Patent Description:
In particular, the present invention relates to diaphragms comprising inner and outer rings, and a plurality of static blades mounted therebetween. Each inner and outer ring is generally split in two halves, along a joint plane of the turbine, for assembly around the rotor of the turbine. The present invention relates particularly to the connection between the upper and lower halves of each ring, and especially of the outer ring of the diaphragm.

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.

The modules generally comprise symmetrical or non-symmetrical single or double flow inner casing enclosing a rotor equipped with mobile blades and supporting fixed or stationary blades forming a diaphragm suspended in said inner casing. 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.

As reactor power is increasing, size of steam turbines is also increasing, leading to casing of huge dimensions. The flexibility of the casing being dependant of its size is also increased. Generally, the casing is made in two halves, slit along a plane joint, so that the turbine comprises an upper half and a lower half. Due to its huge size, it is common to observe an offset between the two halves of the casing after being assembled. Such offset leads to an axial clearance between the upper and lower contact surfaces between the upper and lower halves of the diaphragm and the casing. As the two halves of the diaphragm are rigidly connected together, for example by bolting means, this leads to a gap between the casing and the diaphragm and to leakage of the steam through this gap. Steam may thus flow through such gap, leading to erosion and decrease in performance of the turbine, as steam is not going through the steam path, i.e., through the blades of the diaphragm.

Document <CIT> pertains to a turbine nozzle diaphragm joint for application in e.g., steam turbines. This document discloses a diaphragm that is divided into a first half and a second half. Said first half and said second half are connected together with projections/flanges which include surfaces projecting in the circumferential and the axial downstream direction. Said projections/flanges interact with recesses. Both halves are welded together to provide additional structural support. Turbine diaphragms according to this document have improved aerodynamic performance and are easier to produce. However, halves of the diaphragms cannot move in directions parallel to a halves joint plane.

Document <CIT> pertains to turbine diaphragms. This document introduces inner rings to said diaphragms to prevent deformation of blade units in impulse turbine applications. Thereby, diaphragms contain two rings, i.e., an inner ring and an outer ring.

The outer ring is connected to outer platform portions of blade units and the inner ring mechanically engages with inner platform portions of said blade units. This document discloses that there are two forms of mechanical engagement, i.e., one which is to limit relative movement between the blade units and the ring in directions parallel to the major axis of the ring and another which is to prevent relative circumferential movement between the blade units and the ring and to maintain concentricity of the inner ring with the outer ring during differential thermal growth of the two rings. The first mechanical engagement comprises a circumferential rib projecting radially from the inner ring into a circumferentially aligned grove in the inner ring. Two ribs side-by-side can be used. Said ribs prevents from expansive and time-consuming welding.

Document <CIT> pertains to an alignment apparatus for a steam turbine nozzle assembly. This document discloses nozzles constructions called diaphragms. Said diaphragms contain two halves which are assembled around a rotor and which contain an alignment member. Said alignment member allows for positioning of said diaphragm relative to a casing. The alignment member contains a retaining member and a first and a second vertical alignment member. The vertical alignment members are oriented parallel to a plane that separates said halves. Said alignment member spans across a segment of a diaphragm and a segment of a casing. This alignment member is not suited to connect rings of diaphragm.

It is an object of the present invention to remedy the above drawbacks.

It is a particular object of the present invention to reduce steam leakage inside the turbine by ensuring a proper axial contact between the diaphragm and the casing in any case.

An axial flow turbine comprises a casing, a rotor having an axial rotational axis and rotatably mounted into said casing, at least one set of a plurality of moving blades supported by said rotor, and at least one diaphragm having an outer ring, an inner ring, concentric to the outer ring, and a plurality of static blades mounted therebetween. At least said diaphragm is split in an upper half and a lower half along a horizontal joint plane.

Said turbine diaphragm comprises an assembly system for assembling the upper half to the lower half while allowing the upper half and the lower half to move axially relative to each other.

Thanks to the axial degree of freedom of the diaphragm lower and upper halves, axial contact between the diaphragm and the casing is ensured, preventing any steam leakage.

The assembly system comprises a guiding element for axially guiding the upper half and the lower half, and at least one fastening element on each side for fastening the upper and lower halves together while allowing a relative axial movement of the halves relative to each other, said fastening element being perpendicular to the horizontal joint plane.

The fastening element has a screw head, a smooth shrank portion and a threaded portion.

The diaphragm upper half is formed with a drilling, made along the vertical axis, and having a diameter bigger than the diameter of the smooth shrank portion and the lower half is formed with a threaded bolt hole coaxial with the drilling of the upper half and adapted to receive the threaded portion of the fastening element.

In one embodiment, the assembly system comprises a spacing element provided underneath the screw head of the fastening element, in order to control the clearance underneath said screw head.

In one embodiment, the guiding element of the assembly system comprises a feather key rigidly tightened to the upper half by two screws and an axial groove machined on the joint surface of the lower half and adapted to receive said feather key, an axial clearance being set between each side of the feather key and each axial edge of said axial groove allowing the feather key to slide inside said axial groove. The two halves thus have an axial degree of freedom relative to each other.

In an embodiment, the guiding element of the assembly system comprises at least one cylinder positioned in an axial drilling provided in both the upper and lower halves of the diaphragm, the outer diameter of the cylinder being smaller than the inner diameter of the axial drilling.

Advantageously, a clearance is observed between the screw head and the diaphragm upper half.

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:.

In the further description, terms "horizontal", "vertical", "front", "back", "left", and "right" are defined according to the usual orthogonal benchmark of turbines, illustrated on the figures, and including:.

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 <FIG>, a part of an axial flow steam turbine <NUM>, for example, the low-pressure, the medium-pressure or the high-pressure module of the turbine, comprises a rotor <NUM>, having an axial rotational axis Z, rotatably mounted into a casing <NUM> and supporting a plurality of moving blades <NUM> and a plurality of diaphragms <NUM>. Only one diaphragm is shown on <FIG>. However, it could be possible to provide more than two diaphragms assembled together.

The moving blades <NUM> are supported by the rotor <NUM> by blade roots fixed to a rotor disc <NUM>. The moving blades are known from the man skilled in the art and will not be further described.

As illustrated, the diaphragm <NUM> comprises an outer ring <NUM>, an inner ring <NUM>, concentric to the outer ring, and a plurality of static blades or vanes <NUM> mounted therebetween.

As can be seen on <FIG>, the outer ring <NUM> of the diaphragm <NUM> is split in two halves, an upper half 22a and a lower half 22b, along a horizontal joint plane P. Each of the two halves 22a, 22b has a pair of opposed, joint surfaces 22c, 22d. (Only one of each pair is shown on <FIG>).

The casing <NUM> of the turbine is also split into a lower half 14a surrounding the lower half 22b of the diaphragm's outer ring <NUM> and an upper half (not shown) surrounding the upper half 22a of the diaphragm's outer ring <NUM>. The lower and upper halves of the casing are split along the same horizontal joint plane P.

The upper and lower halves 22a, 22b of the outer ring <NUM> diaphragm are connected together by an assembly system <NUM> allowing the upper half 22a and the lower half 22b to slide relative to each other along the horizontal joint plane P, so that the outer ring of the diaphragm is in axial contact with a radial face <NUM> of the casing. The diaphragm is thus given an axial degree of freedom, ensuring an axial contact between the diaphragm and the casing, thus preventing any steam leakage.

The assembly system <NUM> comprises a guiding element <NUM> for axially guiding the upper half 22a and the lower half 22b, and a fastening element <NUM> adapted to fasten the upper and lower halves 22a, 22b together while allowing a relative axial movement of the halves relative to each other.

As can be seen on the embodiment of <FIG> and <FIG>, the guiding element <NUM> comprises a feather key <NUM> rigidly tightened to the upper half 22a by two screws 38a, 38b and an axial groove <NUM> machined on the joint surface 22d of the lower half 22b and adapted to receive said feather key <NUM>.

An axial clearance ΔZ is observed between each side of the feather key <NUM> and each axial edge of the axial groove <NUM> in order to allow the feather key <NUM> to slide inside said axial groove. The two halves 22a, 22b thus have an axial degree of freedom relative to each other.

As can be seen on the embodiment of <FIG>, the fastening element <NUM> is a joint screw having a screw head 34a, a smooth shrank portion 34b and a threaded portion 34c. The smooth shrank portion 34b is longer than the threaded portion 34c.

Therefore, the diaphragm upper half 22a is formed with a hole or drilling <NUM>, made along the vertical axis Y, accessed by a counter bore or a notch area <NUM> machined in the diaphragm upper half 22a. The bore of the drilling <NUM> is smooth and has a diameter bigger than the diameter of the smooth shrank portion 34b.

The diaphragm lower half 22b is formed with a threaded bolt hole <NUM> coaxial with the drilling <NUM> of the upper half 22a and adapted to receive the threaded portion 34c of the fastening element <NUM>. The diaphragm lower half 22b is further provided with an undercut <NUM> of bigger diameter than the diameter of the threaded bolt hole <NUM>.

The joint screw <NUM> is tightened and torque clamped into the lower half 22b in order to assure a good mechanical strength when torque is exerted on the diaphragm, thus preventing the diaphragm from opening at the joint plane. Therefore, when tightening the joint screw <NUM> into the lower half, the end 34d of the smooth shank portion 34b bears on the lower half, and more precisely on the bottom 48a of the undercut <NUM>.

As illustrated on <FIG>, a spacing element <NUM> is provided underneath the screw head 34a of the joint screw <NUM> in order to control the clearance underneath said screw head 34a. A clearance ΔY is observed between the screw head 34a and the spacing element <NUM>. The spacing element <NUM> illustrated is a washer. As an alternative, any other spacing element may be used, such as, for example, a Belleville spring washer.

Such a particular structure of the joint screw allows the two halves 22a, 22b of the diaphragm's outer ring <NUM> to be assembled together, while allowing a relative axial movement between each other.

The embodiment of <FIG>, in which identical elements bear the same references, differs from the embodiment of <FIG> and <FIG> in the structure of the assembly system of the upper and lower halves 22a, 22b of the outer ring <NUM> of the diaphragm <NUM>.

As illustrated on <FIG>, the assembly system <NUM> comprises a guiding element <NUM> for axially guiding the upper half 22a and the lower half 22b of the diaphragm <NUM>, and a fastening element <NUM> adapted to fasten, respectively the upper and lower halves 22a, 22b together while allowing a relative axial movement of the halves relative to each other. As can be seen on the embodiment of <FIG>, the guiding element <NUM> comprises a cylinder <NUM> positioned in an axial drilling <NUM> provided in both the upper and lower halves 22a, 22b of the diaphragm <NUM>. The outer diameter of the cylinder <NUM> is smaller than the inner diameter of the axial drilling <NUM> so that the halves may slide axially relative to each other. A nitride washer could be added around the cylinder in order to ensure the sliding. As an alternative, a nitriding could be done directly on the cylinder itself.

As another alternative, it is possible to provide a first cylinder in both the upper and lower halves 22a, 22b of the diaphragm <NUM>.

The fastening element <NUM> differs from the fastening element <NUM> of the embodiment of <FIG> and <FIG> in that the fastening element <NUM> is tightened on a cylindrical spacer which is in contact in the counter bore hole, whereas in the embodiment of <FIG> and <FIG>, the fastening element <NUM> is tightened on the lower part. Said fastening element <NUM> comprises a screw head 104a, a smooth shrank portion (not shown) and a threaded portion (not shown). The smooth shrank portion is longer than the threaded portion.

The upper half 22a is formed with a hole or drilling 62a made along the vertical axis Y, accessed by a counter bore or a notch area 62b machined in the upper half 22a. The bore of the drilling 62a is smooth and has a diameter bigger than the diameter of the shrank smooth portion. A cylindrical spacer <NUM> is provided between the outer surface of the shrank portion and the inner surface of the drilling 62a. The inner diameter of the spacer <NUM> is bigger than the outer diameter of the shrank smooth portion of the fastening element <NUM>. A clearance ΔY2 is observed between the screw head 104a and the spacer <NUM>.

The lower half 22b is formed with a threaded bolt hole (not shown) coaxial with the corresponding drilling 62a and adapted to receive the threaded portion of the fastening element <NUM>. The lower half 22b is further provided with an undercut (not shown) of bigger diameter than the diameter of the threaded bolt hole. In this embodiment, when tightening the joint screw <NUM> into the corresponding half, the end 110a of the spacer <NUM> bears on the bottom of the undercut.

Claim 1:
An axial flow turbine (<NUM>) comprising:
• a casing (<NUM>),
• a rotor (<NUM>) having a rotational axis (Z) and rotatably mounted into said casing (<NUM>),
• at least one set of a plurality of moving blades (<NUM>) supported by said rotor (<NUM>), and
• at least one diaphragm (<NUM>) having an outer ring (<NUM>), an inner ring (<NUM>), concentric to the outer ring (<NUM>), and a plurality of static blades (<NUM>) mounted therebetween, wherein said at least one diaphragm (<NUM>) being split in an upper half (22a) and a lower half (22b) along a horizontal joint plane (P),
characterized in that
said at least one diaphragm (<NUM>) comprises an assembly system (<NUM>, <NUM>) for assembling the upper half (22a) to the lower half (22b) while allowing the upper half (22a) and the lower half (22b) to move axially relative to each other,
wherein the assembly system (<NUM>, <NUM>) comprises:
• a guiding element (<NUM>, <NUM>) for axially guiding the upper half (22a) and the lower half (22b), and
• at least one fastening element (<NUM>, <NUM>) for fastening the upper and lower halves (22a, 22b) together while allowing a relative axial movement of the halves (22a, 22b) relative to each other, wherein said fastening element (<NUM>, <NUM>) being perpendicular to the horizontal joint plane (P), and
wherein said at least one fastening element (<NUM>, <NUM>) has:
• a screw head (34a, 104a),
• a smooth shrank portion (34b), and
• a threaded portion (34c), and
wherein the upper half (22a) of said at least one diaphragm (<NUM>) is formed with a drilling (<NUM>, 62a), made along the vertical axis (Y), and having a diameter bigger than the diameter of the smooth shrank portion (34b), and
wherein the lower half (22b) is formed with a threaded bolt hole (<NUM>) coaxial with the drilling (<NUM>, 62a) of the upper half (22a) and adapted to receive the threaded portion (34c) of said at least one fastening element (<NUM>, <NUM>).