THIS INVENTION relates to a gas turbine. More particularly it relates to a method of operating a turbine and to a gas turbine.

In gas turbine machines the flow of working fluid from an upstream end of a turbine rotor towards a downstream end of the rotor exerts a substantial axial load on the rotor and on the turbine shaft. This is as a result of differences on the axial forces on the faces of the turbine rotor as well as blading reaction forces. This situation is exacerbated when the rotor is mounted for rotation about a vertical axis where the axial gas loading on the turbine is combined with the weight of the rotor.

According to one aspect of the invention there is provided a method of operating a turbine having a rotor, at least part of which is mounted for rotation in a housing and an electromagnetic bearing supporting the rotor, which method includes the steps of feeding high pressure gas into a counterbalancing chamber positioned adjacent a downstream end of the rotor to counterbalance axial loading on the rotor and reduce loading on the electromagnetic bearing; inhibiting leakage of gas from the counterbalancing chamber by means of a pressure balancing seal which includes two relatively

displaceable seal components, one of which is fixed to the rotor for rotation therewith; and using the electromagnetic bearing to regulate the clearance between the seal components of the pressure balancing seal.

In the interests of efficiency it is desirable that the volume of high pressure gas being fed to the counterbalancing chamber be minimized.

However, to avoid excessive wear it is important that the relatively rotating seal components do not come into contact with one another.

At the same time to maximize the efficiency of the seal it is desirable that the clearance or spacing between the relatively rotating seal components be relatively small. This matter is complicated by the fact that the turbine shaft may not run perfectly true about its axis of rotation. In addition to both axial and radial movement the longitudinal axis of the turbine may precess.

The method may include monitoring the relative positions of the seal components and adjusting the bearing to maintain a desired spatial relationship between the seal components.

According to another aspect of the invention there is provided a gas turbine which includes a housing; a rotor, at least part of which is mounted in the housing, for rotation about an axis of rotation; an electromagnetic bearing supporting the rotor; a counterbalancing chamber positioned adjacent to a downstream end of the rotor and into which high pressure gas can be fed to

counterbalance axial loading on the rotor and hence reduce loading on the bearing; and at least one pressure balancing seal for inhibiting leakage of gas from the counterbalancing chamber, the seal including two relatively displaceable seal components one of which is fixed to the rotor thereby permitting the spacing between the seal components to be controlled by the electromagnetic bearing.

The pressure balancing seal may be a labyrinth seal, one of the seal components being rotatable together with the rotor and the other seal component being fixed.

The electromagnetic bearing may include a fixed component and a rotatable component which is secured to the rotor, the fixed seal component and the fixed component of the electromagnetic bearing being fixed relative to one another.

As a result, any adjustment of the relative positions of the components of the electromagnetic bearing results in a corresponding adjustment in the relative positions of the seal components. In this way, the relative positions of the seal components, i. e. the seal clearance, can be actively controlled by controlling the electromagnetic bearing.

The seal may be provided at a lower end of the turbine to inhibit the too rapid escape of high pressure gas used to counterbalance downward forces on the rotor due to gas loading and weight of the rotor and thereby reduce the load which is supported by the electromagnetic bearing.

The rotor may include a shaft, the displaceable component of the electromagnetic bearing being in the form of a thrust disc secured to the shaft and on which electromagnets mounted on the fixed component of the electromagnetic bearing act.

In one embodiment of the invention the fixed seal component may be formed by or connected to a surface of the housing.

In another embodiment of the invention the seal components may be formed or connected to the components of the electromagnetic bearing.

The gas turbine may include control means for controlling the operation of the electromagnetic bearing.

The turbine may include sensing means for sensing the relative position of the seal components, the control means being linked to the sensing means and operable in response to signals received from the sensing means.

The turbine may be drivingly connected to an electrical generator.

According to another aspect of the invention there is provided a nuclear power plant which includes a closed loop power generation circuit including a high temperature gas cooled reactor and at least one gas turbine as described above.

The turbine may be located inside a pressure vessel containing helium.

The turbine may be of the pebble bed type containing a plurality of spherical fuel elements.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings.

In the drawings, Figure 1 shows schematically a gas turbine in accordance with the invention; Figures 2 to 5 show schematic sectional elevations, of lower end portions of gas turbines illustrating different seal arrangements.

In Figure 1 of the drawings, reference numeral 10 refers generally to part of a gas turbine in accordance with the invention. The turbine 10 includes a housing or casing 12 within which a rotor 14 is mounted for rotation about a generally vertical axis of rotation 16.

The rotor 14 includes a turbine shaft 18 on which turbine blades 20 are mounted. A counterbalancing chamber 17 is positioned adjacent a downstream end of the rotor 14 and a pressure balancing seal 19 is provided for inhibiting excessive leakage of gas from the counterbalancing chamber 17.

The gas turbine 10 further includes an electromagnetic thrust bearing, generally indicated by reference numeral 26. Referring now to Figure 2 of the drawings, the thrust bearing 26 includes a displaceable component in the form of a thrust disc 28 which is secured to the turbine shaft 18. The thrust bearing 26 further includes a stationary

component or stator 30 which houses a plurality of electromagnets'32 which act on the thrust disc 28.

The seal 19 is in the form of a labyrinth seal and includes an annular seal component 22 which is mounted on the rotor 14 for displacementtherewith. A complementary stationary seal component 24 is secured to the housing 12. Electronic control means (not shown) is provided to control the operation of the electromagnets 32. Further, if required, a sensor 34 is provided to sense the relative positions of the seal components 22,24. The sensor 34 is linked to the control means such that the control means is operative in response to signa ! s received from the sensor 24.

In use, high pressure gas is fed into the counterbalancing chamber 17 at a lower end of the rotor 14 to act on the rotor and counterbalance downward forces on the rotor as a result of the weight of the rotor and the forces applied to the rotor by the working fluid. The thrust bearing 26 supports the rotor 14. In view of the fact that the seal component 22 and the thrust disc 28 are relatively fixed and the seal component 24 and the stator 30 are fixed relative to one another, any adjustment of the position of the thrust disc 28 relative to the stator 30 results in a corresponding adjustment of the position of the seal component 22 relative to the seal component 24. Hence, by sensing the relative positions of the seal components 22,24 via the sensor 34 and feeding the information from the sensor to the control means for the thrust bearing 26, the thrust bearing can be used as an active control system for controlling the clearance between the seal components 22,24 to optimise the operation thereof.

In Figures 3 and 4 of the drawings, reference numerals 40 and 50 refer generally to parts of other gas turbines in accordance with the invention and, unless otherwise indicated, the same reference numerals used above are used to designate similar parts. The main differences between the gas turbines 10,40,50 is in the positions of the seal components 22,24. Hence, in the gas turbines 40,50 the seal component 22 is provided on a seal disc 42 which is secured to the turbine shaft 18. The other seal component 24 is provided by an annuiar protrusion which protrudes radially inwardly from the housing 12.

In Figure 5 of the drawings, reference numeral 60 refer generally to part of another gas turbine in accordance with the invention and, unless otherwise indicated, the same reference numerals used above are used to designate similar parts. In this embodiment of the invention, the pressure balancing seal 19 is formed integrally with the thrust bearing 26. More particularly, the seal component 22 is provided on the thrust disc 28 and the seal component 24 is provided on the stator 30.

It will be appreciated, that the positions of the seal component 22, 24 can be varied as desired.

The Applicant is aware that, previous attempts at actively controlling the relative positions of the seal components 22,24 involved providing the seal components 22,24 with separate electromagnetic adjustment means which operated directly on one or both of the seal component.

By making use of the thrust bearing to actively control the relative positions of the seal component the separate electromagnetic adjustment

means can be omitted. This in turn permits the length of the turbine shaft to be reduced with substantial benefits for the roto-dynamics of the turbine.

The seal arrangement of the invention permits active control of the sealing gap. This will be advantageous during start-up when the gap can be opened to protect the seal from transient displacements. As soon as the rotor reaches operating conditions the gap can be closed under control conditions to enhance the efficiency of the seal. The same considerations apply to thermal expansion, where the axially dynamic sealing face can be controlled to compensate for any axial movement of the rotor.

The inventor further believes that the invention will permit the use of a larger seal face area than that which was available making use of the prior art. This in turn will lead to an increase in the sealing efficiency.

In addition the Applicant believes that the present invention will have substantially fewer components than were used in the prior art which has reliability and cost benefits.