TURBINE DIAPHRAGM The present invention relates to the construction of rings of static blades for axial flow turbines, and in particular, steam turbines .

In particular, the present invention relates to turbine diaphragms .

A steam turbine is a rotating machine intended to convert the thermal of the steam into mechanical energy for driving an alternator, a pump or any other rotary mechanical receiver. Generally, steam turbines comprise a high-pressure module, a medium-pressure module and a low-pressure module.

A steam turbine generally comprises symmetrical or non- symmetrical single or double flow inner body enclosing a rotor equipped with mobile blades and supporting fixed or stationary blades forming a diaphragm suspended in said inner body. The diaphragms are adapted to guide the flow of steam in a specific direction towards the mobile blades of the rotor, thereby accelerating the steam flow.

The present invention is related to known types of construction diaphragms called "spacer band diaphragms" and "platform diaphragms".

In the so called "spacer band diaphragm" type of construction 10, shown in Figure l a, the blade aerofoils 1 1 are fixed radially to an inner ring 12 and to an outer ring 13 by means of inner and outer annular bands 14 , 15 , which are folded from flat strip . Through-holes 16 are cut in said bands 14, 15 , for example by means of laser, to match the cross-section of the aerofoil shape. The ends of the aerofoils 1 1 are then inserted in said through-holes 16 and fillet welded into place. The inner band 14 is in turn welded to the inner ring 12 and the outer band 15 is in turn welded to the outer ring 13. Such type of construction involves a relatively small amount of machining of the blades compared to other types of construction used in steam turbines .

For example, in the so called "platform diaphragms" type of construction 20, shown in Figure l b, the blade aerofoils 21 has a section substantially in the shape of a vane having its two opposite ends integral with radially inner and outer platforms 22, 23. The blade aerofoils and the platforms are machined from solid bars or by forgings . A complete annulus of static blades is built up by assembling successive combined aerofoil-platforms components between an inner and an outer ring (not shown) and by welding the platforms to said rings . Thanks to the platforms, such diaphragm has better mechanical strength compared to the spacer band type, but has much higher manufacturing costs.

The obj ect of the present invention is to remedy the above drawbacks.

It is a particular obj ect of the present invention to provide a type of diaphragm construction which has good performance characteristics as well as being economical to manufacture .

In one embodiment, a static blade for an axial flow turbine comprises an aerofoil portion having a leading edge, a trailing edge, a pressure side and a suction side, and radially inner and outer reinforcement portions integral with said aerofoil portion.

Each reinforcement portion closely follows the shape of the section of the aerofoil portion.

Thanks to the reinforcement portion, the mechanical strength of the spacer band diaphragm is increased without increasing the manufacturing costs.

Advantageously, each reinforcement portion has a section substantially bigger than the section of the aerofoil portion. The section of the reinforcement portion has a rounded and enlarged shape corresponding to the leading edge surrounding the leading edge of the aerofoil portion and a thinner part corresponding to the trailing edge surrounding the trailing edge of the aerofoil portion.

The static blade may be made of an alloy steel material, for example, comprising 12% of chrome .

According the a second aspect, the invention relates to an axial flow turbine diaphragm construction comprising an annulus of a plurality of identical static blades as described previously, an inner and outer spacer bands having though-holes therein shaped to receive the inner and outer reinforcement portions of each static blade, and a radially inner and outer diaphragm rings surrounding the annular spacer bands .

In an embodiment, each reinforcement portions are welded to the corresponding spacer band by welds .

The welds are, for example, located at each leading and trailing edges of each reinforcement portions .

In an embodiment, the inner spacer band is welded to the inner ring and the outer band is welded to the outer ring.

The present invention will be better understood from studying the detailed description of a number of embodiments considered by way of entirely non-limiting examples and illustrated by the attached drawings in which:

- Figure l a illustrates the known spacer band turbine diaphragm type of construction;

- Figure l b illustrates a fixed blade with integral platforms for use in the known platform turbine diaphragm type of construction;

- Figure 2 is a three-dimensional perspective view of a part of a steam turbine diaphragm according to an embodiment of the invention;

- Figure 3 is a three-dimensional perspective view of a static blade for use in the diaphragm construction of Figure 2 ;

- Figure 4 is an upper view of the static blade of Figure 3 ; and

- Figure 5 is a radial cross-section of a static blade of Figure 3 , welded onto spacer bands of the steam turbine diaphragm of Figure 2.

The following detailed description of the exemplary embodiments refers to the accompanying drawings . The same reference numbers in different drawings identify the same or similar elements . Additionally, the drawings are not necessarily drawn to scale.

As illustrated on Figure 2, a part of a steam turbine diaphragm 30 of a turbine comprises a nozzle unit having a plurality of identical static blade aerofoils 32 fixed radially to an inner ring 34 and to an outer ring 36 by means of inner and outer annular spacer bands 38 , 40, which are folded from flat strip .

The inner and outer rings 34, 36 , as well as the inner and outer spacer bands 38 , 40 are concentric.

The inner and the outer spacer bands 38 , 40 are each provided with through-holes 38a, 40a. As illustrated, the through-holes are open at both ends to receive the static blades . The through-holes 38a, 40a may be, for example, cut in said spacer bands 38 , 40, for example by means of laser, to match the cross-section of the aerofoil shape. The ends of the aerofoils 32 are then inserted in said though-holes and fillet welded into place . The inner spacer band 38 is in turn welded to the inner ring 34 and the outer band 40 is in turn welded to the outer ring 36.

As illustrated on Figures 3 and 4, each static blade 32 has an aerofoil portion 44 having an elongated body having an inner end 32a brought into contact with the inner spacer band 38 and an outer end 32b, opposite to said inner end 32a, brought into contact with the outer spacer band 40.

The inner and outer ends 32a, 32b are connected respectively to the inner and outer spacer bands 38 , 40 by welding by way of a weld bead arranged between said ends and said spacer bands. In this way each static blade is welded both to the inner spacer band and to the outer spacer band.

Each static blade 32 has, for example, a section substantially in the shape of a vane, as shown on Figure 4 , having a rounded and enlarged shape corresponding to the leading edge 44a and a thinner part corresponding to the trailing edge 44b .

As illustrated on Figure 3 and 4, the inner and outer ends 32a, 32b of the each static blade 32 are provided respectively with an inner and outer reinforcement portion 46, 48. Each reinforcement portion 46, 48 has a section slightly bigger than the section of the aerofoil portion 44, as shown on Figure 4, having a rounded and enlarged shape corresponding to the leading edge 46a, 48a and a thinner part corresponding to the trailing edge 46b, 48b .

Each reinforcement portion 46, 48 surrounds the periphery of the whole section of the corresponding end so as to have a section substantially bigger than the section of the aerofoil portion 44. The shape of the reinforcement portions 46, 48 thus approximates the shape of the section of the aerofoil portion 44 in its whole, i. e at and near the leading and trailing edges 44a, 44b, as well as the suction side and the pressure side of the aerofoil portion 44. The first and second reinforcement portions 46, 48 are slid into their matching through-holes 38 a, 40a of the spacer bands 38 , 40 as shown on Figure 5 . Once all the static blades 32 have been assembled into the spacer bands 38 , 40, they must be securely welded into position. Each leading and trailing edges of each reinforcement portions are welded to the corresponding spacer band by welds 50a, 50b and 52a, 52b . The welds 50a, 50b and 52a, 52b are shown in hatched lines on Figure 4.

The static blades 32 are made of an alloy steel material, having for example, 12% of chrome.

Thanks to the reinforcement portions provided at each end of the aerofoil portions, the static blade is strengthen.

Thanks to the invention, the diaphragm construction has good mechanical strength, while being economical and easy to manufacture .