Axial flow stream turbines

1. An axial flow steam turbine comprising:

(a) at least one impulse turbine stage having an annular row of static blades extending between a radially outer static ring and a radially inner static ring and an annular row of moving blades provided with blade root portions;

(b) a rotor drum having a plurality of peripherally extending slots, the blade root portions of the moving blades being held in the peripherally extending slots;

(c) a sealing device acting between the inner static ring and the rotor drum; and

(d) a turbine casing surrounding the impulse turbine stage and having a recess, the outer static ring being axially located in the recess and having greater thermal inertia and greater stiffness than the inner static ring, the outer static ring being capable of a limited radial movement relative to the turbine casing so as to thereby accommodate an out-of-round distortion of the turbine casing relative to the outer static ring,

wherein the rotor drum includes an annular recess axially spaced from the peripherally extending slots and having a radial depth less than that of the peripherally extending slots,

wherein the sealing device extends into the annular recess so that a radial extent of the sealing device is disposed at least partly within an outer envelope of the drum rotor, and

wherein a radially inner portion of the inner static ring projects into the annular recess.

2. The axial flow steam turbine as recited in claim 1 , wherein the at least one impulse turbine stage includes a further annular row of moving blades, wherein the annular row of static blades lies between the annular row of moving blades and the further annular row of moving blades, and wherein the annular recess is axially spaced from the annular row of moving blades and the further annular row of moving blades.

3. The axial flow steam turbine as recited in claim 1 , wherein the outer static ring is cross-key located within the turbine casing to facilitate the limited radial movement of the outer static ring relative to the turbine casing.

4. The axial flow steam turbine as recited in claim 1 , wherein the sealing device includes a plurality of sealing elements.

5. The axial flow steam turbine as recited in claim 1 , wherein the sealing device is carried by the inner static ring.

6. The axial flow steam turbine as recited in claim 1 , wherein the outer static ring includes an axial extension and wherein the annular row of moving blades includes a radially outer moving shroud ring element in sealing relationship with the axial extension of the outer static ring.

7. The axial flow steam turbine as recited in claim 6 , wherein a further sealing device projects from the axial extension of the outer static ring towards the moving shroud ring so as to form a sealing contact with the moving shroud ring.

8. The axial flow steam turbine as recited in claim 1 , wherein the sealing device includes at least one of brush seals and fin-type seals.

9. An axial flow steam turbine comprising:

(a) at least one impulse turbine stage having an annular row of static blades extending between a radially outer static ring and a radially inner static ring and an annular row of moving blades provided with blade root portions;

(b) a rotor drum having a plurality of peripherally extending slots, the blade root portions of the moving blades being held in the peripherally extending slots;

(c) a sealing device acting between the inner static ring and the rotor drum;

(d) a turbine casing surrounding the impulse turbine stage and having a recess, the outer static ring being axially located in the recess and having greater thermal inertia and greater stiffness than the inner static ring, the outer static ring being capable of a limited radial movement relative to the turbine casing so as to thereby accommodate an out-of-round distortion of the turbine casing relative to the outer static ring;

at least one reaction stage following the at least one impulse stage;

wherein the at least one impulse stage includes a plurality of impulse stages,

wherein the at least one reaction stage includes a plurality of reaction stages, and

wherein the impulse and reaction stages have an axially constant inner diameter that is the same as a turbine passage annulus of the steam turbine.

10. The axial flow steam turbine as recited in claim 9 , wherein the impulse and reaction stages share the rotor drum.

11. The axial flow steam turbine as recited in claim 9 , wherein a first diameter of the rotor drum adjacent the inner static ring of the at least one impulse stage is substantially the same as a second diameter of the rotor drum adjacent the inner static ring of the at least one reaction stage.

12. The axial flow steam turbine as recited in claim 9 , wherein the at least one impulse turbine stage includes a first impulse turbine stage preceding a second impulse turbine stage, the first impulse turbine stage being a control stage including at least one steam inlet passage for directing steam onto the annular row of moving blades, and a valve element for controlling a flow of the steam.

13. The axial flow steam turbine as recited in claim 12 , wherein the control stage includes a plurality of steam inlet passages, each inlet passage supplying a corresponding circumferentially extending sector of the control stage, and wherein each inlet passage has a valve for controlling a flow of steam to the corresponding sector of the control stage.