GB2239496A

GB2239496A - Hollow metal sealing rings

Info

Publication number: GB2239496A
Application number: GB8928648A
Authority: GB
Inventors: Terence Peter Nicholson
Original assignee: Specialist Sealing Ltd
Current assignee: Specialist Sealing Ltd
Priority date: 1989-12-19
Filing date: 1989-12-19
Publication date: 1991-07-03
Anticipated expiration: 2009-12-19
Also published as: GB2239496B; GB8928648D0

Abstract

A metal sealing ring (7) has a hollow cross section which is open at its radially innermost side and has limbs (9) having portions (11) which converge at the radially innermost side. The radially outermost region (13) of the cross section is provided with reinforcement so as to locally increase its hoop strength. Additionally, the sealing ring (7) and the seating (5) for the ring are provided with complimentary radial cross sectional shapes (21, 23, 25, 27) which cooperate to limit rotation of the sealing ring cross section when axially compressed. <IMAGE>

Description

HOLLOW METAL SEALING RINGS This invention relates to hollow metal sealing rings, and in particular to sealing rings of the so called low-load self-energizing static kind, as used for example in valves, pumps, motors and other apparatus to form leakproof seals between opposed, usually plane, parallel surfaces.

One known form of sealing ring has a radial cross section of C shape, with the open side of the C facing the centre of the ring. Another known seal is that known as the "Ellipseal" (Trade Mark), described in British patent specification 2187805, comprising a radial cross section of modified parabolic form with convergent margins. n further known seal is that of GB 2038961, in which the limbs of the seal cross section have out-turned lips, forming an Q-shaped cross section.

The above-mentioned seals have been very successful in numerous static sealing applications, but are not always entirely successful in meeting the demands of sealing equipment and pipelines used in natural gas fields, where pressures are commonly in the region of 10000 PSI, and may exceed 30000 PSI.

S reason for the lack of success with the known seals when sealing extremely high pressures, is their lack of hoop strength. Because of this, the seal rings expand under the applied internal fluid pressure until they can expand no further because of the restriction imposed by the recesses in which the seals sit. During this change of diameter of the seal ring, the areas of the seal ring surface in contact with the mating faces to be sealed are subjected to a galling action which roughens the surfaces, and in most cases it becomes impossible to establish a satisfactory seal. If the thickness of the seal ring metal is increased, to increase the hoop strength, the flexibility of the seal is substantially reduced, as the seals are made of metal of constant thickness.This then requires larger bolts and increased torque to compress the seal, and makes the seal less able to cope with rotation of the flanges to be sealed, i.e. loss of paralellism, which can occur under the action of the pressure of a contained fluid.

Sn object of the present invention is to provide a self-energizing metal seal capable of overcoming the described shortcomings of the known seals, and in particular capable of prouiding reliable sealing against a fluid at very high pressure.

According to a first aspect of the present invention a metal sealing ring has a cross section which is hollow and open on its radially inner side and has convergent limb regions at this side, and the radially outermost region is provided with reinforcement locally increasing its hoop strength. Preferably the reinforcement of this outermost region is provided by a greater thickness of material than radially inner limb regions which in use engage the surfaces to be sealed.

The increased material thickness in the radially outermost region and resulting increased hoop strength resist expansion of the seal in use, whereas the smaller thickness of inner regions, comprising the flexible limbs which contact the surfaces to be sealed, provides ample flexibility. Relative movement of the seal and sealed surfaces is therefore reduced or eliminated, so that the seal is not subjected to galling and roughening, but the load required to compress the seal is not substantially increased and the seal remains sufficiently flexible to accommodate misalignment and lack of parallelism of the surfaces to be sealed.

In this the present seal contrasts significantly with the conuentional seals, which are made of metal of constant thickness.

In one possible embodiment of the invention, the radial cross section of the seal ring may comprise a circular internal surface, and a generally elliptical or modified parabolic external surface.

The hoop strength of the seal ring can be further increased by broadening (in the axial direction) the outer rim or heel region of the seal ring that is of increased thickness.

The present invention can provide a sea). in which both hoop strength and limb flexibility can be selected within wide ranges, independently of one another.

Another problem that can arise with the known seals, in particular with the seal described in GB 2187805, is that the seal cross section may rotate and expand in diameter, when compressed between the surfaces to be sealed. The pressure contact points at which the clamping pressure effectively acts on the seal cross section, may also move during compression.

According to another aspect of the present invention, a hollow self-energizing metal seal ring and its seating are provided with complementary cross sectional shapes which cooperate to limit rotation of the seal cross section when compressed.

In the case of a seal ring of non-circular external cross section, e.g. as in GB 2187805, the seal ring may be seated in a groove or recess of stepped profile, the step cooperating with the non-circular external profile of the seal ring, particularly in the radially outermost region of the seal ring, to prevent rotation of the seal ring cross-section under load. Slternatively, the radially outermost region of the seal ring may for example be provided with an axially extending projection forming an abutment to cooperate with a grooue or recess of conuentional rectangular cross section; this measure is applicable to seal rings of circular radial cross section. The projection or abutment may also serue to increase the hoop strength of the seal ring.

The second aspect of the present invention is applicable to seal rings in accordance with the first aspect of the invention and also to otherwise conventional seal rings including seals of circular C cross section and seal rings in accordance with GB 2187805.

The present invention will be further described with reference to the accompanying drawings, in which: Figure 1 shows a seal according to a first embodiment of the invention, in position between two surfaces to be sealed, but before compression Figure 2 shows the same seal1 fully compressed Figure 3 shows significant dimensions of the same seal, Figure 4 shows a modification of the seal cross section, Figures 5 to 9 show further modified seal cross sections, and Figure 10 shows the behauiour of a conuentional seal.

when compressed.

Figures 1 and 2 show parts of upper and lower faces 1, 3 to be sealed1 for example pipe end flanges in a natural gas pipeline. The upper flange 1 has a plane surface, the lower flange has a rectangular recess 5 at the end of the pipeline bore, and a seal ring 7 is seated in the recess 5.

The seal ring is of metal with a hollow cross section, open on its radially inner side, that is to say, towards the pipeline bore so that the fluid being conveyed has access to the interior of the seal ring cross section and the pressure of this fluid therefore acts on the interior of the seal ring to force its limbs 9 into contact with the flange surfaces. The limbs 9 have conuergent margins 11, so that the external surface of the ring is conuex when it meets the flange surfaces.

The internal surface of the seal ring cross section is circular. The external surface of the seal ring cross section is non-circular and is such that the thickness of the metal of the seal ring is essentially constant in the conuex limbs 9, but is increased in the radially outermost heel or rim region 13.

The thicker heel region 13 enhances the hoop strength of the seal ring, without reducing the flexibility of the limbs 9.

The relationship between the thickness of the heel and the limbs is selected according to the required hoop strength and flexibility in relation to the intended application of the seal. For fluid pressures in the region of 10000 PSI the thickness ratio may be in the region of 2:1. For extremely high pressures the ratio may be 3:1.

The profile of the external surface is also open to choice according to the desired application and manufacturing considerations. It may for example be elliptical or modified elliptical, parabolic or modified parabolic, or a combination of circular arcs.

It is not essential that the internal surface of the cross section be circular, nor is it essential that the limbs 9 be of constant thickness.

Figure 3 shows one form of seal ring cross section, suitable for sealing fluids of moderately high pressure.

The seal cross section or profile is defined within a square of side . The limb regions are of constant thickness t and the internal surface of the cross section is circular, of radius (n/2 - t), centre 0.375A outwards from the radially inner side of the enclosing square, that is to say, from the radially innermost limit of the seal cross section.

The radially outermost heel region of the cross section has a maximum thickness 2t, and the external surface of the cross section in this region has a radius (4/4 + t).

This arc is connected to the circular arc of radius (n/2) defining the external surfaces of the limbs1 by tangential arcs of radius n connecting the arcs of radius 5/2 to the arcs of radius (5/4 + t).

fis already mentioned, for extremely high pressures the thickness at the heel may be three times the limb thickness. In this case, the position of the centre of curvature of the limbs would be at 0.258 from the radially innermost limit of the cross section.

It is to be understood that, although the described seal has its cross section contained within a square, this is not essential and in particular, the radial dimension of the cross section may be increased relative to its axial dimension in order to increase the fluid pressure the seal is capable of containing.

Figure 3 also shows the compression points at P.

The seal ring described so far has a cross sectional profile based on circular arcs tangential to one another. Figure 4 shows an alternative profile. In this, the thicker heel region has an effectively cylindrical outermost surface 15, joined to the circular arcs defining the limbs by frusto-conical surfaces 17 tangential to the limbs, and radiused transitions 19.

This profile substantially increases the hoop strength of the heel region. In the illustrated seal the heel region has a radial thickness of 3t and its surface 15 has an axial extent of 5/2.

Ss already mentioned, it is desirable that rotation of the seal ring cross section within its seat be prevented. The modified profile illustrated in Figure 3, with its cylindrical outermost surface 15, cooperates directly with the cylindrical radially outer surface of the recess or seat 5 to prevent such rotation.

Figure 5 shows a seat recess modified to cooperate with a seal ring as shown in Figure 1, to prevent such rotation. Specifically, the recess has a stepped profile comprising an arcuate shoulder 21 which cooperates with the non-circular heel of the seal ring to prevent rotation of the seal ring cross section under load.

Figure 6 shows a cylindrical ring 23 brazed or welded to the exterior of the heel, with an axial extent corresponding to or slightly less than the axial dimension of the seal ring when fully compressed. The ring 23 cooperates with a conventional rectangular recess to prevent rotation of the seal ring cross section.

Figure 7 shows a seal ring with an integral, or brazed or welded, axial projection 25 from one side of the heel, serving the same purpose of preventing rotation.

Figure 8 shows a seal ring with integral projections 27 on both axial sides of the heel.

An aduantage of the profiles illustrated in Figures 7 and 8 is that they do not increase the overall radial dimension of the seal ring cross section, in contrast to the seal ring illustrated in Figure 6.

These measures for preventing rotation of the seal ring cross section are also applicable to seal rings made of material of constant thickness, as described for example in GB 2187805. When applied to such rings1 the measures for preventing rotation, illustrated in Figures 6 to 8, haue the further advantage of increasing the hoop strength without reducing flexibility of the limbs.

Figure 9 shows a seal ring in which the outer or heel region has a thickened profile as shown in Figure 3, but the inner ends of the limbs have axially outwardly turned lips 31, so that the profile is approximately that of the greek letter n, resembling in this the seal disclosed in GB 2038961.

These outwardly turned lips have the advantage of preventing the seal from rotating in its seat.

Figure 10 shows the result of a phinite-element analysis of the "Ellipseal" disclosed in GB 2187805. The seal profile before compression is shown in broken lines, the profile after compression in solid lines. The fact that the seal profile has rotated and expanded in diameter during compression is clearly visible, as is the fact that the pressure contact point has moved from P1 outwards to P2. Accordingly, within the scope of the present invention, the rotation-prevnting measures illustrated in Figures 5 to 8 may be applied to the "Ellipseal" of GB 2187805.

The present seals can be manufactured for example by machining from solid; by a combination of machining and rolling; or by initially manufacturing two ring halues and then welding these together on a radial surface perpendicular to the seal ring axis, as shown at 29 in Figure 1.

In the case of a seal ring of welded construction, the indiuidual ring halves can be mad by machining from plate, by pressing, or in any other conuenient way.

Welding can for example be by TIG or micro-plasma welding, but electron beam welding is preferred owing to its lower heat input, making it possible to weld without difficulty sections having a heel thickness up to 35mm.

Particularly in the case of a seal made by electron beam welding, it may be desirable to heat treat the seal after welding, for example by re-solution heat treatment, before any subsequent age hardening treatment. Seals embodying the invention can be made of any suitable metal. High nickel alloys are particularly suitable1 for example Nimonic (registered trade mark) and Inconel (registered trade mark). S suitable alloy for sub-sea sour well applications is Inconel 718.

The spring characteristics of the seal and therefore its recovery factor after compression can be greatly improved by age-hardening.

The seals may be coated before use with a protective and/or low friction coating for example lead, siluer, gold, nickel, PTFE, or a combination of nickel or other metal and PTFE. The last mentioned combination is valuable for reducing galling during compression, when an Inconel seal is compressed between Inconel flanges, or more generally, when nickel alloy seals are used in conjunction with nickel-containing or coated flanges.

Claims

Claims.

1. A metal sealing ring having a radial cross section which is hollow and is open at its radially innermost side and comprises axially spaced first and second limbs resiliently compressible axially towards one another, the said limbs having respective radially inner regions which converge towards one another towards the radially innermost side of the ring cross section and being joined to one another in the radially outermost region of the ring cross section, which region is provided with reinforcement such as locally to increase its hoop strength.

2. A metal sealing ring according to claim 1, wherein at least part of the reinforcement of the radially outermost region is provided by a greater thickness of material in the radially outermost region than in the radially inner regions of the limbs.

3. A metal sealing ring according to claim 1 or 2, wherein at least part of the reinforcement of the radially outermost region is provided by an additional member attached to the sealing ring at or adjacent the said radially outermost region.

4. A metal sealing ring according to any preceding claim, wherein the said cross section comprises a circular internal surface.

5. A metal sealing ring according to any preceding claim, wherein the said cross section comprises an external surface which comprises a combination of circular arcs.

6. A metal sealing ring according to any one of claims 1 to 4, wherein the said cross section comprises a parabolic or modified parabolic external surface.

7. A metal sealing ring according to any one of claims 1 to 4, wherein the said cross section comprises an elliptical or modified elliptical external surface.

8. A metal sealing ring according to any preceding claim, wherein the said respective radially inner limb regions have substantially constant thickness.

9. A metal sealing ring substantially as described herein with reference to Figures 1 to 9 of the accompanying drawings.

10. A hollow self-energising metal sealing ring and a seating therefor; the said sealing ring and seating being provided with complementary cross sectional shapes which cooperate to limit rotation of the sealing ring cross section when axially compressed.

11. A hollow self-energising metal sealing ring and a seating therefor according to claim 10, wherein the said sealing ring comprises the features of a sealing ring according to any one of claims 1 to 8.

12. A hollow self-energising metal sealing ring and a seating therefor according to claim 10 or 11, wherein the radial cross section of the said seating comprises a recess having a stepped profile comprising an arcuate shoulder and the radial cross section of the said sealing ring comprises a non-circular external surface which cooperates with the arcuate shoulder to prevent rotation of the sealing ring cross section under load.

13. A hollow self-energising metal sealing ring and a seating therefor according to claim 10 or 11, wherein the said sealing ring further comprises a cylindrical projection extending axially of the sealing ring from adjacent the radially outermost point of the cross section of the sealing ring, the said projection being adapted to cooperate with the said seating to prevent rotation of the sealing ring cross section under load.

14. A hollow self-energising metal sealing ring and a seating therefor according to claim 10 or 11, wherein said sealing ring further comprises a pair of symmetrically disposed cylindrical projections extending axially of the sealing ring from adjacent the radially outermost point of the cross section of the sealing ring and in opposite axial directions, the said projections being adapted to cooperate with the said seating to prevent rotation of the sealing ring cross section under load.

15. A hollow self energising metal sealing ring and a seating therefor substantially as described herein with reference to Figures 5 to 8 of the accompanying drawings.