GB2477141A - A gas turbine including multiple sealing ring packs - Google Patents

Abstract

A gas turbine includes a rotatable first element 11 contained in a bore of a relatively static second element 10 and sealing ring pack 16 located in a groove 17 on the first element 11 to seal against the second element 10. The sealing ring pack 16 comprises a number (n) of active rings 18 and (n + 1) passive rings 19, wherein each active ring 18 is located and retained between a pair of passive rings 19. The side faces of each active ring 18 are in sliding contact with respective pairs of passive rings 19 wherein the groove is dimensioned to allow axial thermal expansion of the rings.

Description

Multiple Sealing Ring Packs The use of sealing rings in series is a well known technique in situations where a circular component such as a piston or carrier has a limited degree of axial movement in the bore of another member which is static relative to the piston and where it is necessary to provide a fluid seal between the annular spaces on either side of the ring pack.

The ring pack normally consists of individual rings which are split at one radial position and are made to be outspringing so that a fluid seal is formed between the circumferential surface of each ring and the bore of the relatively static member. In such an application the rings may be positioned in annular grooves in the piston or, alternatively, may be close packed together in a single wide groove in the piston. The reason for using the sealing rings as so described in series is to provide an extended leak path for the fluid flowing from the high pressure to the low pressure side of the assembly. Increasing the length and the number of the direction changes in the path reduces the leakage rates as is well known.

Particularly in gas turbine engines, where the sealing rings may be operating in relatively high pressure gradients and high environmental temperatures, the combination of these factors with the high frequency vibrations caused by the rotating assemblies may cause severe wear on both the circumferential and radial surfaces of the rings, particularly at the boundary regions where adjacent surfaces of the rings meet and also on the surfaces of the piston or carrier grooves into which the rings are assembled.

The wear can become particularly severe on the radial surface of the groove which is in the lower pressure side of the seal assembly where the pressure gradient generates an axial force on the sealing ring, the force being balanced by the contact pressure set up between the said radial surface and the portion of the ring abutting the surface.

One of the remedial methods of reducing ring wear is to coat or otherwise harden the ring and/or groove and to grind the rubbing surfaces. Apart from the additional costs of these processes when applied to a high precision ring surface, this still leaves the grooves which located the rings unprotected. The grooves are often machined in piston or carrier components manufactured in materials which have unhardened surfaces so the resulting wear on the sealing and retaining surfaces of such components is significantly high when compared to the wear on the adjacent ring surfaces.

The significant improvement in the working lives of the components performing the fluid sealing functions of the engine is the subject of this invention. The sealing members, typically five in number, consist of two outspringing rings which are designated the "active" rings in the pack and three inspringing rings which are designated as the "passive rings".

In the context of this document, the adjectives "active" and "passive" are used to describe the different functions performed by the rings. In the embodiment being described, the outspringing rings are designated active because the circumferential sealing surface of each ring is in moving contact with the bore of the relatively static member when the engine is running. The inspringing rings are normally in static and therefore passive contact with the base circumferential surface of the piston or carrier groove with one end ring of the pack in static contact with the radial wall of the groove which is situated in the relatively lower pressure region of the assembly.

When the ring pack is fitted onto the piston or carrier an inspringing passive ring is first assembled adjacent to a radial flank surface of the groove, the flank being remote from the piston or carrier end from which the rings are being assembled. In applications where the fluid pressure to be sealed is relatively high, it is advantageous to arrange for this radial flank surface to be located on the lower pressure side of the assembly and to extend the diameter of the piston or carrier to at least the same diameter as the inspringing ring so that there is no unsupported region of the ring radial surface which may initiate coning of the ring.

The next ring to be fitted is an active outspringing ring followed by alternative inspringing and outspringing rings until the last inspringing ring is fitted adjacent to, but not necessarily in contact with the near side radial flank surface of the groove. Both the active and the passive rings are preferably made of the same metal alloy material with no hardening surface treatment, for example an iron based alloy with small additions of chrome and/or nickel. Such alloys, when the touching surfaces vibrate one relative to the other, tend to mutually polish the surfaces and it is found that the wear performance is significantly better than when one hardened surface is in dynamic contact with the softer surface of a different alloy.

In operation an inspringing passive ring abuts the radial surface end of the groove which is in the lower pressure side of the piston or carrier and importantly has no significant dynamic relative movement to the groove surface so wear in this region is either very small or completely absent. Each active outspringing ring now has both flank radial surfaces in contact with the flank radial surface of an inspringing ring and these sealing surfaces therefore are completely divorced from contact with any surfaces on the piston or carrier. This feature is the means whereby the significantly improved wear performance of the complete seal assembly is achieved.

It has been found that some of the improved wear performance is due to the fact that, unlike the existing design of ring seals, each active ring in the pack is in contact with a passive ring surface on both flank surfaces. This feature provides significantly more vibration damping between the active and passive rings in the pack than when only one flank surface of an active ring is in contact.

It will be appreciated that the role of the inspringing and outsprinting rings may be reversed in situations where it is necessary to locate the multiple sealing ring pack in a groove machined in the bore of the relatively static component and the relative axial movement is taking place between the ring pack and the cylindrical surface of a piston or similar shaped component. In such an assembly, therefore, the inspringing rings will be acting as the active rings and the outspringing rings, which will normally number one digit larger than the inspringing rings, will be acting as the passive rings of the pack assembly.

Thus from another aspect the invention consists of a gas turbine in which a first element, having limited degrees of movement in both the axial and radial directions, is contained in the bore of a relatively static second element and a third element, is located in an annular space formed by a groove in the first element to form a fitted seal between the first element and a portion of the cylindrical surface of the bore in the relatively static second element; the third element comprising a pack made up of a number of active and passive rings wherein each active ring is in sliding contact with the radial surface of at least one passive ring and in sliding contact with the bore in the second element, the arrangement thereby being such that no radial surface of an active ring contacts the radial flank surfaces of the groove in the first element.

The contacting radial faces of the rings comprising the third element may be of substantially similar hardness and/or the same material and may be unhardened, coated or uncoated.

Preferably the groove is dimensioned to allow thermal expansion of the rings. In one embodiment at least there may be n active rings and at least n + 1 passive rings with each active ring retained between a pair of passive rings.

Although the invention has been described in the foregoing text, it is to be understood that it includes any inventive combination of the features set out or in

the following description.

In situations where the fluid pressure gradient across the ring pack is substantial and sufficient to force the ring pack against the retaining groove radial flank face which is situated on the lower pressure side of the sealing assembly, it may be possible to dispense with the inspringing ring, situated on the higher pressure side of the sealing assembly: this feature enables the groove width to be shortened by a length equal to the thickness of an inspringing ring. This possible saving in space and weight comes about when the pressure difference across the now first pair of outspringing and inspringing rings, multiplied by the rings' combined exposed annular radial face area generates sufficient axial force to overcome the gripping friction force produced by the inspringing ring. The next pair of rings will behave in a similar manner, the total effect being to compress the whole ring pack against the groove end wall situated in the lower pressure side of the seal assembly. Whilst this design is useful where the axial length of the groove is critical, there will be some loss of the damping effect compared to a normal pack because the outspringing ring on the high pressure side of the pack now has only one annular radial face in contact with an inspringing ring face.

In situations where the fluid pressure gradient is variable and may become eliminated for periods whilst the engine is running, the axial forces generated by the fluid may become low or absent and unable to overcome the gripping forces of the inspringing rings whereby some of the sealing and damping effect between adjacent rings may be reduced or lost. In order to deal with this situation, the groove axial length may be extended and a wave spring or similar spring force generating element may be fitted at the normally higher pressure end of the ring pack in order to generate the axial force necessary to overcome the gripping forces produced by the inspringing rings.

A similar situation, requiring a spring force generating element, may occur in a design where the outspringing active rings are reciprocating in alternative axial directions by a substantial amount sufficient to produce significant axial forces generated by the friction between the rings' outer circumferential surfaces being in sliding contact with the bore of the retaining element.

The invention may be performed in various ways, three specific embodiments of which will now be described with reference to the accompanying drawings in which:-Figure 1 is a local sectional view of an existing sealing ring arrangement; Figure 2 is a prior art picture of a sealing ring configuration; Figure 3 is a local sectional view of the first embodiment of the invention; Figure 4 is a local sectional view showing a second embodiment of the invention in which one inspringing ring has been removed on the higher pressure side of the ring pack; and Figure 5 is a local sectional view showing a third embodiment of the invention in which an additional wave spring washer has been added at the higher pressure end of the ring pack retaining groove.

Figure 1 illustrates a known arrangement for providing a fluid seal between a static member 10 in a gas turbine and a cooperating member 11 which may have limited degrees of movement both axially and rotationally relative to the static member and is contained within a circular cavity or bore in the static member.

A pair of outspringing rings 12 are located in respective circumferential grooves 13 on the cooperating member of carrier 11 and are in contact with at least one annular radial face 14 thereof. The designation active is applied to these outspringing rings because their surfaces contacting both the retaining elements have freedom to move relative to both these elements. The grooves 13 are over-wide relative to the rings to allow for thermal expansion and the manufacturing tolerances, each ring being held against a groove end wall by the action of the fluid pressure on each ring's annular face area. The arrows 15 indicate the direction of the fluid leakage flow being restrained by the presence of the rings spanning the annular cavity so that, in the illustration Figure 1 and in the subsequent figures, the higher pressure region is on the right and the lower pressure region is on the left of the figures as drawn.

Figure 2 is a prior art drawing of sealing rings which has no accompanying description. The left hand-most element is apparently a circlip. It will be seen that the arrangement is not acceptable for gas turbine engines because there is no provision at either end of the multiple ring pack for the substantial thermal expansion which would occur when an engine is run.

A first embodiment of the invention is shown in Figure 3 in which a sealing ring generally indicated at 16 is located in a single wide groove 17 on the carrier and comprises a pair of active outsprinting rings 18 which are located between three passive inspringing rings 19. The designation passive is applied to the inspringing rings because, in this assembly, the rings grip the base of the groove and in normal engine running conditions their inner circumferential surfaces and on one abutting annular surface have no relative movement to the groove surfaces. The inspringing and outspringing rings are made preferably of the same material and may not need to be hardened nor coated where adjacent rings come into contact at the regions indicated by number 20 in Figure 3. This arrangement provides good sealing properties without the need for expensive coatings or hardening to overcome the wear problems induced by the vibration of the active outspringing ring surfaces coming into contact with the carrier groove walls as illustrated in the arrangement shown in Figure 1.

A second embodiment of the invention is shown in Figure 4 which may be used in situations where a relatively high pressure gradient always exists across the ring pack whilst the gas turbine engine is running and the resulting pressure difference across the seal is sufficient to always force the ring pack into contact with wall 21 of the carrier groove which is situated in the lower pressure region of the assembly. In this design there is no inspringing ring fitted at the right hand side of the ring pack which is in the higher pressure region of the assembly.

This enables the groove to be shortened axially by a length equal to one ring thickness. However the right hand outspringing ring now has only one annular radial surface in contact with consequent some loss of the vibration damping effect compared to the remaining outspringing ring or rings in the pack which have both annular radial surfaces in contact.

A third embodiment of the invention is shown in Figure 5 which may be used in situations where the pressure gradient across the seal may be insufficient to overcome the friction forces produced by both the inspringing and outspringing rings being in sprung contact with the base diameter surface of the groove and the bore diameter surface respectively. This design may also be used in situations where the pressure gradient across the seal may be temporally reversed, indicated by the dotted arrows 22 in Figure 5. The carrier groove is extended on the normally higher pressure side of the seal and an axial spring member 23 is provided occupying the extended length of the groove.

Typically, the spring member may be in the form of a wave spring washer where the force produced by compressing the washer will be normally adding to the axial force generated by the pressure gradient.

Claims (11)

1. Claims 1. A gas turbine including a rotable first element contained in a bore of a relatively static second element and a sealing ring pack located in a groove on the first element to seal against the second element, the pack comprising a number (n) of active rings and at least (n +1) passive rings wherein each active ring is located and retained between a pair of passive rings such that the side faces of each active ring are in sliding contact with respective faces of its pair of passive rings and wherein the groove is dimensioned to allow axial thermal expansion of the rings.
2. 2. A turbine as claimed in claim 2 wherein the sliding contacting faces are of substantially identical hardness.
3. 3. A turbine as claimed in claim I or claim 2 wherein the sliding contact faces are the same material.
4. 4. A turbine as claimed in any one of the preceding claims wherein the contact faces are unhardened and uncoated.
5. 5. A turbine as claimed in claim I further including an additional active ring for which the only side face contact is with a single passive ring.
6. 6. A seal for a gas turbine including at least 5 sealing members consist of n outspringing or active rings and n+1 inspringing or passive rings wherein each active ring is disposed between and in sliding contact with a pair of passive rings.
7. 7. A seal as claimed in claim 6 wherein there are two active rings and three passive rings.
8. 8. A seal as claimed in claim 6 or claim 7 including a spring for urging the rings into mutual engagement with their flanks abutting.
9. 9. A seal as claimed in claim 6 including an additional active ring for which the only side face contact is with a single passive ring.
10. 10. A seal substantially as hereinbefore described with reference to Figures 3,4or5.
11. 11. A gas turbine including a seal as claimed in any one of claim 6 to 10.