GB2235023A - Shaft seal - Google Patents

Abstract

A valve stem seal has a fixing region 7 with a flange 9 for mounting in a valve housing, and a flexible seal region 19, with an external groove 17 between these regions to permit the region 19 to flex relative to the fixing region 7. The fixing region 7 has a small radial clearance from the valve stem and acts as a bearing for the latter. The region 19 is an interference fit on the valve stem and has internal labyrinth seal grooves 21. The seal may be made of metal or of plastics material such as PIFE. If made of metal, the region 19 preferably has external grooves 23 in a diamond pattern to provide increased flexibility. The region 19 is externally exposed to the pressure of the fluid in the valve housing so that the seal is self-energizing. <IMAGE>

Description

SHAFT SF4L This invention relates to seals for shafts which are essent:il)y static but which may undergo a limited amount of relatively slow movnt. The invention is particularly, but not exclusively, applicable to stem seals for sealing the stems of valves such as gate valves, ball valves, butterfly valves etc, Such seals must be capable of providing completely reliable sealing against the contents of the valve, which may be under very high pressure and may be aggressive. Th valve stem is mostly stationary, but must be able to move longitudinally and/or rotate for opening, closing and adjusting the valve. The valve stem seal should also be able to tolerate a moderate amount of misalignment or distortion of the valve stem.

It has been found difficult to provide a valve stem seal which nieets all of these requirements.

According to the present invention there is provided a seal for valve stems and other members requiring only limited movement, comprising a sleeve designed to embrace the stem, llith a fixation region on the sleeve adapted t be fixed in place for example on a valve housing, and a further region of the sleeve capable of flexing relative to the fixation region, the further region being adapted to seal internally against the stem, preferably by an interference fit and/or a labyrinth seal.

The seal sleeve may be made of metal, particularly for temperatures over above 2600C, or other material e.g.

polyamide, PTFF.

The fixation region of the sleeve preferably serves as a bearing for the stem, The flexible region of the seal sleeve is preferably an interference fit on the valve stem and accordingly should have an essentially perfect internal-surface.

formed by machining and/or coating.

Between the flexible region and the fixation region, the seal sleeve may have a region of reduced radial thickness to permit relative flexing of these regions.

In the case of a seal made of metal or other relatively rigid material, e.g. plastics incorporating glass fibre, bronze, carbon etc., the flexible region is preferably provided with at least one external cut or groove to enhance the flexibility of the said region.

The external cuts or grooves preferably comprise oblique cuts or grooves of opposite hand, for example helical, forming a "diamond" pattern.

The internal surface of the flexible region is preferably provided with labyrinth seal grooves, particularly for valves to be used with high pressure gasses.

Preferably, the sleeve seal and its seating are arranged so that the external surface of the flexible part of the sleeve is exposed to the working fluid, so that the pressure of this fluid will urge the sleeve into more intimate contact with the stem; that is to say, the seal is self-energizing.

Xn embodiment of the present invention will be further described, by way of example only, in the accompanying drawings, in which: Figure 1 is a partly sectional and cut-away view of a valve stem and stem seal.

Figure 2 is a longitudinal sectional view of a PTFE stem seal Figures 3 to 5 are partial Jongitudinal sections showing details and modifications of the PTFE seal.

Figure 1 shows a valve stem 1 which projects from a valve housing 3, and operates a valve member within the housj ng .

The valve stem is sealed within the housing by a seal sleeve 5 which is seated on the outer end of and proje.cts into the valve housing or body 3.

At its outer end the sleeve has a fixation region 7 which fits tightly within the valve body and is provided with an external flange 9 which rests against an end surface of the valve body and is clamped in place by a retaining cap 11 screwed onto the outer end of the valve body. 4 soft sealing gasket 13 is provided between the valve body and the underside of the flange 9.

The fixation region 7 is provided with an internal annular groove 25 which accommodates a sealing ring 15, providing a seal on the external surface of the valve stem. This seal Inay be an elastomer 0 ring.

Alternatively, for applications with a high fire risk, the sealing ring may be a metal 0 ring or self-energizing sealing ring. If a metal sealing ring is to be fitted, it may be necessary to make the sleeve 5 in two parts which meet at the sealing ring groove, these parts being bonded or braid together after the sealing ring has been fitted in the groove Below, i.e. within, the fixation region 7 is an external circumferential groove 17 defining a sleeve region of reduced thickness relative to the rest of tiii sleeve.

The region 19 of the sleeve below, i.e. within, the groove 17 is of smaller thickness of the fixation region 7 and is radially spaced from the internal surface of the valve body. This region can flex relative to the fixation region 7, by virtue of the groove 17, and also has inherent flexibility. flccordingly the region 19 can be described as a flexing region of the seal sleeve.

The outer portion R of the length of the sleeve, comprising the fixation region 7, the groove 17 and an inner region directly adjacent the groove 17, forms a bearing for the valve stem, which locates the valve stem accurately in relation to the valve body. This portion of the length of the seal sleeve is designed to have a very small clearance relative to the valve stein, of the order of 0.02Smm The flexing region 19, within the bearing region R, provides an interference fit on the valve stem, that is to say, with tolerances, it has a diameter very slightly smaller than the diameter of the stem.

The sleeve is of interference fit diameter substantially throughout the length C of the flexing portion 19. This region has a multiplicity of internal circumferentia] grooves 21 which together form a labyrinth seal. This is particularly advantageous when sealing gasses under high prssur: if there is any leakage between the seal sleeve and the stem, there will be a pressure drop at each groove of the labyrinth seal, providing a satisfactory seal over the length C.

The region 19 has in its external surface a pattern of intersecting helical slots 23 of opposite hand, which run out just below the groove 17.

To establish a satisfactory seal between the sleeve and the stem in the interference-fit region C, mirror finishes are required on the external surface of the stem and the internal surface of the sleeve. It is also highly desirable that the innermost surface of the sleeve in this region be a perfect cylinder without machining marks. Accordingly, this region of the sleeve is preferably internally machined by a tool which extends over the entire length C, this tool being fed radially outwards within the sleeve until the required internal sleeve diameter has been obtained.

Finishing of the sleeve bore by a normal turning procedure with a screw feed of the tool is not recoiiiiiiended because of the machining marks which will be left. However, machining in this manner may be acceptable, if the machined surface is then internally coated to eliminate machining marks. Surface coatings suitable for filling any machining cord or surface asperities include gold, silver, nickel, PTFE, and many others.

fit each end, the sleeve has a tapered region reducing to a thin rim 27 adjacent the valve stein surface, these rims forming wipers to remove from the stem, as it moves, any foreign matter which might damage the mating surfaces of the stem and seal sleeve.

It will be seen that the wall thickness of the seal sleeve in the region 19 is substantial in proportion to the diameter of the sleeve bore. n solid metal sleeve of similar proportions could not provide an interference fit on the stem while allowing movement of the stem.

However, the intersecting helical grooves 23 provide substantial flexibility of and within the region 19, such that the elasticity of the material of the seal sleeve enables the sleeve to stretch over the valve stem, and to return to its original smaller diameter if removed, so that there is at all times a resilient interference fit of the sleeve on the stem, It will be seen that each of the diamonds formed by the intersecting grooves 23 stradles more than one of the labyrinth seal grooves 21. This further increases the suppleness or flexibility of the region 19, while retaining great strength without corresponding hoop stiffness which prevents any distortion of the sealing area.It will be understood that a conventional labyrinth seal region, of relatively thin wall section to provide flexibility, would readily be distorted if subjected to a sudden burst uf high pressure gas for example, and in most cases this would lead to seal failure, The present seal sleeve, in contrast, is strong and durable while being flexible and providing an excellent interference fit nevertheless allowing easy movement of the valve stem.

The thickness-reducing groove 17 enables the region 19 to flex, in case there is any misalignment, however small, between the seal sleeve and the valve stem This ensures that there are no load gradients between these two components, It will be understood that the external grooves 23 could be cut to form a different pattern, for example using longitudinal and circumferential cuts.However it has been found that the described diamond pattern using intersecting helical cuts gives particularly advantageous performance, The seal is self-energizing, because the region 19 is spaced from the valve body by an annular radial clearance 29 which is exposed to the working fluid, so that the pressure of the working fluid presses the seal firmly on to the valve stem. The effectiveness of this is greatly enhanced by the suppleness of the region 19.

The working fluid is for example gas under high pressure, for example in oil and gas production systems.

It is important to prevent any risk of scuffing between the stem and the seal when the stem moves. The risk of scuffing can be reduced by using dissimilar materials for the sleeve and stem, for example a seal of aluminium bronze, to operate on a stainless steel stem. The materials used naturally depend on the fluid being handled and on the environment.

In some applications, the stem and seal have to made of the same material. For example, for sub-sea sour well applications, both components will normally be made of Inconel 718 (registered trade mark). In this case, to avoid scuffing it may be necessary to coat one of the surfaces, preferably the valve stem, with a coating of PTFE, molybdenum sulphide, or other suitable material.

A variety of low-friction coating materials is available, to suit particular applications.

The seal described is of metal, and as such is particularly adapted for applications at relatively high temperatures or where other desirable metallic.

properties are required.

For many applications, other materials, in particular PTFE, are acceptable or preferable.

Figure 2 shows a slf-nrgiiing shaft or stem seal, machined from PTFE bar or mouldings. This seal is particularly suited for the sealing of high pressure gases and a wide range of chemicals to which PTFE is inert. PTFE is also resistant to a wide range of temperatures, from 250/300 down to cryogenic conditions.

The PTFE seal shown in Figure 2 has a configuration generally similar to the metal seal shown in Figure 1, but the seal ring 15 and groove 25 may be omitted, and for inmost applications, it will not be necessary to provide a pattern of grooves in the external surface corresponding to the grooves 23 shown in Figure 1. PTFE has much greater inherent flexibility than metal and consequently, in general, it is not necessary to enhance the flexibility of the interference-fit region of the seal by providing such external grooves. On the contrary, because PTFE -is readily compressed, the region 19 may become over-compressed when used in circumstances of extremely high pressure. Over-compression would reduce the performance of the seal or increase the torque necessary to rotate the shaft or stem to an unacceptable level, possibly enough to cause damage to the seal itself. It may therefore be desirable, at least for high-pressure applications, to fit the metal sleeve 31 on the exterior of the region 19.

Ttle composition or formulation of the PTFE sleeve can be varied to provide particular characteristics, for example to provide maximum resistance to abrasion or chemical attack, or to enhance lubrication.

Figure 3 shows a modification of a seal shown in Figure 2, in which one. or both of the wiper lips 27 is replaced by a metal scraper ring 33 set into the end of the sleeve. The thin-section metal ring ensures that any coarse or hard foreign matter that may accumulate on the shaft is wiped away, thus preventing scoring of the shaft and damage. to the seal. The metal. scraper ring may be bonded in place in the seal sleeve.

The seal sleeve is provided with labyrinth seal grooves 21, which may be of rectangular cross section as shown in Figure 4, or flattened saw-toothed cross section as shown in Figure 5.

An incidental advantage of the presence of the labyrinth seal grooves is that the effort required to rotate or axially move the shaft is reduced.

It will be noted that Figures 2, 4 and 5 show clearly the variation in internal diameter between the upper, outer region of the seal, which provides a bearing for the shaft, and the lower or inner region of the seal which is an interference fit. The change in diameter occurs immediately above or outside the labyrinth seal region.

Claims (1)

1. .1. n seal for a shaft or like member, comprising a tubular sleeve adapted to encircle the said member, a fixation region on the sleeve adapted to fix the sleeve in position on a mounting, and a further region of the sleeve, capable of flexing motion relative to the fixation region and adapted to seal internally against an external surface of said member.

   2. A seal as claimed in claim 1 in which the fixation region has an internal bearing surface adapted to act as a bearing for the said member.

   3. X seal as claimed in claim 1 or 2 in which the further region has an internal surface of smaller radius than that of the fixation region, for making an interference fit on the said member.

   4. n seal as claimed in claim 3 in which the further region has an internal surface of true cylindrical form.

   seal as claimed in any preceding claim in which the further region has in its internal surface at least one circumferential groove for prouiding a labyrinth seal against the said member.

   6. A seal as claimed in any preceding claim haying between said regions an intermediate region of reduced thickness permitting said flexing.

   7. Q seal as claimed in any preceding claim hauing in the external surface of the said further region at least one groove extending at least partly longitudinally of the sleeve, to prouide the said region with flexibility.

   8. n seal as claimed in claim 7 in which the groove is or grooues are helical.

   9. ii seal as claimed in claim 7 or 8 hauing said grooues of opposite hand and intersecting.

   10. n seal as claimed in any preceding claim, made of plastics material and prouided with an external metal sleeve embracing the exterior of the said further region.

   11. n seal as claimed in any preceding claim having, at at least one end, a scraper for cleaning the said member.

   12. X seal as claimed in any preceding claim, in which the fixation region is provided with an external flange for axial clamping.

   13. n seal as claimed in any preceding claim further including a seal ring seated in the internal surface of the sleeve in or adjacent the fixation region.

   14. n seal substantially as herein described with reference to any of Figures 1 to 5 of the accompanying drawings.

   15. In combination, a seal as claimed in any preceding claim and a shaft or like member encircled by the scal.

   the diameters of the said member and seal being such that between the said member and seal there is a radial clearance in the fixation region and an interference fit in the further region.

   16. In combination, a seal as claimed in any of claims 1 to 14 and a mounting therefor, there being a radial clearance between the external surface of the seal in the further region and the mounting, for exposing the said surface to the pressure of a fluid within the mounting.

   17. The combination claimed in claim 15, together with a mounting which secures the fixation region, there being a radial clearance between the external surface of the seal in the further region and the mounting, for exposing the said surface to the pressure of a fluid within the mounting.