Title of Invention

GAS SEAL ASSEMBLY

Abstract

A gas seal assembly comprises a pair of mutually rotatable sealing members 2 4 and 36, each of which has a front face adjacent a gap between the two members which constitutes a leakage path through the seal assembly. The two sealing member are urged, in use, in a direction to close the gap and a gas bleed arrangement is provided for allowing gas from a high pressure side 16 of the seal to bleed into the gap so as to apply a force tending to separate the members. The gas bleed arrangement comprises a bore 60 extending between the front face and a rear face cf one of the sealing members 36, a generally radial groove 64 formed in the rear face of the sealing member 3 6 and a piare 66 overlying the rear face to cover the groove 64 and thereby define between the rear face of the sealing member and the plate a bleed passage connecting the bore 60 to the high pressure side of the seal. In the invention, the plate 66; is formed as a shim that is sufficiently flexible be deflected by the gas pressure into mating contact with the rear face of the sealing member (36) so as to confine the gas flow of the bleed passage to the radial groove. Figure 1.

Full Text

GAS SEAL ASSEMBLY This invention relates to a non-contacting gas seal assembly for restricting leakage of a fluid about a shaft extending through a housing, such as a turbine or a compressor and in particular gas under low and high pressures. Rotary fluid film seals, also known as non-contacting face seals are usually applied to high speed rotating equipment, where the use of ordinary mechanical contacting face seals is not possible due to excessive heat generation and very rapid wear leading to catastrophic failure. Non-contacting gas seals are also known as dry gas seals of more simply as gas seals. The operation of gas seals relies on a thin film of gas being generated between the two sealing faces; one of which rotates and the other is stationary. The gas film keeps the two sealing faces apart, by maintaining a very small gap between them, thus providing a sealing restriction and avoiding undesirable face contact during operation. At the same time, the gas film cools and lubricates the seal. The sealing gas can either be the process gas from the machine onto which the gas seal is fitted, such as a compressor or s turbine, or it can be supplied form an external source elsewhere in the plant installation. There are a number of ways of accomplishing non-contacting operation. One of the most common includes the application of shallow spiral grooves pattern to one of the faces to generate some hydrodynamic lift and thereby separate the sealing faces. The sealing faces are normally flat and smooth. The face area between the sealing faces is normally called the sealing interface or sealing zone and the clearance between the two that is created and maintained for non-contacting operation is known as the sealing gap or sealing clearance. Separation is achieved by allowing sealing fluid into the sealing gap. In the case of the spiral grooves this is achieved by the grooves pumping the gas in when the face rotates. Separation can also occur due to pressure differential alone. As gas enters the restriction, pressure builds up. This generates a separation force, forcing the two faces apart. Particularly critical to seal operation are the startup and shutdown conditions. It is under these conditions that the sealing gap in a conventional spiral groove is at its minimum and the risk of sealing face contact is at its greatest. It is therefore important to be able to achieve full sealing face separation at very low fluid sealing pressures and low rotational speeds while still maintaining robust non-contacting operation under all other duty parameters within the specified design operating range. It is known to provide gas under pressure to the sealing gap to maintain sealing face separation at low rotational speeds through a gas bleed arrangement which includes a throttled passage that opens into the sealing gap. EP-0 961 059 teaches supplying sealing gas from an external pressurised supply. GE 2 375 148, discloses a gas seal assembly, as set out in the preamble of Claim 1. The seal assembly comprises a pair of mutually rotatable sealing members, each of which has a front face adjacent a gap between the two members which constitutes a leakage path through the seal assembly, the members being urged, in use, in a direction to close the gap. A gas bleed arrangement is provided to allow gas from a high pressure side of the seal to bleed into the gap so as to alcove a force tending to separate the members. The gas bleed arrangement comprises a bore extending between the front face and a rear face of one of the sealing members, a generally radial groove formed in the rear face of the sealing member, and a plane overlying the rear face to cover the groove and thereby define between the rear face of the sealing member and the plate a bleed passage connecting the bore to the high pressure side of the seal, By "generally radial" it is only meant that the groove should have a radial component to connect the rear end of the bore to the high pressure side of the seal. Thus the groove could be a spiral groove. The intended purpose of a gas seal assembly is to prevent uncontrolled escape of gas from the high pressure side of the seal. Because non-contacting operation is achieved in a gas seal by paradoxically allowing some gas to bleed out to maintain a gap between two sealing members, is important for the bleed passage which feeds gas to the gap in order to keep the sealing members apart to be heavily throttled to permit only a controlled amount of leakage. In practice, this requires a bleed passage of very small cress-section, its width being typically only a few microns. In GB 2 375 146, a bleed passage is produced by forming one or more radial grooves in the rear face of the sealing member and covering the rear face with a separate rigid plate. However, if it is the grooves that are to dictate the controlled leakage, it is essential for the clearance between the plate and rear face of the sealing member to be significantly smaller that the grooves. Machining the rigid plate and the rear face of the sealing member to such high tolerances, both m terms of surface finish and flatness is difficult and costly to achieve. With a view to mitigating the foregoing problem, the plate in the present invention is formed as a shim that is sufficiently flexible to be deflected by the gas pressure into mating contact with the rear face of the sealing member so as to confine the gas flow of the bleed passage to the radial groove. The flexibility of the shim removes the need for it and the rear face of the sealing member to be perfectly flat as the shim can deflect to take up any inaccuracy in the flatness of the rear face of the sealing member. It is preferred to urge the shim resiliently against the rear face of the sealing member. A metal spring energised polymer or o-ring may be positioned for this purpose behind the sealing shim in an open groove. Under low pressure operation, the spring force is sufficient re-main taint the plate in contact with the rear face of the sealing member. At elevated pressures, the process gas can be used to clamp the two components together thus maintaining the bleed passage geometry under ail operating conditions. Usually, to achieve an even distribution of the bleed gas over the sealing gap, a gas seal will include several bleed passages, each comprising an axial bore and a radial groove in the rear face of the sealing member. From the axial bores, the gas is preferably distributed over the sealing gap by a series of distribution grooves at predetermined position. The front distribution groove geometry may comprise a series of periodic circumferential grooves. Alternatively, the distribution grooves may be joined together in a continuous circumferential groove at a given diameter. The seal design is hydrostatic in its nature of operation. If desired additional hydrodynamic elements may be added by a series of radial or angled grooves in the front face of one of the sealing members. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a section through a gas seal of the invention, Figure 2 is a view of the rear face of the rotating sealing member in Figure 1, Figure 3 is a view of the front face of the rotating sealing member in Figure 1, and Figure 4 is a view of the front face of the relating seal member of an alternative embodiment of the invention. Fig. 1 is an axial section through a gas seal module designed or seal gases across a range of pressures, from 15 to about 4 50 bar, in a pump, compressor or turbine housing. The drawing shows a gas seal module generally-designated 10 arranged between the shaft 12 and housing 14 of a compressor. The seal is constructed as an assembly that is located at one end of the shaft 12 to prevent gas on the high pressure side 16 of the seal from escaping to the low pressure side 18 which may be under atmospheric pressure. The drawing shows a single seal assembly but it is not uncommon for two such seal assemblies to be arranged axially one behind the other to form a tandem seal. The seal assembly comprises a non-rotating sealing member 2 4 and a rotating sealing member 3 6 which in operation. are separated by a small sealing gap. The gap ensures that the sealing members do not wear but is small enough to limit the escape of gas from the high pressure side 16 to a small bleed that is required to maintain the gap between the two sealing members. The two sealing members 24 and 36 form part of sub-assemblies or cartridges that can be slipped over the shaft 12 and into the end of the housing 14, respectively. The stationary sub-assembly includes an annular retainer 20 sealed relative to the housing 14 by means of a seal 22. As with the other seals to be mentioned below, the seal 22 is preferably a metal spring energised polymer (MSEP) seal, but it is alternatively possible to use an O-ring. A balance diameter sleeve 44 is inserted into the retainer 20 and sealed relative to it by a seal 4 6. The sleeve A A supports the non-rotating sealing member 2 4 while allowing it to more axially. A set of springs 4 0 acts on uhe sealing member 2 4 by way of a pair of rings 42, 4 3 which serve to distribute the spring forces evenly about the circumference of the sealing member 24 and are sealed relative to the balance diameter sleeve 4 4 by means of a further seal 48. The area of the front face of the sealing member 2 4 that faces the sealing member 36 is larger than the area on its opposite side exposed to high gas pressure. This imbalance in the diameters creates a net force that urges the sealing member 24 in a direction to increase the width of the sealing gap. This force is opposed by the springs 40' which act to reduce the gap. The rotating sub-assembly, on the other hand, comprises a sleeve 3 0 that is mounted on the shaft 12 and sealed relative to it by means of a seal 32. The sleeve has a radially projecting collar 3 4 which supports the sealing member 36 that rotates with the shaft 12. The sealing member 36 is held against the support collar 34 by means of a looking sleeve 51 which is secured axially to the end of the sleeve 30. The locking sleeve 50 is slightly spaced from the sealing member 3 6 and a further seal 54 is arranged between it and the sleeve 30. A bleed arrangement as will be described in more detail below allows gas from the high pressure side to bleed from the rear face of the sealing member 3 6 into the sealing gap between the members 2 4 and 36. As so-far described the gas seal is known, as is its principle of operation, from GB 2 375 148. Gas from the high pressure side acts to urge the sealing members 2 4 and 3 6 apart with a force that decreases as the gap increases. At the same time, the springs 40 act to close the gap and the balance of forces is to maintain a small clearance that is sufficient to save wear but is effective in limiting the leakage of the gas from the high pressure side to ambient. To operate effectively, the gas seal recruires a bleed arrangement that accurately controls the gas flow to the gap between the sealing members 24 and 36. This is achieved in the embodiment of the invention shown in Figures 1 to 3 zy forming three axial bores 60 in the sealing member that communicate with three circumferential distribution grooves 62 on the front face of the sealing member (see Figure 3) and with three radial grooves 64 that are etched into the rear face of the sealing member 36 (see Figure 2). A flexible plate 66, also herein referred to as a shim, is urged by an MSEF seal 68 against the rear face of the sealing member 36. A further MSEP seal 70 seals between the sealing member 3 6 and the collar 34. The shim 66 is sufficiently flexible to conform to the accurately machined rear surface of the sealing member 3 6 and to close the grooves. The etched grooves 64 are only a few microns deep and therefore act as an effective throttle tc limit the volume of gas that that can flow to the gap between the sealing members. At low pressures, the spring of the MSEP seal 68 is alone sufficient to maintain the shim 6 6 in sealing contact with the rear face of the sealing member 36. This spring force is supplemented by the gas pressure acting on the shim 6 6 so that it is firmly held in place when the gas pressure increases. The following dimensions have been found to be suitable and are given by way of example. Effective depth of bleed groove 64 : 4-20 microns With of bleed groove 64 : 2-10 microns Depth of distribution groove 62 : 5-200 microns Radial width of distribution groove 62 : 0.5-2 mm. While it would be permissible to secure the shim 66 to the rotating sealing member 36, this is not simple to achieve in practice because the sealing ring is made of engineered ceramic or treated steel. It is therefore preferred for the shim 66 to remain separable from the sealing member 36. The distribution grooves 62 are capable of sustaining unequal pressures and divide the sealing ring 36, into sectors. The pressures in the grooves 62 responds inversely to the gap over the sectors and causes the axial separating force of each sector to respond in an inverse fashion. In that way any inclination of the sealing members 3 6 and 2 4 invokes a restoring moment, The seal design of Figures 1 to 3 relies exclusively on hydrostatic effects to maintain the gap the sealing member 24 and 36. It is alternatively possible, if desired, to supplement hydrodynamically the force acting to separate the sealing members. An embodiment operating in this manner is shown in Figure 4. In Figure 4, three axial bores 60' open Additionally, grooves 61' extending generally radially inwards of the distribution groove 62 ' serve to provide hydrodynamic lift. The grooves 61' may be spiral, as illustrated, or radial. The above described seal has particular benefits when used in compressors and gas turbines which have a shaft diameter in excess of 5 cms. The seal is of a radial design with the process gas being located at the outside diameter of the sealing gap and the atmospheric gas at the inside diameter of the sealing gap. Of course, the converse could equally apply, that is to say the process gas may act on the inside diameter. The ability of the gas seal to maintain a sealing gap-under all conditions, results in extremely low torque during full pressure stops and starts. This in turn eliminates face lock up and subsequent damage to the seal parts. CLAIMS 1. A gas seal assembly (10) comprising a pair of mutually rotatable sealing members (24, 36 i each of which has a front face adjacent a gap between the two members which constitutes a leakage path through the seal assembly (10), the members (24,36) being urged, in use, in a direction to close the gap, and a gas bleed arrangement (60,64) for allowing gas from a high pressure side (16) of the seal to bleed into the gap so as to apply a force tending to separate the members (24,36), the gas bleed arrangement comprising a bore (60) extending between the front face and a rear face of one of the sealing members (36), a generally radial groove (64) formed in the rear face of the sealing member, and a plate (66) overlying the rear face to cover the groove (64) and thereby define between the rear face of the sealing member and the plate a bleed passage connecting the bore (60) to the high pressure side (16) of the seal, characterised in that the plate (66) is formed as a shim that is sufficiently flexible to be deflected by the gas pressure into mating contact with the rear face of the sealing member (36) so as to confine the gas flow of the bleed passage to the radial groove. 2. A gas seal assembly as claimed in claim 1, wherein 3. the groove (64) in the rear face of the sealing member is 4. formed by etching. 5. A gas seal assembly as claimed in claim 1 or 2, 6. wherein the plate (6 6) is separable from the sealing member (36) and is urged into mating contact with the rear face of the sealing member (36) by means of a spring (68) and the gas or. the pressure side of the seal. 4. A gas seal assembly as claimed in any preceding claim, wherein the sealing member (36) formed with a grooved rear face is associated with a rotatable shaft (12) and the second sealing member {24) is associated with a stationary casing (14). 5. A gas seal assembly as claimed in claim 4, wherein 6. said second member (24) is spring (40) loaded to urge it in 7. a direction tending to close the gap. 8. A gas seal assembly as claimed in claim 4 or 5, 9. further comprising a seal ring (46) for sealing between said 10. second member (24) and the casing (14). 7. A gas seal assembly as claimed in any preceding claim, wherein the bore (50) cooperates with formations (62; m said front face to distribute the bleed gas over said front face. 8. A gas seal assembly as claimed in claim 7, wherein said formations (62) comprise one or more generally arcuate or circumferential grooves in said front face. 9. A gas seal as claimed in Claim 7 or 8, wherein the formations ( 62 ' ) in said front face include generally radial grooves (61') serving to generate hydrodynamic lift.

Documents:

3998-CHENP-2006 AMENDED PAGES OF SPECIFICATION 29-10-2012.pdf

3998-CHENP-2006 AMENDED CLAIMS 29-10-2012.pdf

3998-CHENP-2006 ASSIGNMENT 29-10-2012.pdf

3998-CHENP-2006 EXAMINATION REPORT REPLY RECEIVED 29-10-2012.pdf

3998-CHENP-2006 FORM-1 29-10-2012.pdf

3998-CHENP-2006 FORM-18.pdf

3998-CHENP-2006 FORM-3 29-10-2012.pdf

3998-CHENP-2006 FORM-6.pdf

3998-CHENP-2006 OTHER PATENT DOCUMENT 29-10-2012.pdf

3998-CHENP-2006 POWER OF ATTORNEY 29-10-2012.pdf

3998-CHENP-2006 POWER OF ATTORNEY.pdf

3998-CHENP-2006 CORRESPONDENCE OTHERS 25-07-2012.pdf

3998-chenp-2006-abstract.image.jpg

3998-chenp-2006-abstract.pdf

3998-chenp-2006-claims.pdf

3998-chenp-2006-correspondnece-others.pdf

3998-chenp-2006-description(complete).pdf

3998-chenp-2006-drawings.pdf

3998-chenp-2006-form 1.pdf

3998-chenp-2006-form 3.pdf

3998-chenp-2006-form 5.pdf

3998-chenp-2006-pct.pdf