contact between the diffusion barrier 1 1 and the shaft 2, or correspondingly between the diffusion barrier 1 1 and the valve 1 , may cause additional fluid leakages (along the shaft or valve, as illustrated by arrows 8 and 6 in Figure 1 ), it may be preferable to leave a small gap between the shaft 2 and the diffusion barrier 1 1 and between the valve 1 and the diffusion barrier 1 1 . Therefore, the entire first surface 12 is not necessarily covered by the diffusion barrier 1 1 , but a small non-covered (ring-shaped) area may be present along the shaft contact surface 13 and/or along the opposite contact surface 14. Therefore, the diffusion barrier 1 1 preferably covers 'substantially' the entire first surface 12.

[0027] A diffusion barrier layer is preferably present only on one surface of the seal, such as on the bottom surface of the seals 4 shaped into rings in Figure 1 . In this way, it can be ensured that contact between two metallic diffusion barrier layers does not cause leakage at the interface between two seals.

[0028] In order to protect the shaft 2 and to ensure that a minimum of torque is needed to turn the shaft 2, a lubricant may be provided on the shaft contact surface. Such a lubricant may include MoS (Molybdenum disulfide) or WS2 (Tungsten Disulfide), for instance.

[0029] Figures 4 and 5 illustrate a second embodiment of a seal. The embodiment of Figures 4 and 5 is very similar to the one described in connection with Figures 2 and 3. In the following the embodiment of Figures 4 and 5 will mainly be described by pointing out the differences between these embodiments.

[0030] In Figure 4 a part of a seal 4' shaped into a half-ring is illustrated, and in Figure 5 a part of the seal 4' is illustrated (in more detail than in Figure 4) before shaping it into a ring. Similarly to the previous embodiment, the seal 4' comprises a braid 9' retaining fire retardant particles 10 in the empty space in the center part of the braid 9'. [0031] In the embodiment of Figures 4 and 5, however, the braid 9' is metallized by a metallic coating on at least a part of the braid or on at least some of the plurality of filaments. The metallization may include Silver, Fe (Ferrite), Ni (Nickel) or Au (Gold), for instance. A first advantage with such metallization is that once the seal 4' is compressed by the forces F illustrated in Figure 1 , for instance, the metallic particles are pressed together to form a compact, almost continuous diffusion barrier throughout the entire seal. Such a diffusion barrier efficiently prevents leakage through the seal 4' along arrow 7 illustrated in Figure 1 . A second advantage is improved fire resistance, as the metallization in diffusion barrier 1 1 or braid 9 ^' slows down burning process in sealing materials by preventing external oxygen from getting inside sealing structure. Thus, loss of sealing material in high temperature can be minimized. In addition, the metallization may have significantly better heat resistance than other parts of the braid 9'. In practice the braid 9' may include carbon fibers, aramid fibers or metallic filaments in addition to the metallization.

[0032] In order to avoid that the metallized braid 9' of the seal damages the shaft 2 by scratching, for instance, the shaft contact surface is provided with a soft layer 15' preventing the metallized braid 9' from contacting the shaft 2. One alternative to implement such a soft layer 15' is to provide the seal with a second braid of non-metallized filaments. In that case, the filaments may be of aramid fibers or carbon fibers, for instance. Similarly, as in the embodiment of Figures 2 and 3, the surface contacting the shaft is preferably provided with a lubricant.

[0033] The non-metallized soft layer 15' is preferably made relatively thin as compared to the thickness of the entire seal 4' in order to ensure that if the material of the soft layer 15' is destroyed due to fire or high temperatures, only a small leakage will occur, as the main part of the seal 4' is still able to block most of the annular space between the shaft 2 and the valve 1 .

[0034] Figure 6 illustrates a third embodiment of a seal 4". In Figure 6 only the front surface of the seal is illustrated. The embodiment of Figure 6 is very similar to the one described in connection with Figures 4 and 5. Therefore the embodiment of Figure 6 will be explained mainly by pointing out the differences between these embodiments.

[0035] In Figure 6 the left part of the seal 4" with the metallized braid 9', the fire retardant particles 10 and the barrier layer 1 1 is implemented as explained in connection with Figures 4 and 5. However, the soft layer 15' on the shaft contact surface 13 is, in the embodiment of Figure 6, implemented as a hollow second braid 9" where the hollow center part of the second braid 9" is filled with fire retardant particles 10, similarly to the braid 9'. The second braid may be made of non-metallized filaments in order to avoid scratching a shaft 1 coming into contact with the shaft contact surface 13. In that case the filaments may be of aramid fibers or carbon fibers, for instance. The second braid 9" is preferably also lubricated. In the embodiment of Figure 6, the diffusion barrier 1 1 layer, is implemented such that it also extends across the second braid 9" between the opposite contact surface 14 and the shaft contact surface 13.

[0036] The second braid 9" may be attached to the braid 9' such that threads or filaments of the braid 9' and the second braid 9" intermesh in order to ensure that no gap allowing fluid leakage is present at the interface between the braid 9' and the second braid 9".

[0037] Figures 7a to 7f illustrate a method for manufacturing a seal. The illustrated method may be utilized to manufacture a seal according to the embodiments of Figures 1 to 5, for instance.

[0038] In Figure 7a a carbon fiber filament 16 (or alternatively a plurality of filaments) is metallized by an electrolytic process. This may be implemented by connecting the carbon fiber filament to a positive potential, and by feeding it into a bath 17 containing a solution with silver, which is connected to a negative potential. After such metallization the metallized carbon fiber filament 16 may be used to produce a braid 9', for instance.

[0039] In case the braid is produced of a material that is not electrically conductive, the braid or single filaments of the braid can be metallized by a suitable deposition process, such as by a PVD (Physical Vapour Deposition) process.

[0040] In Figure 7b the braid 9' of the seal 4' has been produced by braiding, twisting or stranding, for instance, and the hollow center part of the braid 9' is filled with fire retardant particles 10, such as with graphite particles.

[0041] In Figure 7c the seal 4' is cut into a suitable length and its ends are brought together to form a ring-shaped seal 4', which is placed in a mould 18. At this stage a special tool may be used to compress the seal 4' in order to ensure that it obtains the exact desired dimensions defined by the mould 18.

[0042] In Figure 7d it is, by way of example, assumed that the diffusion barrier 1 1 layer is produced by an electrolytic process, which is naturally one of a plurality of possible solutions for growing the diffusion barrier 1 1 layer. If so, the bath 17 may contain a solution with metal (e.g. silver) which is connected to a negative potential while the seal 4' in the mould 18 is connected to a positive potential. During the electrolytic process the seal 4' is kept in the solution such that only the part of the seal 4' where the diffusion barrier 1 1 should be grown is kept in contact with the solution. An alternative way to produce the diffusion barrier is a PVD (Physical Vapour Deposition) process, for instance.

[0043] Figures 7e and 7f illustrate grooves shaped in the diffusion barrier 1 1 layer. In Figure 7e the seal 4' is shown in the mould 18, and Figure 7f is a cross-sectional view of the seal 4'. The grooves 19 may be shaped in the diffusion barrier 1 1 layer by using a steel sleeve having a corresponding shape on the front surface, for instance. In that case, the sleeve may be placed on top of the diffusion barrier 1 1 layer and pressed with a high force against the diffusion barrier layer (by hitting with a hammer, for instance). Grooved surface gives more flexibility and adaptivity to diffusion barrier 1 1 layer. Diffusion barrier 1 1 must remain solid and imprenetable to ensure adequate emission performance against hazardous toxic industrial gases (TIC) or volatile organic compounds (VOC).

[0044] An advantage of providing the diffusion barrier 1 1 with the illustrated grooves 19 is that when several such seals are placed on top of each other, as illustrated in Figure 1 , the ring-shaped seals are set very tightly against each other, and the grooves 19 minimize possible fluid leakage in the interfaces between the seals.

[0045] Figures 7a to 7f do not illustrate the soft layer on the shaft contact surface. However, such a soft surface may be provided to the seal in connection with braiding of the braid 9', as the soft layer may be braided to the braid 9' by using suitable fibers at the same time as metallized fibers are used for the braid 9'. In any case, it is preferable to firmly attach the soft layer to the braid 9'. If the soft layer is implemented as a separate loose ring arranged inside the braid without firm attaching, a risk of fluid leakage in the interface between the two rings is present during subsequent use of the seal.

[0046] Figures 8 and 9 illustrate a fourth embodiment of a seal. Figures 8 and 9 are very similar to the previously explained embodiments. Therefore the embodiment of Figures 8 and 9 will in the following be explained mainly by pointing out the differences between these embodiments. [0047] Figure 8 illustrates a braid 29 with a plurality of filaments 26 retaining fire retardant particles 10, and Figure 9 illustrates a seal 24 manufactured of a plurality of such braids 29.

[0048] The filaments 26 may be carbon fiber or aramid fibers which intermesh with each other to form elongated, tubular braids 29 which are filled with fire retardant particles 10, such as graphite particles. The filaments 26 may be metallized as explained in connection with Figure 7a, for instance. Alternatively, metal filaments 26 may be included in the braid 29. Still another alternative is that the entire braid 29 with the fire retardant particles 10 is metallized in a similar way as explained in connection with Figure 7a.

[0049] Figure 9 illustrates a plurality of braids 29 which intermesh with each other to obtain a seal 24. This seal may subsequently be shaped into a ring form, as explained in connection with the previous embodiments.

[0050] The braids 29 of the seal 24 may be separately metallized, as explained previously. Alternatively, the braids may be non-metallized, and the seal 24 as illustrated in Figure 9 may be metallized as explained in connection with Figure 7a. Alternatively, the seal 24 may be arranged in a mold to obtain a ring shape, and a single surface of the braid 24 may be metallized to obtain a diffusion barrier surface, as explained in connection with Figures 7c and 7d.

[0051] In case the braids 29 of the seal 24 are metallized, the seal structure comprising such metallized braids 29 provides a diffusion barrier 1 1 by itself. Once such a seal is compressed in a valve or in a pump, for instance, the metal particles are pressed against each other which results in a very compact, fluid tight metal structure within the seal 24. However, it is naturally possible to additionally provide one or more surfaces of the seal 24 with a diffusion barrier layer which is grown on a surface of the seal 24, as has been explained in connection with the previous embodiments.

[0052] In order to avoid that the metallized parts of the seal come in contact with a shaft, the seal 24 can be provided with a soft layer 13, as explained in connection with Figures 2 and 6, for instance, and this soft layer 13 may be lubricated.

[0053] Figure 10 illustrates a fifth embodiment of a seal. The embodiment of Figure 10 is very similar to the one explained in connection with Figures 8 and 9. Therefore the embodiment of Figure 10 will be mainly explained by pointing out the differences between these embodiments. [0054] In Figure 10 an outer hollow braid 39 with a plurality of fibers 31 surrounds a plurality of inner braids 29. The inner braids 29 are similar as the one explained in connection with Figures 8 and 9. The fibers 31 of the braid 39 may be aramid fibers of carbon fibers, for instance.

[0055] In case the inner braids 29 are metallized in one of the alternative ways explained in connection with Figures 8 and 9, then a sufficient diffusion barrier 1 1 may be obtained simply by compressing the seal 34 such that the metal particles are pressed against each other and the metal particles within the braid 39 form the diffusion barrier 1 1 . However, as an alternative or in addition to a diffusion barrier within the braid 39, a diffusion barrier layer may be grown on an outer surface of the seal 34, as has been explained in connection with Figures 7c and 7d, for instance.

[0056] Similarly, as in the previously explained embodiments a soft layer may be arranged on the shaft contact surface, and the shaft contact surface may be lubricated.

[0057] Figures 1 1 to 14 illustrate a sixth embodiment of a seal. The embodiment of Figures 1 1 to 14 is very similar to the previously explained embodiments. In the following the embodiment of Figures 1 1 to 14 will be explained by pointing out the differences between these embodiments.

[0058] Figures 1 1 to 14 the seal structure of the seal 44 comprises an elongated strip 41 of fire retardant particles 10 or with a layer of fire retardant particles 10 covering substantially the entire surface of the elongated strip 41 . At least one surface of the elongated strip 41 is provided with a metallized layer 42. The metallized layer may be provided electrochemically or by a deposition process, similarly as has been previously explained.

[0059] The metallized layer 42 may cover entirely one surface of the elongated strip 41 , or alternatively as illustrated in Figure 1 1 , the opposite ends of the elongated strip 41 may have regions which are not metallized.

[0060] In Figure 12 the elongated strip 41 has been wound into a ring. The metallized layer 42 is indicated with a thicker line than the part of the elongated strip 41 which is not metallized. From Figure 12 it can be seen that due to the non-metallized regions at the ends of the elongated strip 41 , an inner and outer non-metallized soft layer has been formed at the shaft contact surface 13 and at the opposite contact surface 14. If necessary, these soft layers may be lubricated, as previously explained. [0061] Figure 13 is a cross section A - A along line A - A in Figure 12. Figure 13 illustrates the situation before the seal 44 is compressed and Figure 14 illustrates the same cross section after the seal 44 has been compressed.

[0062] When comparing Figures 13 and 14 with each other one can observe that after compression the metallized layer 42 forms a structure efficiently working as a diffusion barrier 1 1 . In case the seal is still further compressed, the separate metallized layers 42 will come into contact with each other and even more efficiently prevent diffusion.

[0063] Naturally, in case an additional diffusion protection is needed, it is possible to grow an additional diffusion barrier layer on the top or bottom surface of the seal ring, as has been explained in the previous embodiments.

[0064] It is to be understood that the above description and the accompanying figures are only intended to illustrate the present invention. It will be obvious to a person skilled in the art that the invention can be varied and modified without departing from the scope of the invention.