GB2259121A - Hollow metal sealing rings - Google Patents

Abstract

A self-energizing metal sealing ring has a generally C-shaped cross section open on its radially inner side, with limbs (9) which are convergent at least on their axially outer surfaces (11). The radially outermost region (13) is of increased thickness; and is shaped to provide a broad cylindrical outer rim or heel surface (15). The free ends of the limbs have axially outwardly projecting tips (31), the axial distance being the maximum axial dimension (A) of the ring in its relaxed state. The limb regions have, adjacent to the said projections, regions (27) of reduced thickness at which the limbs flex during axial compression. <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. A further known seal is that of GB 2038961, in which the limbs of the seal cross section have out-turned lips, forming an 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.

A reason for the lack of success with these known seals when sealing extremely high pressures, is their lack of hoop strength. Because of this, the seal rings can 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 many 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.

Our GB-A-2239496 (8928648.8) describes self-energizing metal seals capable of overcoming the described shortcomings of the known seals, and in particular capable of providing reliable sealing against a fluid at very high pressure.

According to GB-A-2239496 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 conventional seals, which are made of metal of constant thickness.

The hoop strength of the sealing 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 radial thickness.

The hoop strength and limb flexibility can be selected within wide ranges, independently of one another.

According to another aspect of GB-A-2239496, 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.

The present invention concerns a further improved hollow metal sealing ring, applicable to situations in which there is exceptionally high pressure to seal, and/or a tendency to separation of the flanges or other surfaces to be sealed owing to the extremely high pressures.

According to the present invention, in a metal sealing ring of the kind generally set forth in GB-A-8928648. 8, the said reinforcement of the radially outermost region is provided by an increase of the thickness of the ring material in this region; this thicker region is shaped to provide an axially broad outer rim or heel surface generally parallel to the axis of the ring; the radially innermost free ends of the limb regions have axially outwardly projecting tips which provide the maximum axial dimension of the ring in its relaxed state; and the limb regions preferably have, radially external to the said projections, regions of reduced axial thickness providing points of preferential flexing of the inner ends of the limb regions during axial compression.

Preferably the maximum axial dimension is no greater than, preferably less than the radial dimension of the ring cross section between its radially innermost and outermost surfaces.

In one form of ring embodying the invention, the limb regions are convergent on their axially outer surfaces, for example along circular or other arcs blending into the regions of least thickness, whereas the internal profile of the ring cross section comprises axially inner limb surfaces which, in the relaxed condition, are substantially parallel to each other, being interconnected at their radially outer sides by a smoothly blended part-circular or other arc.

One form of seal ring embodying the present invention will be described, by way of example only, with reference to the accompanying drawing, which is a cross section on a radial and axial plane of one side of a sealing ring seated in a recess.

The drawing shows part of a lower flange 3 to be sealed against an upper flange (not shown), for example pipe end flanges in a natural gas pipeline. The upper flange has a plane surface, the lower flange has a rectangular recess 5 at the end of the pipeline bore 1, and a hollow metal sealing ring 7 is seated in this recess. The overall axial dimension A of the sealing ring in its relaxed condition is greater than the axial depth of the recess, so that one axial side of the ring stands proud of the flange surface and is compressed by the opposite flange in use. In the illustrated case, the depth of the recess is 0. 88 A.

With the exception of the radial extent B of the ring cross section, all of the dimensions of the ring cross section are related to the maximum axial dimension A, and a preferred set of relationships between the ring dimensions is illustrated in the drawing. B may be equal to A, or greater than A to give greater strength.

The present illustrated ring combines certain aspects of the rings shown in Figures 4 and 9 of GB-A-2239496 but is modified to cope with higher pressures and greater flange separations.

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 arcuate convergent external margins 11, so that the external surface of the ring is convex where it meets the flange surfaces.

The internal surface of the seal ring cross section is U-shaped with parallel surfaces 21 joined by a semi-circular surface 23. 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 increased progressively into 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 use.

The thicker heel region has an effectively cylindrical outermost surface 15, joined to the circular arcs 11 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 surface 15 has an axial extent of 0. 6 A.

It is desirable that rotation of the seal ring cross section within its seat be prevented. The cylindrical outermost surface 15 cooperates directly with the cylindrical radially outer surface of the recess or seat 5 to prevent rotation.

The inner ends of the limbs have axially outwardly turned lips 31, so that the profile is approximately that of the greek letter n.

These outwardly turned lips also prevent the seal from rotating in its seat.

The outwardly turned lips 31 have axially facing flat surfaces 25, which converge with one another in the radially outward direction. Thus, when these lips are put under compression between the flanges, it is the radially innermost edges or angles of the lips 31 which first come under compression, as these define the maximum axial dimension A of the sealing ring.

Because the internal surfaces 21 of the ring cross section are plane radial surfaces whereas the outer surfaces 11 are convergent arcuate surfaces, regions 27 of minimum thickness of the limbs are defined, immediately radially outside the lips 31.

When the illustrated sealing ring is compressed axially, the points E at the radially innermost extremities of the lip surfaces 25, are the first to contact the mating faces of the flanges. Under compression, initially the lips 31 are compressed, flexing about the regions 27 of least thickness, until the flanges make contact with the next widest part of the ring cross section, at the positions C on the convex portion of the ring profile.

At this time, the surfaces 25, having rotated under the initial compression, lie substantially flat against the flange surfaces.

Accordingly, at this time there is a primary seal at positions E (surfaces 25) and a secondary seal at positions C. Because the limb thickness at points C is greater than in regions radially inwards from these points, and increases further, radially outwards of the points C, the contact pressure at the points C is much greater than that at the points E. During further compression of the sealing ring, now effecting compression at the points C, the limbs flex at the radiused diameter of surface 23 of the internal groove 29 in the sealing ring.

To cope with extremes in flange separation or rotation, as may occur in the sealing of pressure vessels, the radial distance between points C and E can be increased as necessary.

It will be seen that the present ring provides a two-stage sealing action, in which sealing contact is initially made only at the relatively flexible lips 31, forming a primary seal, and after initial compression a further, stronger, secondary seal is formed at the position C. This, together with the increased radial extent of the seal ring cross section and in particular of the flattened heel region, enable the sealing ring to cope.with the most extreme sealing conditions.

Preferably, the surfaces 25 are tengential to the convex arcuate secondary seal regions, in the relaxed state; this is indicated by broken lines in the drawing. As already stated, surfaces 25 will be tangential to regions C after compression when they lie agaisnt the sealed surface.

By providing that the primary seal surfaces 25 are tangential to the secondary seal surfaces, we ensure the optimum progression of sealing action from initial contact at the corners of the surfaces 25, to the condition in which the surfaces 25 lie flat against the sealed surfaces and the secondary sealed regions are in contact with the sealed surfaces. This also ensures particularly good sealing in the final compressed condition.

In the illustrated embodiment, the limbs of the seal ring cross section have minumum thickness at the positions 27, immediately adjacent the outwardly turned lips. This has the important advantage that the lip regions flex about the regions of minimum thickness, relative to the portions of the limbs which are of greater radius from the centre of the ring.

this design in which the tip regions of the limbs flex preferentially at the adjacent regions 27, provides particularly high elestic recovery after compression, typically a recovery of 43% is achieved with the illustrated ring profile, which enables the sealing ring to be re-used.

However, it is also possible to use a seal ring in accordance with the present invention, having a constant limb thickness, in the region between the outwardly turned lips 31, and the secondary seal regions. In this case the internal surfaces 21 are not paralle to each other, but instead, each is parallel to the respective outer surface 11 of the same limb. However a ring with this kind of cross section profile does not provide the same high degree of recovery after compression as the ring illustrated in the drawing, and it provides a smaller sealing contact pressure, therefore it cannot be used in very highly demanding applications, as can the ring illustrated in the.drawings.

The surfaces 25 should be given a high surface finish and close tolerances, for example by machining and lapping.

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 halves and then welding these together on a radial surface perpendicular to the seal ring axis.

In the case of a seal ring of welded construction, the individual ring halves can be made by machining from plate, by pressing, or in any other convenient 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 suitable, for example Nimonic (registered trade mark) and Inconel (registered trade mark). A 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, silver, 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 (7)

1. A self-energizing sealing ring, in which the axial cross section of the ring is generally C-shaped and open on its radially inner side, the radially outermost region is of increased thickness and is shaped to provide an outer rim or heel surface generally parallel to the axis of the ring, and the radially inner free ends of the limb regions of the cross section have axially outwardly turned tips, which in the relaxed state of the ring define the maximum axial dimension of the ring.

2. A self-energizing metal sealing ring having a cross section which is hollow and open on its radially inner side and has at this side limb regions which are convergent at least on their axially outer surfaces; and in which the radially outermost region of the cross section is of increased thickness and is shaped to provide an axially broad outer surface generally parallel to the axis of the ring; the radially innermost free ends of the limb regions have axially outwardly projecting tips which provide the maximum axial dimension of the ring in its relaxed state; and the limb regions have, radially external to and adjacent the said projections, regions of reduced axial thickness providing points of preferential flexing of the inner ends of the limb regions during axial compression of the ring.

3. A seal ring as claimed in claim 1 or 2 in which the said maximum axial dimension of the ring cross section is not greater than the maximum radial dimension of the ring cross section.

4. A sealing ring as claimed in claim 1, 2 or 3 in which the internal profile of the ring cross section comprises axially inner limb surfaces which, in the relaxed condition, are substantially parallel to each other, being interconnected at their radially outer sides by a smoothly blended arc.

5. A seal ring as claimed in any of claims 1 to 4 in which the outwardly projecting tips of the limb regions have axially outermost surfaces which, in the relaxed condition of the ring, extend obliquely relative to the radial direction and converge with one another in the radially outward direction.

6. A seal ring as claimed in claim 5 in which, in the relaxed condition, the said axially outermost surfaces are substantially tangential to outer convex surfaces of the ring.

7. A seal ring substantially as herein described with reference to the accompanying drawing.