Turbine diaphragm construction

1. An axial flow turbine diaphragm including an annular array of blade units, each blade unit comprising:

an aerofoil;

radially inner and outer platforms integral with the aerofoil, the radially inner platform having a segment of the inner diaphragm ring and the radially outer platform having a segment of the outer diaphragm ring, at least the outer ring segment including engagement features that mechanically engage with complementary engagement features on neighbouring outer ring segments in the annular array of blade units, the engagement features acting to interlock neighbouring outer ring segments and produce a self-supporting turbine diaphragm, wherein the engagement features on the outer ring segment of each blade unit include hook features on both circumferentially facing sides of the outer ring segment that engage with complementary features on neighbouring outer ring segments of adjacent blade units, the hook features including radially extending grooves and being oriented to maintain axial location of each blade unit relative to its neighbours;

wherein the engagement features on the outer ring segment of each blade unit include tongue and groove features that engage with complementary features on the outer ring segments of adjacent blade units, the tongue and groove features being oriented to maintain a radial location of each blade unit relative to its neighbours, and the tongue and groove features comprise:

a groove on a circumferentially facing first side of the outer ring segment, the groove being formed as a gap between a radially outer part of a corresponding hook feature and a radially outer circumferentially projecting lip portion of the outer ring segment; and

a circumferentially projecting tongue projecting from a circumferentially facing second side of the outer ring segment in exact opposition to the groove on the first circumferentially facing side.

2. An axial flow turbine diaphragm according to claim 1 , in which the inner ring segment of each blade unit also comprises engagement features that mechanically engage with complementary features on neighbouring inner ring segments in the annular array of blade units and that are operative to produce a self-supporting turbine diaphragm in cooperation with the engagement features on the outer ring segments.

3. An axial flow turbine diaphragm according to claim 2 , in which the engagement features on the inner ring segment of each blade unit comprise hook features having a radially extending groove that engage with complementary hook features on neighbouring inner ring segments of adjacent blade units, the hook features being oriented to maintain axial location of each blade unit relative to its neighbours.

4. An axial flow turbine diaphragm according to claim 3 , in which the hook features are a first hook, formed by the radially extending groove proximate the pressure side of the aerofoil, and a second hook, formed by the radially extending groove proximate the suction side of the aerofoil.

5. An axial flow turbine diaphragm according to claim 4 , in which the radially inner sides of the radially inner ring segments are configured as a seal, or are configured to retain a seal, such seal being operative to restrict leakage between relatively high and low pressure sides of the diaphragm.

6. An axial flow turbine diaphragm according to claim 3 , in which the radially inner sides of the radially inner ring segments are configured as a seal, or are configured to retain a seal, such seal being operative to restrict leakage between relatively high and low pressure sides of the diaphragm.

7. A blade unit for an axial flow turbine diaphragm according to claim 3 .

8. An axial flow turbine diaphragm according to claim 2 , in which the radially inner sides of the radially inner ring segments are configured as a seal, or are configured to retain a seal, such seal being operative to restrict leakage between relatively high and low pressure sides of the diaphragm.

9. A blade unit for an axial flow turbine diaphragm according to claim 2 .

10. An axial flow turbine diaphragm according to claim 1 , in which the radially inner sides of the radially inner ring segments are configured as a seal, or are configured to retain a seal, such seal being operative to restrict leakage between relatively high and low pressure sides of the diaphragm.

11. A blade unit for an axial flow turbine diaphragm according to claim 1 .

12. A method of assembling the turbine diaphragm of claim 1 , comprising:

(a) producing the individual blade units to their final shape;

(b) placing a first blade unit on a flat surface ready for coupling with further blade units;

(c) sliding a second blade unit axially into engagement with the first blade unit and the flat surface so that engagement features on the outer ring segment of the second blade unit mate with the complementary engagement features on the outer ring segment of the first blade unit; and

(d) successively sliding further blade units axially into engagement with blade units that are already engaged with each other and the flat surface until the annulus of the diaphragm is complete.

13. An axial flow turbine diaphragm according to claim 1 , in which the radially inner sides of the radially inner ring segments are configured as a seal, or are configured to retain a seal, such seal being operative to restrict leakage between relatively high and low pressure sides of the diaphragm.

14. A blade unit for an axial flow turbine diaphragm according to claim 1 .