GB2409002A

GB2409002A - Thrust balance piston fitted between high and low pressure paths in a turbine.

Info

Publication number: GB2409002A
Application number: GB0328266A
Authority: GB
Inventors: Chris Briggs
Original assignee: Siemens Power Generation Ltd; Siemens PLC
Current assignee: Siemens PLC
Priority date: 2003-12-08
Filing date: 2003-12-08
Publication date: 2005-06-15
Also published as: WO2005059314A1; GB0328266D0

Abstract

A rotor is provided for use in a turbine having high and intermediate pressure blade paths. A balancing piston arrangement is provided between the blade paths. The rotor is particularly useful for retrofitting existing turbines.

Description

A Turbine Rotor and turbine This invention relates to a turbine rotor and

a turbine arrangement in particular a steam turbine arrangement.

Steam Turbine designs are classified into two main types depending on the fundamental blading principle applied. Impulse turbines (also known as Low- reaction turbines) use impulse blading where the pressure drop across a blade stage occurs over the stationary blading. Reaction turbines use reaction blading (typically 50% reaction) where the pressure drop occurs over the stationary and moving blades.

Steam Turbines comprise a number of elements, typically a high pressure (HP) element, an intermediate pressure (IP) element, and a low pressure (LP) element.

Each turbine element comprises a bladepath made up of a number of blade stages. Each blade stage is comprised of a stationary blade row and a moving blade row.

The stationary blade rows of each element are mounted within casings or carriers and the moving blade rows are mounted on the turbine rotor. In addition to holding the stationary blades, the casings or carriers also form pressure partitions either between different pressure zones within the turbine, or between the internal steam pressure and the external atmospheric conditions.

The turbine elements are arranged to form a turbine train comprising each of the previously mentioned elements. However, the configurations in which the elements are arranged vary widely, depending upon the size and application of the design, the design philosophy employed, the age of the unit, and the particular turbine manufacturer.

This invention applies specifically to a turbine configuration known as a combined high pressure (HP) / intermediate pressure (IP) turbine. In a combined HP/1P turbine the HP and 1P bladepaths are mounted on a single rotor and contained within the same outer casing. In most cases the two bladepaths are arranged so that the inlets to each are in the centre of the rotor, and the steam flows are in opposite directions toward each end of the rotor. Gland seals are located in the centre of the rotor to form a pressure partition between the HP and 1P inlet pressures, and also at the ends of the rotor to form a pressure partition between HP exhaust pressure and atmospheric pressure, and IP exhaust pressure and atmospheric pressure.

In all steam turbines, axial thrust forces act on the turbine rotor. These thrusts are generated by the steam pressure acting upon the axial faces of the rotor and bladepath(s). Forces acting on individual sections of the rotor can be large, depending upon the magnitude of the steam pressure and the area of the axial face, and if not balanced by opposing forces acting on other sections, the resulting net thrust acting on the total rotor will be excessive. One requirement of a turbine design is that all the thrust forces acting on a rotor are balanced under all operating conditions, so that the resultant net thrust is controlled within reasonable limits. This practice is known as 'thrust balancing'. The small net thrusts are then constrained by means of a thrust bearing which is located somewhere along the turbine train.

For Impulse turbines, where the pressure drop over each blade stage occurs predominantly over the stationary blade rows, the axial thrust forces acting on a rotor are typically small, and thrust balancing is relatively straightforward. On the other hand, for reaction turbines where the pressure drop over each stage occurs over both the stationary and moving blade rows, the forces acting on the rotor are much larger. In this case it is necessary to balance the forces produced in the bladepaths by means of 'balance pistons' (also known as 'dummy pistons'). A balance piston is a disk machined onto the rotor body at a predetermined diameter which provides additional axial faces by which the bladepath thrusts can be balanced. Gland seals are mounted on the circumferential faces of these pistons and seal against a corresponding face of a stationary component, for instance an inner casing or 'dummy carrier'. These seals provide for a pressure drop across the balance piston, therefore enabling a net thrust to be produced which balances the thrust produced by the bladepaths.

For a reaction-bladed combined HP/IP turbine, where the HP and lP flows are arranged as described above, three (3) balance pistons are required to achieve satisfactory thrust balance under all operating conditions. In previous designs, either one or two of these balance pistons have been positioned at the HP exhaust end of the rotor, and require a dummy carrier to be mounted in the casing at this position. This is shown in prior art figure 1.

According to the invention there is provided a rotor for a turbine having a high pressure inlet and an exhaust outlet which rotor comprising a number of blades arranged in High Pressure and Intermediate Pressure bladepaths to be driven by the input pressure to rotate the rotor and wherein balancing pistons are provided on the rotor at a position between the HP and lP bladepaths.

The invention also provides a turbine including a rotor in accordance with the invention.

A specific embodiment of the invention will be described, by way of example only, with reference to the figures in which: Fig. 1 shows a typical conventional reaction-bladed combined HP/IP turbine with balance piston adjacent to HP exhaust. (Note: Longitudinal section with HP exhaust at right-hand side); and Fig.2: shows a rotor and a turbine in accordance with the invention with all balance pistons arranged in centre of rotor.(Note: Plan View with HP exhaust at left-hand side).

As is shown in figure 2, the invention is a novel arrangement of the balance pistons where all the pistons are located in the centre of the rotor, between the HP and IP bladepaths. The arrangement (Fig.2) comprises an HP balance piston, adjacent to the HP inlet, which balances the thrusts generated by the HP bladepath; an IP balance piston, adjacent to the IP inlet, which balances the thrusts generated in the IP bladepath; a centre gland which provides a pressure partition separating the HP and IP sections. Additionally, the arrangement comprises HP and IP dummy return pipes: the HP dummy return pipes connect the pressure zone between the outlet of the HP balance piston and the centre gland with the HP bladepath exhaust pressure; the IP dummy return pipes connect the pressure zone between the outlet of the 1P balance piston and the centre gland with the IP bladepath exhaust pressure.

This invention has particular application when retrofitting a reactionbladed combined HP/IP design into an existing impulse-bladed combined HP/IP, as it avoids the need to locate any balance pistons at the HP exhaust end of the rotor, and therefore allows the existing outer casing to be re-used.

Claims

WHAT IS CLAIMED IS: 1. A rotor for a turbine having a high pressure inlet

and an exhaust outlet which rotor comprising a number of blades arranged in high pressure and intermediate pressure blade paths to be driven by the input pressure to rotate the rotor and wherein a balancing piston is provided on the rotor at a position in use between the high pressure and the intermediate pressure bladepaths.
2. A rotor as claimed in claim 1 including a circumferential sealing surface disposed between the balancing pistons to use engage an annular sealing gland.
3. A rotor for a turbine substantially as herein before described with reference to the drawing.
4. A turbine including a rotor as claimed in any proceeding claim.
5. A turbine as claimed in claim 4 wherein the sealing gland comprises a radially innermost annular sealing surface to engage the circumferential sealing surface on the rotor.
6. A turbine as claimed in claim 5 wherein the sealing gland has a radially outwardly extending annular portion of axially extending width less than the axially extending width of the annular sealing surface.
7. A turbine substantially as herein before described with reference to and as illustrated by the drawing.