GB2030270A - Valve seats - Google Patents

Abstract

Very hard valve seats, for example of hard carbon, are necessary in ball valves for use under extremes of pressure and temperature. The hard seat is preferably resiliently supported to avoid excessive loading of the seat. A seat assembly, to be inserted in a conventional valve housing, comprises a seat 4 of hard carbon supported by a resilient annular cushion 3 of expanded graphite material, a metal retaining ring 1 extending around the outside of the cushion to retain the seat and the cushion. The expanded graphite material of the cushion has a density in the range 0.7 to 1.6 gram/cc and a compressive elastic modulus in the range 5 to 500 MPa. The seat may be supported by a frusto-conical surface of the cushion. The cushion is formed by compression of powdered expanded graphite material, or by winding a tape of expanded graphite material around a former. <IMAGE>

Description

SPECIFICATION Valve seats This invention relates to seats for valves, primarily for ball-type valves although it may be applied also to seats for other types of valve.

In many applications of ball valves, in particular those involving high temperature high pressure steam or other liquids which will readiiy form gases, or, at the other end of the temperature scale, cryogenic liquids which evaporate when the pressure is lowered, it is often necessary to make the or each seat which the ball engages of a very hard material such as a ceramic, or tungsten carbide or stellite in order to reduce erosion by so-called "wire drawing" as the valve is operated.

These seat materials are virtually rigid, with effectively no resilience, and the seats have to be manufactured very accurately so that, when the valve is assembled, there is no excessive clamping load imposed on the seat by the valve closure member as this can lead to very high friction which resists operation of the valve or, in the worst case, causes fracture of the seat.

Furthermore seats of such hard materials, when rigidly located, make no allowance for wear.

These problems are sometimes overcome by bedding the seat on a cushion in the form cf an elastomeric '0' ring to accommodate toierance variations, but this restricts use to those fields where the poor thermal resistance and susceptibility to chemical attack of the elastomer are acceptable.

It has also been proposed, in U.S. Patent No.

4006 881, to support a hard carbon seat in a ball valve, with a resilient washer formed from a carbonaceous sheet material. The carbonaceous material is formed from expanded graphite and carbon fibres, and is said to have good thermal resistance. However, because of the porosity of the carbonaceous material a circumferential portion of the washer has to be compressed to avoid leakage through the washer. For this compression of the washer the valve housing must be specially designed.

According to the invention there is provided a valve seat assembly comprising a rigid annular seat member of a hard material, for making sealing contact with a closure member of a valve in which the assembly is mounted, a resilient annular cushion of an expanded graphite material supporting the seat member, and a retaining ring extending around the radially outer periphery of the cushion.

The density of the expanded graphite material of the cushion is preferably in the range of 0.7 to 1.6 gram/cc, with the resilience of the cushion being of the order of 8% to 1 1%. However, the resilience of the cushion could be in the range of 7% to 14%. The compressive elastic modulus of the expanded graphite material of the cushion is preferably in the range of 5 to 500 MPa. The seat member may be of any suitable material, for example ceramic or tungsten carbide, but is preferably of hard carbon having a compressive elastic modulus, for example, of the order of 10,000 MPa.

The retaining ring can be of metal, and in preferred arrangement comprises two axially spaced, radially inwardly projecting, flange portions arranged to retain the seat member and the cushion in the ring. The assembly may comprise also a second retaining ring, extending around the radially inner periphery of the cushion.

In one preferred form, the seat member is supported by a frusto-conical surface of the cushion. The seat member may extend radially inwardly beyond the cushion, and the cushion may extend axially beyond the seat member in a direction away from a face of the seat member arranged to make sealing contact with a valve closure member.

The invention also provides a method of making a valve seat assembly, as set out in the last preceding paragraph but three, comprising locating a retaining ring in a cavity of a die, introducing expanded graphite material in powder form in to the cavity, compressing the expanded graphite material in the die cavity to form a cushion, and mounting and securing a seat member so that it is supported by the cushion.

Alternatively, a valve seat assembly may be made by a method comprising forming an annular cushion by a method comprising winding a tape of expanded graphite material under tension around a former, and assembling a retaining ring, the cushion and a seat member to form the valve seat assembly.

In either of the methods set out in the last two preceding paragraphs, after the seat member has been mounted on the cushion, opposite end portions of the retaining ring may be turned radially inwardly to form flange portions arranged to retain the seat member and the cushion in the ring.

As will be apparent from the description which follows, there are various ways in which the hard seat member may be mounted in or on the graphite cushion.

Several embodiments of the invention will now be described, with reference to the accompanying drawings, to illustrate the invention by way of example.

In the accompanying drawings:- Figures 1 to 5 show longitudinal cross-sections through five different forms of annular valve seat assembly; Figures 6, 7 and 8 illustrate stages in the manufacture of one valve seat assembly and Figures 9 and 10 show cross-sections through two further valve seat assemblies, in position in valve housings.

All seven seat assemblies illustrated in Figures 1 to 5, 9 and 10 are suitable for bali-type valves, and are in the form of a ring to be received in an annular recess in a valve housing, a rigid annular seat member of each assembly having a frustoconical operative face F against which the spherical valve closure member makes line contact. [In other embodiments of the invention the seat member could be contoured to conform with the valve member over a substantial area.] In all seven examples, furthermore, there is a metal retaining ring of L-section, U-section or C-section, shown at 1. In the examples of Figures 3, 4 and 5 there is also a second metal retaining ring shown at 2.Against the metal ring, or between the two radially spaced metal rings, there is a resilient annular cushion 3 of an expended graphite material, for example that material sold under the Registered Trade Mark 'Graftite', supporting the seat member. The retaining ring, or an outer one of the retaining rings extends around the radially outer periphery of the cushion and (except in the embodiment shown in Figure 1) comprises two axially spaced, radially inwardly projecting, flange portions by which the seat member 4 and the cushion 3 are retained in the ring. In the embodiment of Figure 1, the ring comprises only one flange portion.

The seat member itself, which is in each of the examples of hard carbon, is shown at 4. In each case it has a frusto-conical working face and rests with one rear face or two against the cushion 3.

By virtue of the resilience of the cushion the seat member is able to accommodate tolerances and take up at least some degree of wear, and is furthermore very much less likely to be fractured under extreme conditions than if rigidly mounted.

In Figures 9 and 10 are shown two seat assemblies mounted in an annular recess of a housing of a valve. In these two assemblies, as also is the case in the seat assembly shown in Figure 4, the seat member 4 is supported by a frusto-conical surface of the cushion 3. The seat member 4 in each assembly extends radially inwardly beyond the cushion 3, and the cushion extends axially beyond the seat member in a direction away from the operative face F of the seat member arranged to make sealing contact with the valve closure member 1 0. The seat member 4 is axially spaced from an axially facing annular end face 11 of the recess, and the housing comprises an axially projecting annular knib, in the form of a wall portion 12, bounding the radially inner periphery of the end face 11. The knib projects axially to overlap the seat member 4, to guard against erosion of the cushion 3 in the passage of fluid through the valve. In other embodiments, the knib might be omitted and the gap between the end face 11, of the recess, and the seat member 4 reduced to protect the cushion from erosion. The seat member 4 must in any case be spaced sufficienily from the end face 11 to accommodate deflection of the seat member upon compression of the cushion 3.

One possible method of forming the cushion 3 comprises winding a tape of expanded graphite material under tension around a former. In the valve seat assembly shown in Figure 10 the cushion 3 comprises two portions, a portion 3a made from tape, and a portion 3b made by compression of powdered expanded graphite material. The two portions are adhered together.

The purpose of the portion 3b is to protect against extrusion of the cushion into the space between the seat member 4 and the end face 11 of the recess, as could tend to happen where the cushion is made entirely, as the portion 3a, from tape.

Figure 6 shows a simple form of die for manufacturing a seat assembly similar to that shown in Figure 5. The die is formed by an outer sleeve 5, a core 6 and a ring-shaped end plate 7 to define an annular cavity into which are inserted, and located in suitably machined recesses in the members 5 and 6, inner and outer metal retaining rings similar to the rings 1 and 2 of Figure 5. Then a measured quantity of loose expanded graphite powder 8 is placed in the cavity 5 (see Figure 7) and compressed by a punch 9 until the density of the expanded graphite material is in the range of 0.7 to 1.6 gram/cc.

The resulting assembly of the rings and the formed cushion is then removed, by taking the die apart, and a separately formed hard seat member is inserted between the metal rings, as shown in Figure 8. It is a slide fit between the metal rings, so that it can move under load against the resilience of the graphite cushion, and is retained in place by pressing or spinning over opposite end portions of the metal rings to form flange portions similar to those in Figure 5. The seat member is thus mounted and secured in place supported by the cushion.

Because of the significant structural difference between the expanded graphite cushion and the hard carbon seat member, the former will have a far lower compressive modulus of elasticity than the latter. This would still apply even if pressures higher than 3 tons/in2 were to be used. The compressive elastic modulus of the graphite cushion is preferably in the range 5-500 MPa (typically 50 MPa) whereas a suitable value for the carbon seat member may be 10,000 MPa.

Claims (19)

1. A valve seat assembly comprising a rigid annular seat member of a hard material, for making sealing contact with a closure member of a valve in which the assembly is mounted, a resilient annular cushion of an expanded graphite material supporting the seat member, and a retaining ring extending around the radially outer periphery of the cushion.

2. An assembly according to claim 1 in which the density of the expanded graphite material of the cushion is in the range of 0.7 to 1.6 gram/cc.

3. An assembly according to either one of claims 1 and 2 in which the compressive elastic modulus of the expanded graphite material of the cushion is in the range of 5 to 500 MPa.

4. An assembly according to any one of the preceding claims in which the seat member is of hard carbon.

5. An assembly according to any one of the preceding claims in which the retaining ring is of metal.

6. An assembly according to any one of the preceding claims in which the retaining ring comprises two axially spaced, radially inwardly projecting, flange portions arranged to retain the seat member and the cushion in the ring.

7. An assembly according to any one of the preceding claims comprising a second retaining ring which extends around the radially inner periphery of the cushion.

8. An assembly according to any one of the preceding claims in which the seat member is supported by a frustoconical surface of the cushion.

9. An assembly according to any one of claims 1 to 8 in which the seat member extends radially inwardly beyond the cushion.

10. An assembly according to claim 9 in which the cushion extends axially beyond the seat member in a direction away from a face of the seat member arranged to make sealing contact with a valve closure member.

11. A valve comprising a housing with an annular recess, and a valve seat assembly according to claim 10 mounted in the recess with the seat member axially spaced from an axiallyfacing annular end face of the recess, the housing comprising an axially projecting annular wall portion bounding the radially inner periphery of said end face.

12. A valve according to Claim 11 in which said wall portion projects axially to overlap the seat member.

13. A method of making a valve seat assembly according to any one of claims 1 to 10 comprising locating a retaining ring in a cavity of a die, introducing expanded graphite material in powder form into the cavity, compressing the expanded graphite material in the die cavity to form a cushion, and mounting and securing a seat member so that it is supported by the cushion.

14. A method of making a valve seat assembly according to any one of claims 1 to 10 comprising forming an annular cushion by a method comprising winding a tape of expanded graphite material under tension around a former, and assembling a retaining ring, the cushion and a seat member to form the valve seat assembly.

1 5. A method according to either one of claims 13 and 14 in which after the seat member has been mounted on the cushion, opposite end portions of the retaining ring are turned radially inwardly to form flange portions arranged to retain the seat member and the cushion in the ring.

16. A valve seat assembly substantially as hereinbefore described with reference to any one of Figures 1 to 5.

17. A valve seat assembly substantially as hereinbefore described with reference to either one of Figures 9 and 10.

18. A valve substantially as hereinbefore described with reference to either one of Figures 9 and 10.

19. A method of making a valve seat assembly substantially as hereinbefore described with reference to Figures 6, 7 and 8.