GB2493737A - Turbo-machine automatic thrust balancing - Google Patents

Abstract

A turbo-machine automatically balances its internally generated fluid dynamic thrust forces. A balance piston 5 is located around a downstream end of a rotor shaft 3. A balance chamber 7 receives leakage flow from the turbo-machine during operation thereof, and is in communication with a downstream side of the balance piston. A balance flow return line 8 connects the balance chamber with an inlet 1 of the turbo-machine. A control valve 25 is connected to the balance flow return line and is controlled by rotor shaft proximity/position sensors 30. The balance piston may be cylindrical and disposed within a cylindrical bush 6, with an annular clearance there between to allow leakage flow into the balance chamber. The turbo-machine may be a centrifugal compressor, an axial compressor, a centrifugal pump, a turbo gas expander (turbine), and may comprise two or more stages.

Description

Turbo-machine thrust balancer and rotor positioner.

The invention relates to an automatic thrust balancer and rotor positioning system that eliminates the thrust bearing in turbo machines, The turbo-machine thrust bearing is a very critical component, handling high thrust loads at thrust collar speeds as high as 150 metres per second, which makes it very susceptible to failure. The invention resolves the problem, by providing an automatic thrust balancing system which additionally, always maintains the rotor in the design axial position relative to the stationary diaphragms', without the need for a thrust bearing.

Turbo-machine reliability, performance and safety will be enhanced by the invention.

The invention will now be described by using a multi stage centrifugal compressor as an example and with the aid of the following figures: Figure I.shows a conventional multi-stage compressor, fitted with a double acting thrust bearing for handling bi-directional residual thrust loads.

Figure 2. shows the invention, which automatically balances the internally generated fluid dynamic thrust loads and which maintains the rotor in the design axial position relative to the stationary diaphragms without the need for a thrust bearing.

Figure 3.shows the conventional design practice for thrust balance, resulting in bi-directional residual thrust forces on the thrust bearing.

Figure 4. shows the thrust balance practice used in the invention which incorporates an increased diameter balance piston, resulting in uni-directional residual forces with the control valve hilly open.

A conventional multi-stage compressor with a balance piston 5 designed in accordance with standard design practice and double acting thrust bearing 9, is shown on figure 1. Gas enters at suction pressure at flange 1 and is compressed by impellers 2 driven by shaft 3, Gas exits at discharge pressure at nozzle 4 The impellers discharge the flow into static components called diaphragms 28, the purpose of which, is to convert the kinetic energy of the impeller flow into pressure, before returning the flow to the next impeller. Each impeller diaphragm combination, is called a stage 29 The shaft, impellers, balance piston 5 and thrust disk 10, are assembled together to form a rotor 32.

The diaphragms, balance piston seal 6, inter-stage seals 35 and other static components. are stacked together to form a horizontally split inner casing, comprising top and bottom halves 3 3-34, The rotor is installed in the bottom half inner casing 34, the top half 33 then encloses the rotor at the centre line joint, to form a complete inner assembly called a bundle 16.

To complete the compressor unit, the bundle is inserted in the outer barrel casing 11 and is closed and statically sealed by an end cover 13. The shaft is supported by journal bearings 36 and is sealed as it protrudes through the easing and end cover, by dry gas seals 14.

The residual thrust forces on the rotor are handled by a double acting thrust bearing 9.

A balance piston 5, is attached at the discharge end of the shaft and performs the following functions; * Controls the leakage flow from discharge to suction pressure by means of a small annular clearance between the balance piston and a bush 6 fitted in casing 11 Leakage flow enters the balance chamber 7 whence it is returned to suction by the balance flow return line S * Provides a thrust force which balances the internally generated forces of the impellers.

Conventional design practice is to finely balance the thrust forces at rated conditions, so that only a very small residual force is transferred from the rotor by way of the thrust collar onto the active thrust bearing 17. The design point is noted as Td on figure 3 Considerable variation in residual thrust occurs according to changes in compressor flows, pressures and turbo-machine condition as illustrated by the operating envelope abcd on figure 3.

Referring to figure 3, it can be seen that the residual thrust forces act bi-directionally for different process and machine conditions which can result in high loads on both active 1 7 and in-active 15 thrust bearings.

Figures 2 and 4. illustrate the invention, which represents a radically change in traditional design practice, by increasing the diameter the balance piston in order to over-compensate for the impeller thrusts for all process and machine conditions, This means, that the balance piston force can be decreased to exactly equal the impeller thrust force, by increasing the pressure in balance chamber 7 with a control valve 25, The unique feature of the invention, is that by increasing the diameter of the balance piston, the residual thrust force,with a fUlly open control valve 25, is always uni-directional for all conditions.( figure 4 operating envelope abcd) This feature, enables the control valve to increase the balance chamber pressure to reduce the balance piston force, so as to always balance the impeller and balance piston forces, thereby eliminating the residual thrust for all conditions (A-A controlled thrust on figure 4.) Proximeter probes 30, are used to control the rotor axial position relative to the diaphragms, sending signals to the rotor position controller 31, which conveys a signal to control valve 25.

The rotor can thus be controlled in its design axial position relative to the diaphragms, and with the elimination of residual thrusts.

The thrust bearing and the associated problems can therefore be eliminated.

Apart from enhanced reliability, additional advantages are reduced power losses and a smaller lubricating oil system.

Claims (1)

1. <claim-text>Claims.A turbo-machine structured for automatic balancing of fluid-dynamic thrust forces generated within the machine and for the axial positioning of the rotor 32 in the machine by automatic means, comprising: -a balance piston 5 of increased diameter compared to conventional design practice, arranged around the discharge end of the shaft 3 of the turbo machine for counter balancing thrust force generated by the impellers 2 and ensuring that residual forces on the rotor 32 are uni-directional.-a balance chamber 7 structured for receiving the discharge flow emanating from the turbo-machine during operation thereof, said balance chamber 7 communicating with the downstream side of the balance piston; -a balance flow return line 8 connecting the the balance chamber 7 to the suction side of the turbo -machine and for conveying said leakage flow; and -a control valve 25 connected to the balance flow return line 8 for regulating the balance chamber pressure, characterized in that the control valve 25 is connected to a rotor positioner controller 31 for regulating the degree of opening of the control valve and -wherein the rotor position controller 3 I is connected to,and receives control signals, from at least one proximeter probe 30 targeted on the shaft 3 structured to sense and forward the position of the rotor; thereby allowing the control valve 25 to increase the balance chamber pressure Pb and reduce the counter thrust force of the balance piston until it exactly equals the fluid-dynamic thrust forces of the impellers 2 for all operating and machine conditions.</claim-text> <claim-text>2. The turbo-machine according to claim I, characterized in that the said balance piston 5 is cylindrical and is disposed within a cylindrical bush 6,so as to provide an annular clearance between the balance piston bush6,the clearance of which is capable of passing the leakage flow through the annular clearance and into the balance chamber 7.</claim-text> <claim-text>3. The turbo-machine according to claims I or 2, characterized in that the turbo machine is comprised of a centrifugal compressor.</claim-text> <claim-text>4, The turbo machine according to claims I, or 2 characterized in that the turbo machine is comprised of an axial compressor.</claim-text> <claim-text>5. The turbo-machine according to claims l,or 2 characterized in that the turbo-machine is comprised of a centrifugal pump.</claim-text> <claim-text>6. The turbo-machine according to claim l,or 2 Characterized in that the turbo-machine is comprised of a turbo gas expander.</claim-text> <claim-text>7. The turbo-machine according to claim 3,4,5 or 6 Characterized in that the turbo-machine is comprised of two or more stages.</claim-text>