Patent Application
20120163742
The present invention relates to a gas bearing for rotors in rotating machinery combining the properties of a balance piston and thrust bearing disk.
A conventional rotor for rotating machinery such as a compressor, for instance, is supported by oil lubricated bearings. The bearings are located in atmospheric bearing housings. Therefore the bearings must be separated from the compressor impellers exposed to gas at high pressure by dry gas seals.
Turbo machinery with dedicated bearing, balance piston and seals has existed more than 100 years. Common to all are the requirement for complex and vulnerable support systems.
Radial and thrust bearings used in turbo machinery are bearings typically having shoes or pads on pivots. When the bearing is in operation, the rotating part of the bearing carries fresh oil in to the pad area. Fluid pressure causes the pad to tilt slightly, thereby building a wedge of pressurized fluid between the shoe and the other bearing surface. The pad tilt adaptively changes with bearing load and speed. Various design details ensure continued replenishment of the oil to avoid overheating and pad damage.
Due to the pressure rise developed through the impeller, a pressure difference exists across the hubs and covers involving the impellers have a net thrust in the direction of the compressor inlet. This effect is counteracted by means of a balance piston, see
An important step in the direction against an improved solution is shown by WO-A1 2008/018800 disclosing a combined bearing system wherein the rotor is provided with radial bearings and associated seals. Each radial bearing and sealing point for the rotor is in the form of a bearing and seal combination being formed of a stator located within the machinery housing, and the stator is formed with a bore.
By providing an axial bearing in the form of a cylindrical disk/impeller on the rotor resting against an associated portion of the stator, a gas film can be formed with rigidity and damping according to the same principle as in a radial gas bearing with desired dynamic rigidity and damping. Alternatively, the axial bearing can be formed according to the hydrostatic principle, which involves a flow restriction before and after the bearing surface, so as to obtain rigidity with accompanying damping. The axial bearing can also be formed using a combination of the two principles.
The main objective of the present invention is to replace conventional balance piston and thrust bearings with a simplified axial gas thrust bearing wherein the axial movement in the shaft reduces the distance between the rotating disk/impeller and the stationary radial wall, i.e. fluid forces increases that stops further shaft movement in the axial direction. The radial length of the axial thrust bearing and, thus, the gap between stator and disk, is depending on the pressure ratio against the machine, and the radial location of the gas bearing area, creating fluid forces, should be optimised further with respect to frictional losses and load capability.
This objective is achieved by an axial bearing for rotors in rotating machinery, wherein the bearing is comprising a at least one radial disk, integral with or fastened to the rotor and one fixed seal facing each disk or two fixed seals positioned to surround each disk, lower portions of the seals being situated in distance from the rotor to allow the inflow of compressed fluid passing in the gap between the respective disk and seals, thereby combining the properties of a balance piston and thrust bearing disk.
The air gap geometry formed inside the axial gas bearing include but not limited to, a) converging, b) diverging, and c) parallel. Further, the gas supplied to the air gap in the axial gas bearing may come from the radial inner or outer side of the bearing. It is also to be understood that the term “disk” shall include an impeller and the like.
The rotating disk or stator roughness can be in the form of different special defined geometries, such as but not limited to pockets, honeycomb (HC) or hole pattern (HP), not illustrated.
There are multiple configurations possible:
Other favourable aspects of the present invention are to be understood from the dependent claims and the discussion below.
The rotor can be shorter and stiffer giving rise to better rotor-dynamic performance and/or shorter and thinner involving weight savings. Conventional centrifugal gas compressors or compact hermetically sealed motor driven compressors are two useful but not the only application wherein the invention is expected to have advantages.
The present invention will now be discussed in more detail with the aid of preferred illustrative embodiments shown in the drawings, in which:
Although a compressor is mentioned in the discussions here, all other forms of rotating machinery are applicable such as pumps, turbines and expanders wherein a fluid, for instance gas, is to be given an increased or reduced pressure.
The axial bearing requires pressure differential to function. An arrangement for start/stop may therefore be required. This could be achieved by aerostatic action pulling gas from an accumulator or by use of a reduced capacity back-up bearing of suitable type, not illustrated.
The present invention is disclosing a axial gas thrust bearing for rotors 4 in rotating machinery, wherein the bearing comprises at least one radial disk 5, integral with or fastened to the rotor 4 and one fixed seal 2 facing each disk or two fixed seals 2 positioned to surround each disk. The lower portions of the seals are situated in distance from the rotor to allow the inflow of compressed fluid passing in the gap between the respective disk and seals. Thus, the inventive concept is combining the properties of a balance piston and thrust bearing disk into only one component. This new concept conduct the work required identical to the old solution but with less space and no support system.
Thus, it is presupposed use of a standard type of radial disk 5, or impeller wheel as mentioned above, with a plane surface face to face with the respective seal, or the disk can alternatively be enhanced with a groove type of disk to obtain a higher load capacity in axial direction, not illustrated. To ensure the gas high pressure being divided equally to both sides, the radial disk itself can contain at least one balanced hole 6, thereby allowing for equal pressure between the two sides of the disk. As shown in
In
The gas with increased pressure from the compressor is entering a radial seal 2. In a favourable configuration of such seals, the rotating disk 5 should be smooth whereas the stator surface should be rough to reduce leakage and enhance dynamic coefficients of stiffness and damping. The stator roughness can be in the form of honeycomb (HC) or hole pattern (HP) tapered seal, not illustrated. As depicted in
When the gas flows across the seal surface a pressure force is generated in the axial direction producing stiffness and damping. This stiffness and damping is trying to maintain a centre shaft position between the two seals. When the gas has left the exit of the two radial 2 seals it return backs to suction 3 of the compressor as a normal compressor balance piston system.
If extreme thrust forces are ongoing it is possible to balance the bearing by applying a longer radial length for the radial HP or HC seals 2 in the direction requiring additional force, i.s active or passive thrust. The distance between the rotor 4 and the lower end of at least the seal facing the impellers can also be varied to adjust the inflow of gas along the sides of the disk 5.
Thus, the present invention uses gas forces generated between a rotating disk and two radial seals to balance a turbo compressor in axial direction. The integrated solution provides thrust bearing properties, i.e. stiffness, damping and load capacity, between the disk and seals. The balance in axial direction can be achieved by adjusting one of the radial seals to provide more or less bearing properties.
By moving the balance piston from leaking in axial direction to radial a significant positive effect is expected in term of stability of the rotor i.e. rotor dynamics effect. The shaft length is reduced drastically which is favouring for critical speed and compact machines. The machinery is not sensitive to radial vibration because the seal is located in axial direction, normally radial seals get damage over time. Due to the disk length it is expected a considerable amount of load capacity in this specific design.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20092379 | Jun 2009 | NO | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/NO2010/000237 | 6/22/2010 | WO | 00 | 3/9/2012 |
