DESCRIPTION TECHN ICAL FIELD

Embodiments of the subject matter disclosed herein relate to methods of assembl ing a set of impellers, impellers and turbomachines.

BACKGROUND ART

Assembling of a set of impellers may be done in different ways.

One known way consists in providing axial through holes in the impellers, placing all the impellers axially adjacent to each other, inserting an axial tie rod in the holes so that it protrudes both from the first impeller and from the last impeller, applying axial forces on the first impeller and last impeller by means of the tie rod so to tightly hold all the impellers together.

SUMMARY

Such solution is simple and effective, but it has some drawbacks.

When the impellers assembly heats up due to the operation of the machine and to the fluid in contact with the impellers, also the tie rod heats up and looses the impellers somewhat; this may cause relative rotations of the impellers and/or unbalance of the rotor and/or high vibrations of the machine and/or low power generation/absorption and/or fretting and wear of the connections between impellers. This drawback is proportional to the number of impellers and to the length of the tie rod . This drawback depends on the temperatures of the tie rod and of the impellers during the operation of the machine. This drawback depends also on the materials used for the tie rod and for the impellers in particular because of to the different thermal expansion coefficients.

When the tie rod is long, damping devices need to be associated to the axial tie rod and placed at some points between its two ends, this means inside the axial holes of the impellers. Such damping devices are subject to wear and/or damage and so they are a source of reliability reduction of the machine using them; furthermore, as such damping devices are located inside the impeller axial holes, their maintenance operation requires complete disassembling of the machine.

Therefore there is a need for an improved way of assembl ing a set of impellers.

The basic idea is to use a plurality of axial tie rods, typically two or three or four.

A first aspect of the present invention is an impeller.

The impeller of a turbomachine comprises an axial through hole, a first portion with a first axial hole having a first cross-section and a second portion with a second axial hole having a second cross-section, said first cross-section being smaller than said second cross-section, wherein said first and second axial holes constitute said axial through hole and said first portion is couplable to an end of a first axial tie rod and said second portion is couplable to an end of a second axial tie rod .

A second aspect of the present invention is a turbomachine. The turbomachine comprises:

- at least one impeller comprising an axial through hole (6B), wherein at least one impeller has a first portion with a first axial hole having a first cross-section and a second portion with a second axial hole having a second cross-section said first cross-section being smaller than said second cross-section, wherein said axial through hole comprises said first and second axial holes; - at least a first and a second axial tie rods located at least in part inside the axial through holes;

- at least one nut; wherein the at least one impeller is coupled to an end of said first axial tie rod by means of said nut and is directly coupled to an end of said second axial tie rod .

A third aspect of the present invention is a method of assembling a set of impellers.

The method is used for assembling a rotor for a turbomachine comprises the steps of:

- providing a plurality of impellers having respective axial through holes; - disposing said impellers axially adjacent to each other;

- providing at least a first and a second axial tie rods;

- providing at least one connection element;

- using said connection element for securing, at one side of its axial through hole, an end of said first axial tie rod and, at the other side of its axial through hole, an end of said second axial tie rod .

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated herein and constitute a part of the specification, illustrate exemplary embodiments of the present invention and, together with the detailed description, explain these embodiments. In the drawings:

Fig .1 shows a simpl ified cross-section view of a set of impellers assembled through only one axial tied rod, Fig .2 shows a simpl ified cross-section view of a set of impellers assembled through three axial tied rods,

Fig .3 shows a simpl ified and partial cross-section view of an impeller having a simple axial through hole,

Fig .4 shows a simpl ified and partial cross-section view of an impeller having a shaped axial through hole, and

Fig .5 shows the impeller of Fig .4 as it is used in the arrangement of Fig .2.

DETAILED DESCRIPTION

The following description of exemplary embodiments refers to the accompanying drawings.

The following description does not limit the invention . Instead, the scope of the invention is defined by the appended claims.

Reference throughout the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed . Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. Fig.1 shows a set of five impellers 1 A, 1 B, 1 C, 1 D, 1 E of a centrifugal compressor that is a five-stage centrifugal compressor. Impellers 1A, 1B, 1C, 1D, 1E are axially adjacent to each other and have respective axial through holes 2A, 2B, 2C, 2D, 2E; in particular, these holes are, for example, cylindrical and have the same diameter. Axial through holes 2A, 2B, 2C, 2D, 2E are axially aligned and a single axial tie rod 3 is inserted in these holes so that it protrudes (at least somewhat) both from the first impeller 1A and from the last impeller 1E. Axial forces are applied on the first impeller 1A and last impeller 1E by means of the tie rod 3 (as well as of two elements, according to this embodiment) so to tightly hold all the impellers 1A, 1B, 1C, 1D, 1E together.

Fig.2 shows an arrangement similar to that of Fig.1, but wherein three axial tie rods 4A, 4B, 4C are used to tightly hold all the five impellers together, namely 1A, 5B, 1C, 5D, 1E. The first impeller (i.e. impeller 1A), the central impeller (i.e. impeller 1C) and the last impeller (i.e. impeller 1E) of this embodiment of Fig.2 are identical to those of the embodiment of Fig.1.

The second impeller 5B is shaped so that tie rod 4A tightly holds the first impeller 1A and the second impeller 5B together; the fourth impeller 5D is shaped so that tie rod 4B tightly holds the second impeller 5B and the third impeller 1C and the fourth impeller 5D together; the fourth impeller 5D and the fifth impeller 1E are held together by tie rod 4C through a further element 10 that will be described later on. According to this embodiment, each axial tie rod is used to hold together (for example) two or three impellers only and not all of them; therefore, the risk of loosening the impellers and the need of dampers for the tie rods are much reduced.

In general, the assembling of a set of impellers is realized through at least one connection element that is axially adjacent to two impellers respectively at its two sides and that has an axial through hole; the connection element is used for securing , at one side of its axial through hole, an end of a first axial tie rod and, at the other side of its axial through hole, an end of a second axial tie rod . Advantageously, said connection element is one of the impellers of the set to be assembled ; in the embodiment of Fig .2, impellers 5B and 5D act also as connection elements and are very similar to each other; Fig .4 shows in detail impel ler 5B, by way of example, with its axial through hole 6B . Advantageously, one of the impellers is at least part of said connection element.

Advantageously, the impel lers 1 A, 5B, 1 C, 5D, 1 E of the set are tangentially coupled to each other by respective h irth joints 7A, 7B, 7C, 7D located around their axial through holes 2A, 6B, 2C, 6D, 2E; h irth joints assure a very good coupl ing and have the advantage of allowing exactly the same reciprocal position of the impellers even after several assembl ing and d isassembl ing operations (due to e.g . maintenance).

I mpeller 5B comprises an axial through hole 6B; a first hole portion 6B1 of the axial through hole is located at a first side of the impel ler and has a first cross-section; a second hole portion 6B2 of the axial through hole is located at a second side of the impel ler and has a second cross-section ; the first side is opposite to the second side; the first cross-section (see 6B1 ) is smaller than the second cross-section (see 6B2); the first hole portion 6 B1 may be used as reference for tie rod 4A centering through nut 1 1 A. A flat surface 6B3 connects the internal surfaces of the first and second hole portions 6B1 and 6B2 and is adapted to be coupled to an end of an axial tie rod ; Fig .5 shows surface 6 B3 coupled to a n ut 1 1 A of axial tie rod 4A. The second hole portion 6B2 is adapted to be coupled to an end of another axial tie rod ; Fig .5 shows axial tie rod 4B screwed in hole portion 6B2 (that is threaded); in particular, there is a threaded shank of an enlarged (specifically radially enlarged) end 12 of the axial tie rod 4B. The enlarged end 12 of axial tie rod 4B has a recess 13 (specifically an axial recess) for housing an end (specifically the tip of the end) of axial tie rod 4A; in this way, a very good connection may be achieved in a smaller axial length still allowing precision assembly and tightening. It is to be noted that a partial or total wall may be placed between hole portions 6B1 and 6B2

Impeller 5B is provided with teeth 7A2 of a first hirth joint 7A located around axial through hole 6B at a first side of the impeller, and teeth 7B1 of a second hirth joint 7B located around axial through hole 6B at a second side of the impeller.

Fig.3 shows an embodiment of impeller of the set to be assembled that does not act as connection element; impellers 1A, 1C and 1E are very similar to each other. Impeller 1C comprises an axial through hole 2C that is, for example, cylindrical. Impeller 1C is provided with teeth 7B2 of a second hirth joint 7B located around axial through hole 2C at a first side of the impeller, and teeth 7C1 of a third hirth joint 7C located around axial through hole 2C at a second side of the impeller. According to the embodiment of Fig.2, the axial tie rods 4A, 4B, 4C are arranged in series; the first axial tie rod 4A of the serial arrangement is connected to an element 9 acting as head for tensioning the axial tie rod 4A and located in front of the first impeller 1A of said set. For example, an end of tie rod 4A is screwed in a threaded hole of element 9, and element 9 is connected to impeller 1A by means of a hirth joint 8A.

According to the embodiment of Fig.2, the axial tie rods 4A, 4B, 4C are arranged in series; he last axial tie rod 4C of the serial arrangement is coupled to an element 10 through a nut 11 C of the tie rod 4C; element 10 is axially adjacent to the last impeller 1E at one of its sides and has an axial through hole 1 0F (specifically a shaped hole) for receiving the last axial tie rod 4C. For example, element 1 0 is connected to impeller 1 E by means of a hirth joint 8B.

It is to be noted that elements 9 and 1 0 may have different shapes and sizes; in particular, they could comprise: journal bearings, shaft end seals, balance drums, thrust collars.

Assembling of the arrangement of Fig .2 is carried out gradually and for example as follows:

- rod 4A is fixed to element 9, - rod 4A is inserted in hole 2A of impeller 1 A till coupling of joint 8A,

- rod 4A is inserted in hole 6B of impeller 5B till coupling of joint 7A,

- nut 1 1 A is screwed on rod 4A till it is tightened, - rod 4B is screwed in hole 6B till it is tightened,

- rod 4B is inserted in hole 2C of impeller 1 C till coupling of joint 7B,

- rod 4B is inserted in hole 6D of impeller 5D till coupling of joint 7C, - nut 1 1 B is screwed on rod 4B till it is tightened,

- rod 4C is screwed in hole 6D till it is tightened,

- rod 4C is inserted in hole 2E of impeller 1 E till coupling of joint 7D, rod 4C is inserted in hole 1 0F of element 1 0 till coupling of joint 8B,

- nut 1 1 C is screwed on rod 4C till it is tightened .

At this point the arrangement is fully assembled . It is to be noted that the above description of the assembling procedure is not intended to specify which parts are moved and which parts are maintained stationary.