NO20151379A1

NO20151379A1 - Improved Seal Assembly

Info

Publication number: NO20151379A1
Application number: NO20151379A
Authority: NO
Inventors: Alexander Gundersen
Original assignee: Nli Asker Subsea As
Priority date: 2013-03-15
Filing date: 2015-10-12
Publication date: 2014-09-18
Also published as: GB2527253A; GB201518135D0; GB201304786D0; WO2014140379A1

Description

Improved Seal Assembly

Technical field

The present invention relates to an improved seal assembly.

In particular it relates to a seal assembly that has application in the oil and gas production industry where it can be used, for example, to seal a valve stem or in conjunction with fluid lines on a stab connector, as well as being useful for other shaft sealing situations, particularly where there is rotary or reciprocating movement or both, but equally for use in static applications.

It also relates to devices, such as valves and connectors, incorporating the new seal assembly.

Introduction

Seal assemblies for fluid lines in the oil and gas industry have to be extremely reliable and work well across a wide range of extreme conditions.

Temperatures, for example, can vary by several hundred degrees from the ambient sea water conditions, which could be below 0°C, to well head temperatures which can be in excess of a few hundred degrees Centigrade. Temperatures of the fluids can also drop substantially where there is a sudden expansion of gas. On the topside of a platform, external temperatures can reach many tens of degrees below 0°C. Subsea equipment may have to be proven to operate reliably at temperatures down to -20°C or even -44°C. The temperatures experienced by devices like valves and connectors, for example, can cycle quickly between these various extremes.

With regards to operating pressures, in deep water environments the external pressure can be extremely high, e.g., several hundred bars (e.g., tens of megapascals) and the internal pressures from the fluids within the subsea fluid lines will be much higher. The pressures experienced by the seal can fluctuate markedly during use, particularly where there is an interruption in the supply of the fluid.

The seal assembly may also be subject to wearfrom rotational and/or reciprocating movement, e.g., during use and/or connection, and it is important that minimum levels of wear resistance are achievable for the entire life of the product under such harsh physical as well as chemical conditions.

Typically, the seals for subsea valves and stab connectors are made of polymers. They usually comprise a pair of axially spaced, O-ring seals, each retained in a recess to seal against a respective surface. These arrangements and materials have a proven track record, including meeting minimum operating conditions and levels of use without significant wear (some minimum service requirements are described, for example, in ISO 10423 F1.11). However, the polymer seals still require frequent maintenance and tend to degrade in harsh environments. Of course, engineers are always looking for ways to extend the performance and operating ranges of such seal assemblies.

Metal-to-metal seals are known to offer an alternative to polymer seals in certain applications, but there has been some concerns about the a bi I i ty of metal seals to resist wear, particularly on items like subsea valves or stab connectors where the reliability of the seal is paramount to any product acceptance.

It would be desirable to provide an improved seal assembly with good reliability that can be used on valve stems and stab connectors as well as other pieces of equipment for the oil and gas production industry where there are dynamic and static shaft sealing applications.

Summary of the invention

Viewed from a first aspect, the present invention can be seen to provide a seal assembly for sealing between a shaft and a housing. The seal assembly is positioned in an annular region between the shaft and the housing. The seal assembly comprises at least a pair of lip seals that are arranged within the annular region. Each lip seal comprises an energising ring which is partially encapsulated within an annular jacket. The jacket of each lip seal further has a lip-shaped part for engagement against one of the shaft or housing. The at least one pair of lip seals are arranged spaced in a radial direction of the shaft. A first of the lip seals provides an inner lip seal where its lip-shaped part seals against the shaft, and a second of the lip seals provides an outer lip seal where its lip-shaped part seals against the housing.

The annular region may comprise an annular recess on the shaft or an annular recess in the housing, where the annular recess accommodates the majority of the seal assembly (the lip-shaped part or parts protruding out of the annular recess to seal against an internal surface of the housing or a surface of the shaft respectively). The annular region may also comprise a region between the shaft and housing that is defined on an inner and an outer circumferential side by the respective surfaces of the shaft and housing, and in an axial direction by axially spaced thrust surfaces of thrust members accommodated between the shaft and housing at opposite axial ends of the seal assembly. These thrust members may be in the form of compression rings, at least one of which may be moveable. The thrust members may also be provided as a coilar on the outside of the shaft or the inside of the housing. They could also be part of a plate or other component. In some preferred embodiments, one or other of the thrust members (or both) may be integrally formed with the shaft or housing.

The lip seals may be of a form as described in EP-B-1688649. They are available as a commercial product from High Tech Metal Seals (H.T.M.S.) for some shaft sealing applications. Compared to an earlier metal-to-metal seal known from EP-A-0529928, the lip seal in EP-B-1688649 is said to be better able to accommodate shrinkage and expansion of the parts during temperature cycling. Its use on a valve stem or a stab connector, however, is not suggested in EP-B-1688649 or other brochure materials.

When the present inventor experimented with using these lip seals in such applications, it was found that there were some performance problems that needed to be resolved. The solving of these problems has led to the new seal assembly described above, where the first of the pair of lip seals provides an inner lip seal that seals against the shaft, and a second of the lip seals provides an outer lip seal that seals against the housing.

A set preload may be applied to the inner and outer lip seals collectively by tightening a compression ring provided at one end of the annular region or recess, thereby compressing the pair (or pairs) of lip seals between a thrust surface provided by the compression ring and an opposing thrust surface provided by a thrust member or a shoulder forming an opposite end of the annular recess. Thrust rings may be provided between consecutive pairs of lip seals.

Thus the design of the seal assembly makes it possible to use multiple lip seals in one annular region or envelope (e.g., a groove provided in the housing or the shaft). Through this, reliable bi-directional sealing and double barriers can be achieved without the need for a plurality of such regions and individual systems for preloading the seals, something which would add considerably to the size and complexity of the seal assembly.

The inner and outer lip seals of each pair may be arranged concentrically. Thus a central plane of the inner lip seal is preferably aligned with a central plane of the outer lip seal, both central planes extending perpendicularly to the longitudinal axis of the shaft.

The energising ring of both the inner lip seal and the outer lip seal have a profile substantially of a ring torus, with the outer lip seal håving a larger inner diameter than the outer diameter of the inner lip seal, and the diameter of the respective ring cross-sections preferably being the same. The thickness of the jackets around the energising rings are preferably the same too. Hence the overall depth of the inner lip seal in the axial direction of the shaft is preferably equal to that of the outer lip seal.

By providing at least a pair of radially spaced, preferably concentrically arranged, lip seals, improved sealing is achievable and a broader range of temperatures and pressures can be accommodated by the devices incorporating the seal assembly. Reduced wear on the seal assembly during use can also be achieved, allowing recognised limits for the oil and gas industry to be met.

When viewed in cross-section, the lip-shaped part of the inner lip seal may 'peel away' from its respective energising ring in a different direction

(clockwise/anticlockwise) to the lip-shaped part of the corresponding outer seal, with both of the lip-shaped parts extending in substantially the same axial direction from the energising rings. The pair of lip seals are preferably in axial alignment (but spaced radial ly), though embodiments are also envisaged where the inner and outer lip seals are staggered axially with respect to each other too. This may require steps or the like to be provided in the opposing thrust surfaces of the annular region, for example, in a compression ring and an opposing thrust member or shoulder. In some applications it may also be desirable to have lip seals or pairs of lip seals with energising rings of different cross-sectional diameter.

In a further arrangement, the seal assembly comprises a stack of more than one pair of lip seals, each pair of lip seals providing an inner and an outer lip seal within the annular region. Thrust rings may be provided between the pairs of lip seals.

Thus, in one configuration of the seal assembly, at least two pairs of lip seals are arranged within the annular region. The lip seals may be provided as a stack of such lip seals, all located within the annular region (e.g., within an annular recess that is provided in either the shaft or the housing of the device and common to all the lip seals). A set preload may be applied to the seals collectively by tightening a thrust member, such as a compression ring which is provided at one end of the annular region, thereby compressing the lip seals collectively between the compression ring and an opposing thrust member or shoulder forming an opposite end of the annular region. Thrust rings may be provided between consecutive pairs of lip seals.

When viewed in cross-section, and in particular looking at the lip-shaped parts of two pairs of the lip seals where the jackets separate from the respective energising ring, preferably the inner lip seal of the first pair 'peels away' from the underlying jacket in the same direction (e.g., clockwise or anticlockwise) as for the inner lip seal of the second pair, and similarly the outer lip seal of the first pair 'peels away' from the underlying jacket in the same direction as for the outer lip seal of the second pair. The inner lip seals preferably 'peel away' in an opposite direction to the outer lip seals so that the lip-shaped parts of each pair extend in the same axial direction along inner and outer circumferential sides of the seal assembly (e.g., against a surface of the shaft or housing).

Thus, the seal assembly may have a stack of at least two pairs of lip seals, each of the lip seals håving a lip-shaped part arranged for engagement against the shaft or housing, wherein the lip-shaped parts extend in the same axial direction.

The lip seals may be arranged with this lip-shaped part extending towards the side which is predominantly at the higher pressure, so that the fluid pressure can act to help urge the lip-shaped part into sealing engagement with the respective surface of the shaft or housing (in other words, the concave side of the lip-shaped part preferably faces towards the high pressure side).

By providing more than one lip seal in a given sealing direction, leakage of fluids can be minimised as a result of the redundancy provided by the additional lip seals. In this way, if one of the lip seals should leak, the leakage would be limited and this would create a pressure drop across the lip seal. The fluid on the lower pressure side is then less likely to push around a next lip seal, both in the radial and axial directions. The more layers of lip seals that are provided, the greater this labyrinth effect will be, helping to prevent leaks occurring.

The seal assembly may also include additional pairs of lip seals in the seal stack which have a lip-shaped part extending in the other axial direction to the first two (or more) pairs of lip seals, to provide sealing in the opposite direction. This may be useful when, for example, the device is used in a deepwater application and the flow of fluids is interrupted. The seal assembly may comprise two or more pairs of lip seals håving a first sealing direction and one or more pairs of lip seals, preferably two or more pairs of lip seals, håving a second sealing direction.

The lip seals may comprise an energising ring which is in the form of a helical spring, preferably a metal helical spring.

The jacket for each lip seal may also be of metal and encapsulate more than half of the energising ring. This gives a metal-to-metal seal with the respective surface of the shaft or housing, as well as with the other points of contact for the seal. A metal-to-metal seal can offer reduced creep at high temperatures and so provide better sealing across a broader range of temperatures. In addition to the lip-shaped part which seals against the shaft or the housing, the jacket may form a seal with two abutting surfaces provided by one or more of a thrust member, compression ring, shoulder, thrust ring or an adjacent seal. The jacket may further include a coating, such as a soft, lubricating, corrosion resistant metal, e.g., like si Iver or gold, or a polymer-based friction reducing material, e.g., like PTFE. The jacket may also be formed of other (non-metal) materials where appropriate.

The surface of the shaft or housing that the lip-shaped parts engage, and/or any other part of the annular recess which the jacket engages, may include a coating to improve the hardness and/or reduce friction. Such coatings include carbide and nitride based coatings, with tungsten carbide being a particular preferred example.

The seal assembly may be provided in a valve housing of a valve in order to seal a valve stem.

In further applications, the seal assembly is provided on a male stab of a stab connector. In another arrangement, the seal assembly is provided in the housing of a stab connector, e.g., on the female part of a stab connector.

The seal assembly may be used for sealing all types of fluids, such as gases, liquids, mixtures of liquids and gases, etc.

Brief description of the drawings

Certain preferred embodiments of the present invention will now be described in greater detail, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 illustrates a cross-sectional view of a known seal arrangement taught in EP-B-1688649; Figure 2 illustrates, in cross-section, an enlarged view of the seal described in EP-B-1688649; Figure 3 illustrates a cross-sectional view of a seal assembly comprising a seal stack of four lip seals accommodated in an annular recess of a housing; Figure 4 illustrates a cross-sectional view of another seal assembly comprising a seal stack with a different arrangement of four lip seals accommodated in an annular recess of a housing; Figure 5 illustrates a cross-sectional view of a further seal assembly comprising a seal stack of six lip seals provided in an annular recess of a housing; Figure 6 illustrates a cross-sectional view of yet another seal assembly comprising a seal stack with eight lip seals provided in an annular recess of a housing; Figure 7 illustrates a cross-sectional view of a similar seal assembly being used instead in an annular recess of a shaft for sealing against a housing; and Figure 8 illustrates a further variation of a seal assembly where the seal assembly is accommodated between a pair of compression rings positioned between a shaft and a housing.

A lip seal and a seal arrangement with just a single lip seal is known from EP-B-1688649. Figure 1 shows the known lip seal 1 arranged to seal between a shaft 2 and a housing 3.

As shown in more detail in Figure 2, the lip seal 1 comprises two components. The first, innermost part is an energising ring 4 that has the profile substantially of a ring torus. The energising ring 4 may be of circular or oval cross-section. It is intended to be compressible under a preload and is preferably made as a helical spring. The energising ring 4 is gripped within the second component, namely a jacket 5 that encapsulates over half of the energising ring's curved surface. The jacket 5 additionally includes a lip-shaped part 6 where it departs or 'peels away' from the energising ring 4 to extend in a substantially axial direction (the shaft has an axis A-A which defines the axial direction, this extending perpendicularly to the plane of the seal B-B).

The single lip seal 1 is retained within an annular region 7 in the form of a recess 7 that extends around an inner surface 8 of the housing 3. In use the lip seal 1 is 'energised' through a preload that is applied to the lip seal 1 by tightening a compression ring 9. The compression ring 9 doses off one end of the annular recess 7, and as it is brought down onto the housing 3 by tightening the bolts 10, the lip seal 1 is compressed between the compression ring 9 and an opposing shoulder 11 of the annular recess 7. With the correct tolerances, good seals can be created at points A and B around the lip seal between the jacket 5 and the corresponding surfaces of the annular recess 7 provided by the compression ring 9, and the opposing shoulder 11 through preloading the lip seal 1 (i.e., energising it), these points being at 180° to each other. A seal is also formed at point C between the lip-shaped part 6 and the shaft 2. By increasing the amount of preload, the lip-shaped part 6 is urged into tighter and tighter contact with the shaft 2.

A seal assembly of the present invention is located within an annular region, which may comprise an annular recess 7 of a shaft 2 or housing 3, to provide a seal between that component and the other of the shaft 2 or housing 3. The shaft 2 and/or housing 3 may rotate and/or reciprocate with respect to each other, for example, during connection and/or during use. The seal assembly is also suitable for static or predominantly static sealing situations too.

In a first embodiment illustrated in Figure 3, the seal assembly comprises two pairs of lip seals 1, that are arranged in an annular recess 7 of the housing 3 as a seal stack 12 to seal against the shaft 2. Thus the annular recess 7 is sized to house more than one lip seal 1, for example, a pair of lip seals, two pairs, three pairs, four pairs or more than four pairs of lip seals 1. The lip seals 1 are accommodated collectively within the common annular recess 7. Each of the lip seals 1 of a given pair comprises an energising ring 4, like the seal shown in Figure 2. The energising ring is preferably a helical spring that has an approximate ring torus profile. The energising ring 4 is partially encapsulated within an annular jacket 5, the jacket 5 extending around and in contact with over half of the outer curved surface of the energising ring 4. The annular jacket 5 of each lip seal 1 also has a lip-shaped part 6, where the annular jacket 5 departs or 'peels away' from the energising ring 4 to extend in a direction parallel to the axis A-A of the shaft 2, for engagement against one of the shaft 2 or housing 3.

In the embodiment shown in Figure 3, the first and second pairs of lip seals 1 each provide inner and outer lip seals 13, 14, that are spaced in a radial direction of the shaft 2 (see the cross-sectional view in Figure 3). The lip seals 1 of each pair are aligned axially and arranged concentrically, with each pair being spaced in the axial direction A-A from the next. Together they provide a seal stack 12 håving two inner lip seals 1, each with a lip-shaped part 6 to seal against the shaft 2, and two outer lip seals 14, each with a lip-shaped part 6 to seal against the housing 3. The first and second pairs of lip seals 1 are separated by one or more thrust rings 15.

These one or more thrust rings 15 may be of planar or substantially planar form. In other arrangements the thrust ring(s) 15 may additionally include an annular rib 16 (or pair of ribs) protruding in the axial direction A-A, the annular rib extending between an inner lip seal 13 and an outer lip seal 14 of a given pair. The thrust ring 15 in Figure 3 has an annular rib 17 (or pair of ribs) projecting from the opposite side too, giving the thrust ring 15 a X-shaped annular section. The annular rib 16,

17 may also provide additional sealing functionality.

Embodiments are also envisaged where the thrust rings 15 have a different profile or are absent.

In order to energise the lip seals 1, a thrust member in the form of a compression ring 9, which is provided at one end of the annular recess 7, is tightened to preload the pl ura I i ty of lip seals 1 collectively.

The valve stem seal assembly shown in Figure 3 is provided with two pairs of lip seals 1. Any miss-function of the first pair of lip seals 1 is caught by the second pair of lip seals 1, in order to prevent a leak occurring. This arrangement, where more than one lip seal 1 is used, not only provides redundancy but also helps to distribute the sealing and frictional forces which develop during use of the device, resulting in reduced wear. The arrangement of seals also creates an extended labyrinth for any escaping fluids to navigate past. At each lip seal there would be an associated drop in pressure for the escaping fluid, making any leakage from a subsequent lip seal unlikely, thereby improving the reliability of the seal assembly. This may allow the metal lip seals 1 to be used reliably in devices like valves and stab connectors for the oil and gas production industry.

As illustrated in the cross-sectional view of Figure 3, the lip-shaped part 6 of each inner lip seal 13 may 'peel away' from its respective energising ring 4 in a different direction (clockwise/anticlockwise) to the lip-shaped part 6 of the corresponding outer lip seal 14, with all of the inner and outer lip-shaped parts 6 extending in substantially the same axial direction A-A from the energising rings 4. The lip-shaped parts 6 extend towards the high pressure side, i.e., the side where the high pressure fluids would escape from (the high pressure side is the lower part of the arrangement shown in Figure 3). In this way, the pressure of the fluid may also help to urge the lip-shaped parts 6 into sealing engagement either with the surface of the shaft 2 for the inner lip seals 13 or the surface of the housing 3 for the outer lip seals 14.

For subsea applications in particular, it may be desirable to provide at least a pair of lip seals 1 that are arranged in a first sealing direction and at least a further pair of lip seals 1' that are arranged in an opposite sealing direction. An example of this is shown in Figure 4. In this way, when the flow of the high pressure fluids is interrupted, the lip seals 1' arranged to seal in the opposite sealing direction can prevent the ingress of sea water into the pipeline or fluid line.

Other combinations of pairs of seals are also envisaged, for example, two in one direction and one in another, three in one direction and two, three or four in another, etc.

In the embodiment, the lip seals 1,1' are separated in an axial direction A-A by a thrust ring 15. The thrust ring includes a rib 16, 16' that extends in an axial direction A-A from each thrust face 17. As shown, the thrust ring 15 may have an X-shaped annular cross-section.

The other features of this embodiment are analogous to those of the first embodiment and explanations are being omitted here for conciseness. However, for the avoidance of any doubt, any feature disclosed in connection with one of the embodiments can be used with any one of the other embodiments even if it is not expressly recited herein.

Figure 5 shows another embodiment for use in a valve where the seal assembly is used to seal a valve stem. In this embodiment, the seal assembly is positioned within an annular region comprising an annular recess 7 provided in the housing 3. The seal assembly comprises three pairs of lip seals 1,1' each pair of lip seals 1,1' providing an inner lip seal 13 where the lip-shaped part 6 engages the shaft 2, and an outer lip seal 14 where the lip-shaped part 6 engages an inner surface 8 of the housing 3. A first and second pair of the lip seals 1 are shown arranged in the same sealing direction. However, as with the stem seal of Figure 4, for certain applications, such as for subsea use, the valve stem seal may require one or more of the pairs of seals to be arranged in an opposite sealing direction. Thus in the embodiment of Figure 5, one of the pairs of lip seals 1' is provided in an opposite sealing direction, in order to seal against the hydrostatic water pressures when the pressure of the fluid being carried by the fluid line drops.

The lip seals 1,1' are preloaded collectively in the same way as the other embodiments, by tightening the compression ring 9 to shorten the length of the annular recess 7. This compression ring 9 may be provided in the form of a plate or a lid, depending on the structure of the housing 3 and the way in which the lip seals 1 are to be compressed.

In the embodiment of Figure 5, the lip seals 1 are shown separated in the axial direction by a plurality of thrust rings 15,15'. Each thrust ring 15, 15' includes a rib 16 that extends in an axial direction A-A between a pair of lip seals 1. This rib helps to centralise the thrust ring with respect to each pair of the lip seals 1 and compartmentalise any fluid which escapes. The same function could be provided by two thinner parallel ribs.

As shown, one of the thrust rings 15 has a X-shaped annular cross-section and the other thrust ring 15' has a T-shaped annular cross-section. This helps the fitting of the lip seals 1 in the annular recess 7 of the seal assembly during the fabrication process.

Other arrangements of X and T-shaped thrust rings 15, 15' are also envisaged. For example, the order of the various thrust rings 15, 15' could be varied or two T-shaped rings may be used back-to-back in place of an X-shaped thrust ring. The inner lip seals 13 could also use separate thrust rings 15,15' to the outer lip seals 14.

Figure 6 shows an example of a further preferred embodiment where the seal stack 12 has a first set of lip seals 1 (two pairs) arranged to seal in one direction and a second set of lip seals 1' (two pairs) arranged to seal in the other direction.

All the other features of the embodiment are generally similar to those of Figures 3, 4 and 5, and references made to those embodiments can be read against this one accordingly.

In Figures 3, 4, 5 and 6, the seal assembly is shown positioned in an annular recess 7 on the housing 3 for sealing against a surface of the shaft 2. However, embodiments also are envisaged where this is the other way around, for example, as shown in Figure 7, where the annular recess 7 is provided in the shaft 2 for sealing against an inner surface 8 of the housing 3.

Figure 7 shows how the seal stack 12 can be adapted for use on a stab connector or the like where the seal stack 12 is provided in an annular recess 7 of the shaft 2 for sealing against the inner surface of the housing 3. In the embodiment shown, the seal stack 12 comprises eight lip seals 1,1' arranged as four pairs of inner and outer lip seals 13, 14. However, any of the seal assemblies previously described could be used in conjunction with a stab connector depending on the specific requirements. Thus the stab connector seal assembly could comprise two, three, four, five, six or more pairs of lip seals 1,1' arranged in the seal stack 12.

The shaft 2 is hollow to provide a bore 18 for fluids (which could be oil and/or gas but are more likely to be for other media like methanol, water, hydraulic oil, cutting injections, etc, that are used to support the production operations) to flow through at high pressure. The female part of the stab connector comprises a housing 3 that the male stab is inserted into that is of corresponding shape and size. The female part may be located on a Christmas tree or other installation and the male part attached to an end of a flow line. By way of example, the flow lines typically have an internal diameter of anywhere between about 0.5 to 8 inches (e.g., 1 to 20 cm).

The compression ring 9 may be retained on the end of the shaft 2 by way of a screw thread, the screw thread allowing the amount of preload to be adjusted. In other arrangements the compression ring 9 is welded or otherwise fixed in place once a set preload has been applied. In further arrangements a mechanical adjustment device can be provided to adjust the pressure applied by the compression ring 9. For example, it may be desirable to only apply a certain level of preload once the shaft 2 has been introduced into the housing 3 and not before, in order to avoid the lip-shaped parts 6 from interfering with the assembly or connection. In such arrangements, a moveable compression ring 9 may be provided at the other end of the recess 7 to urge the lip seals 1 towards a shoulder 11 at the distal end of the connector when preload is applied to energise the lip seals 1.

Figure 8 shows how the seal assembly can be accommodated within an annular region 7 that is positioned between a shaft 2 and a housing 3, råtner than comprising an annular recess formed in the shaft 2 or housing 3. Thus the annular region 7 is defined on inner and outer circumferential sides by the surfaces of the shaft 2 and housing 3, and in the axial direction by opposed thrust surfaces 19 provided by a pair of compression rings 20 also located between the shaft 2 and housing 3.

The other features of this embodiment are generally similar to Figures 6 and 7 and the preceding figures. The previous explanations of those features apply to this embodiment equally. To energise the lip seals 1, one or both of the compression rings 20 would be displaced toward the other to reduce the axial length of the annular region 7 and apply pressure to the lip seals 1.

Another variation on this embodiment (not shown) is where one of the compression rings 20 is provided as a coilar on the shaft 2 or housing 3. The coilar may be an integral part of the shaft 2 or housing 3, to give a thrust surface 19 in the form of a shoulder of the shaft 2 or housing 3 for the seal assembly 1 to be compressed against. Further the two compression rings 20 in Figure 8 could be provided as a first coilar on the shaft 2 and a second coilar on the housing 3, where axial displacement of the shaft 2 with respect to the housing 3 is used to adjust the axial length of the annular recess 7 to control the compression of the seal assembly and energising of the lip seals 1.

While the inner surface 8 of the housing 3 has been shown in the embodiments as cylindrical, other shapes are also envisaged, depending on the application. For example, the inner surface 8 of the housing 3 may include steps or tapered regions. The position and relative sizes of the lip seals 1,1' might be varied accordingly.

The valve stem and valve housing of Figures 3 to 6, the male and female parts of

the stab connector of Figure 7, and the shaft, housing and thrust members of Figure 8, may be of metal, for example, a nickel based superalloy. An example of such an alloy is Inconel 718. The parts may also include a coating, particularly in the region of the connection and where the lip seals 1,1' are intended to engage. An example of such a coating is tungsten carbide. The coating may have a hardness of 800 HV or greater. It may be polished to a surface finish Ra of 0.2. The thickness of such a coating, by way of example, could be from 4 to 50 thousandths of a millimetre,

depending on the method used to deposite the coating.

The improved seal assembly of the present invention, of course, has application beyond valves and stab connectors, and could be used to seal all manner of situations where a shaft 2 is located within a housing 3, particularly where allowance must be given for rotary or reciprocating movement or both. The term "shaft" used herein extends also to parts of a component håving a shaft-like section where it is necessary to provide a reliable seal between that section and a corresponding region of a housing, for example, situations where there is a lid or a blind flange where bi-directional sealing or double barriers are required. For these shaft-like parts there may be no relative rotational movement. The improved seal assembly offers particular utility for a range of devices, not limited to valves and connectors, that are exposed to high hydrostatic pressures.

Viewed from another aspect, the present invention can also be seen to provide an annular seal assembly for sealing between a first metal body and a second metal body when the first metal body is arranged coaxially within the second metal body, the seal assembly comprising a stack of two or more pairs of annular metal seals which are spaced axially within an annular region, the pairs of seals comprising an inner seal håving a portion arranged to seal against an inner cylindrical surface of the first metal body and an outer seal håving a portion arranged to seal against an outer cylindrical surface of the second metal body when the first metal body is positioned coaxially within the second metal body, wherein the inner and outer seals each comprise an energising ring retained within an annular jacket. Each annular seal is preferably a lip seal as described previously in this specification. For the avoidance of all doubt, the previously described preferred features are equally applicable to this aspect too and can be read in conjunction with this aspect as if each feature had been explicitly described herein in combination with this aspect.

Claims

1. A seal assembly for sealing between a shaft and a housing, the seal assembly being positioned in an annular region between a surface of the shaft and a surface of the housing, wherein the seal assembly comprises at least one pair of lip seals arranged within the annular region, each lip seal comprising an energising ring which is partially encapsulated within an annular jacket that has a lip-shaped part for engagement against one of the shaft or housing, and wherein the at least one pair of lip seals is arranged within the annular region spaced in a radial direction of the shaft, to provide an inner lip seal with a lip-shaped part to seal against the shaft, and an outer lip seal with a lip-shaped part to seal against the housing.

2. A seal assembly as claimed in claim 1, wherein the seal assembly comprises a stack of two or more pairs of lip seals, each pair of lip seals providing an inner and an outer seal within the annular region.

3. A seal assembly as claimed in claim 2, wherein the seal assembly comprises a pair of lip seals arranged in a first sealing direction and at least a further pair of lip seals arranged in an opposite sealing direction.

4. A seal assembly as claimed in claim 2 or 3, wherein the seal assembly comprises three pairs of lip seals housed within the annular region.

5. A seal assembly as claimed in any preceding claim, wherein the seal assembly comprises four pairs of lip seals housed within the annular region.

6. A seal assembly as claimed in any preceding claim, wherein thrust rings are provided between consecutive pairs of lip seals.

7. A seal assembly as claimed in claim 6, wherein the thrust rings include a rib that extends in an axial direction between a pair of lip seals.

8. A seal assembly as claimed in any preceding claim, wherein the annular region accommodating the seal assembly comprises an annular recess on the shaft or an annular recess in the housing.

9. A seal assembly as claimed in any preceding claim, wherein the lip seals each comprise an energising ring which is in the form of a helical spring.

10. A seal assembly as claimed in any preceding claim, wherein the jackets of the lip seals are of metal.

11. A device comprising a shaft and a housing, the device including a seal assembly as claimed in any preceding claim, wherein the device includes a compression ring at one end of the annular region, the compression ring enabling the lip seals to be preloaded collectively.

12. A device as claimed in claim 11, wherein a surface of the shaft or housing that the lip-shaped parts of the lip seals engage against is provided with a coating to improve the hardness and/or reduce friction relative to the base material of the shaft or housing.

13. A device as claimed in claim 11 or 12, wherein the device is a valve and the seal assembly is used to seal against a valve stem.

14. A device as claimed in claim 11 or 12, wherein the device is a stab connector and the seal assembly is provided on a male stab for sealing within a female stab connector.

15. A device as claimed in claim 14, wherein the shaft is hollow and comprises a bore for conveying a fluid.