TECHNICAL FIELD
[0001] The present invention relates to a gas turbine assembly and to a method for operating
said gas turbine assembly. In particular, the gas turbine assembly of the present
invention can be part of a plant for the production of electrical energy.
BACKGROUND
[0002] As is known, a gas turbine assembly for power plants comprises a compressor, a combustor
and a turbine.
[0003] In particular, the compressor comprises an inlet supplied with air and a plurality
of rotating blades compressing the passing air. The compressed air leaving the compressor
flows into a plenum, i.e. a closed volume delimited by an outer casing, and from there
into the combustor. Inside the combustor, the compressed air is mixed with at least
one fuel and then the fuel is combusted. The resulting hot gas leaves the combustor
and expands in the turbine. In the turbine the hot gas expansion moves rotating blades
connected to a rotor, performing work.
[0004] Both the compressor and the turbine comprise a plurality of stator assemblies axially
interposed between rotor assemblies.
[0005] Each stator assembly comprises a plurality of stator vanes supported by a respective
stator structure (casing or carrier) and a stator ring arranged about the rotor.
[0006] Each rotor assembly comprises a rotor disk rotating about a main axis and a plurality
of blades supported by the rotor disk.
[0007] During rotation, the tips of the blades are spaced from the stator. However, said
spacing can change due to thermal expansions and mechanical backlashes depending on
the operational conditions of the gas turbine assembly.
[0008] It is therefore necessary to properly control the dimensions of said gap in order
to avoid an excessive spacing between the tips of the blades and the stator or a dangerous
excessive reduction of the gap that may lead to a contact between the tips and the
stator.
[0009] A proper control of the gap between the tip of the blades and the stator improves
the reliability and the efficiency of the gas turbine assembly.
SUMMARY
[0010] The object of the present invention is therefore to provide a gas turbine assembly,
wherein the gap between the tip of the blades and the stator is controlled in a simple
and reliable way.
[0011] In particular, it is an object of the present invention to provide a gas turbine
assembly extending along a longitudinal axis and comprising:
a stator defining a working channel;
a rotor comprising a plurality of rotor stages; each rotor stage comprising a plurality
of blades rotatable in the working channel about the longitudinal axis; each blade
being provided with a tip;
at least one movable element which is coupled to the stator by means of at least one
retaining device and moves radially between the stator and at least one tip of the
blades for regulating a clearance gap between the at least one tip and the movable
element;
wherein the movable element is coupled to the stator so as to create a regulating
gap between the stator and the movable element, which is in fluidic communication
with the working channel; the movable element being provided with at least one first
sealing element dividing the regulating gap in at least two chambers; one first chamber
of the at least two chambers being in fluidic communication with the working channel
upstream the rotor stage and one second chamber of the at least two chambers being
in fluidic communication with the working channel downstream the rotor stage.
Thanks to presence of the first sealing element, the resulting force acting on the
movable element is regulated by the pressures in the chambers of the regulating gap
and by the pressures in the clearance gap.
The pressures in the chambers of the regulating gap are strictly linked to the pressures
upstream and downstream of the rotor stage. In this way, the clearance gap is regulated
naturally, without the need of active control means. This implies low realization
costs and high performances of the gas turbine assembly.
[0012] According to a variant of the present invention, the movable element is provided
with an inner face, facing, in use, the at least one tip of the blade and with an
outer face, facing, in use, the regulating gap; on the outer face the movable element
being provided with at least one first recess housing the first sealing element.
[0013] Preferably, the axial dimension of the first recess is greater than the axial dimension
of the sealing element.
[0014] According to a variant of the present invention, the first sealing element is transversal
to a longitudinal plane containing the longitudinal axis, preferably orthogonal to
the longitudinal plane containing the longitudinal axis.
[0015] According to a variant of the present invention, the the first sealing element is
a brush seal.
[0016] According to another variant of the present invention, the first sealing element
comprises a metal sheet.
[0017] According to a variant of the present invention, the movable element is provided
with a second sealing element arranged at a distance from the first sealing element
so as to create an intermediate chamber between the first sealing element and the
second sealing element; the intermediate chamber being arranged between the first
chamber and the second chamber.
[0018] Preferably, the intermediate chamber is in fluidic communication with the clearance
gap.
[0019] More preferably, the intermediate chamber is in fluidic communication with the clearance
gap by means of a through hole made in the movable element.
[0020] According to a variant of the present invention, the intermediate chamber is in fluidic
communication with the first chamber and/or the second chamber through at least one
passage at the first sealing element and/or at the second sealing element.
[0021] According to a preferred embodiment of the present invention, the retaining device
is configured to reduce the regulating gap when the turbine assembly is not operating.
[0022] More preferably, the retaining device is configured to limit the radial movement
of the movable element between a minimum displacement and a maximum displacement.
[0023] The retaining device can comprises at least one spring, or at least one bimetallic
element, or at least one bellow filled with heat-sensitive material or a magnet, or
a combination thereof.
[0024] It is also an object of the present invention to provide a method for operating a
gas turbine assembly, which is able to properly control the gap between the tip of
the blades and the stator in a simple and reliable way.
[0025] In particular, it is an object of the present invention to provide a method for operating
a gas turbine assembly according to claim 15.
[0026] In this way the pressure fields acting on the movable element regulates the position
of the movable element so that a proper clearance control is obtained (i.e. the net
force acting on the movable element and determined by the pressure fields is zero
at a target radial width of the clearance gap).
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The present invention will now be described with reference to the accompanying drawings,
which illustrate some non-limitative embodiment, in which:
- Figure 1 is a schematic sectional front view, with parts removed for clarity, of a
gas turbine assembly according to the present invention;
- Figure 2A is a schematic sectional front view, with parts removed for clarity, of
a first detail of Figure 1;
- Figure 2B is a schematic sectional lateral view, with parts removed for clarity, of
a second detail of Figure 1;
- Figure 3A is a schematic representation of the pressure trends along the axial direction
acting on a movable element of the first detail of figure 2 in a first operating condition
(large clearance condition);
- Figure 3B is a schematic representation of the resulting force acting on the movable
element of the first detail of figure 2 in the first operating condition of figure
3a;
- Figure 4A is a schematic representation of the pressure trends along the axial direction
acting on a movable element of the first detail of figure 2 in a second operating
condition (very small clearance condition);
- Figure 4B is a schematic representation of the resulting force acting on the movable
element of the first detail of figure 2 in the second operating condition of figure
4A;
- Figure 5 is a schematic sectional front view, with parts removed for clarity, of a
first detail of Figure 1 according to a variant of the present invention;
- Figure 6A is a schematic representation of the resulting force acting on the movable
element of the first detail of figure 5 in a first operating condition (large clearance);
- Figure 6B is a schematic representation of the resulting force acting on the movable
element of the first detail of figure 5 in a second operating condition (very small
clearance condition).
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0028] In figure 1 reference numeral 1 indicates a gas turbine assembly (schematically shown
in Figure 1).
[0029] The gas turbine assembly 1 comprises a compressor 3, a combustion chamber 4, a turbine
5.
[0030] In particular, the gas turbine assembly 1 comprises a stator 6, defining at least
a compression working channel 8 and an expansion working channel 9.
[0031] The compressor 3 and the turbine 5 are mounted on a same shaft to form a rotor 11,
which is housed in the stator 6 and extends along an axis A.
[0032] In greater detail, the rotor 11 comprises a front shaft 12, a plurality of rotor
stages 13 and a rear shaft 14.
[0033] Each rotor stage 13 comprises a rotor disk 16 and a plurality of rotor blades 17
coupled to the rotor disk 16 and radially arranged.
[0034] The plurality of rotor disks 16 are arranged in succession between the front shaft
12 and the rear shaft 14 and preferably clamped as a pack by a central tie rod 18.
As an alternative, the rotor disks may be welded together.
[0035] A central shaft 20 separates the rotor disks 16 of the compressor 3 from the rotor
disks 16 of the turbine 5 and extends through the combustion chamber 4.
[0036] Further, stator stages 22 are alternated with the compressor rotor stages 13.
[0037] Each stator stage 22 comprises a stator ring 24 and a plurality of stator vanes 25,
which are radially arranged and coupled to the stator ring 24 and to a respective
structure of the stator 6.
[0038] In figure 2A an enlarged view of a rotor stage 13 is illustrated. The rotor stage
13 illustrated in figure 2A is housed in the compression working channel 8 of the
compressor 3. However, the same features here below detailed can be applied, mutatis
mutandis, to a rotor stage 13 housed in the expansion working channel 9 of the turbine
5 wherein a hot gas flows.
[0039] The rotor blade 17 extends along a radial direction r with respect to longitudinal
axis A and is provided with a base 28 coupled to the rotor disk 15 and with a free
tip 29, arranged radially opposite to the base 28.
[0040] The blade 17 comprise a leading edge LE, which is the edge first meeting the incoming
flow and which separates the flow, and a trailing edge TE, which is the edge at which
the air divided by the leading edge LE meets again.
[0041] Arrow D indicates the direction of the air flow flowing in the compression working
channel 8.
[0042] The gas turbine assembly 1 comprises at least one movable element 30, which is coupled
to the stator 6 by means of at least one retaining device 32 and moves radially between
the stator 6 and at least one tip 29 of the blades 17 for regulating a clearance gap
33 between the at least one tip 29 and the movable element 30.
[0043] In particular, the movable element 30 is coupled to the stator 6 so as to create
a regulating gap 35 between the stator 6 and the movable element 30, which is in fluidic
communication with the expansion working channel 8.
[0044] With reference to figure 2B, the gas turbine assembly 1 preferably comprise a plurality
of movable elements 30 for each rotor stage 13. The plurality of movable elements
30 are preferably arranged circumferentially about the longitudinal axis A at a distance.
[0045] Each movable element 30 has a radial length Lr and circumferential length Lc.
[0046] With reference again to figure 2A, the movable element 30 is provided with at least
one first sealing element 37 dividing the regulating gap 35 in at least two chambers
38: one first chamber 38a is in fluidic communication with the working channel 8 upstream
the rotor stage 13 and one second chamber 38b is in fluidic communication with the
working channel 8 downstream the rotor stage 13.
[0047] The movable element 30 is provided with an inner face 40, facing, in use, the at
least one tip 29 of the blade 17 and with an outer face 41, facing, in use, the regulating
gap 35.
[0048] Preferably, on the outer face 41 the movable element 39 is provided with at least
one recess 43 housing the first sealing element 37.
[0049] Preferably, the axial dimension of the recess 43 is greater than the axial dimension
of the sealing element 37 in order to allow the sealing element 37 to move inside
the recess 43.
[0050] In use, in fact, the sealing element 37 should maintain the division between the
chambers 38 in all the positions of the movable element 30.
[0051] Therefore, when the movable element 30 moves radially, the sealing element 37 can
consequently slide radially in the recess 43. For example, when the movable element
30 is in the position closest to the stator 6 (i.e. radial width of the regulating
gap 35 is at the minimum and radial width of the clearance gap 33 is at the maximum)
the sealing element 37 substantially engages completely the recess 43 touching the
bottom of the recess 43. When the movable element 30 is in the position farthest to
the stator 6, the sealing elements 37 is radially spaced from the bottom of the recess
43. Said radially spacing between the sealing element 37 and the bottom of the recess
43 defines the maximum radial width variation of the regulating gap 35.
[0052] The sealing element 37 can be a brush seal having inclined movable bristles or a
sheet metal that can slide radially at the shoulder of the recess 43.
[0053] As we will see in detail later, due to the pressure difference between the chambers
38, in use, the sealing element 37 is pressed against one shoulder of the recess 43.
[0054] Preferably, the sealing element 30 extends transversal to a longitudinal plane containing
the longitudinal axis A.
[0055] More preferably, the sealing element 30 is arranged orthogonal to a longitudinal
plane containing the longitudinal axis A.
[0056] According to a variant not shown, the outer face of the movable element is not provided
with a recess and the first sealing element is coupled to the outer face.
[0057] The retaining device 32 is preferably configured to limit the radial movement of
the movable element 30 between a minimum displacement and a maximum displacement.
[0058] In other words, the retaining device 32 is configured to safely limit the movement
of the movable element 30 in order to avoid an excessive approaching of the movable
element 30 towards the tip 29 of the blade 17 (in order to avoid dangerous rubbing
between the tip 29 and the movable element 30) or an excessive approaching of the
movable element 30 towards the stator 6 (in order to avoid an excessive reduction
of the regulating gap 35).
[0059] The retaining device 32 is also preferably configured to reduce the regulating gap
35 when the gas turbine assembly 1 is not operating. In other words, when no pressures
field is acting on the movable element 30 the retaining device 32 moves the movable
element 30 away from the tip 29 of the blade 17 in order to avoid rubbing between
the tip 29 and the movable element 30.
[0060] Thus, at standstill conditions, in fact, a clearance gap 33 is provided and it is
possible to turn the rotor slowly (e.g. rotor barring) without rubbing of the tip
29 of the blade 17.
[0061] The retaining device 32 is also preferably configured to damp the potential oscillations
of the movable element 30.
[0062] In the non-limiting example here disclosed and illustrated, the retaining device
32 comprises two springs 45 coupled to the stator 6 and to the outer face 41 of the
movable element 30.
[0063] The movable element 30 has eigenfrequencies due to the presence of the springs 45
and therefore can vibrate due to flow induced excitation. A damping device is therefore
advantageous. The damping can be obtained by the friction of the sealing element 37
on the shoulders of the recess 43. Other methods to create friction are possible.
[0064] According to a first variant not shown, the retaining device may comprise at least
one bimetallic element.
[0065] According to a second variant not shown, the retaining device may comprise at least
one bellow filled with heat-sensitive material.
[0066] According to a further variant not shown, the retaining device may comprise at least
one magnet or a combination of the options above.
[0067] With reference to figure 3A-3B 4A-4B, the movable element 30, in operation moves
under the effect of the pressure field acting on it.
[0068] In particular, on the inner face 40 of the movable element 30, the pressure field
depends mainly on the dimensions of the clearance gap 33 and on the operating conditions.
[0069] On the outer face 41, the pressure field depends on the dimensions of the chambers
38 created by the sealing element 37.
[0070] In the first chamber 38a, in fact, the pressure is identical to the pressure P1 in
the working channel upstream of the rotor stage 13.
[0071] In the second chamber 38b, the pressure is identical to the pressure P2 in the working
channel downstream of the rotor stage 13.
[0072] In figure 3A it is represented the pressure trend on the inner face 40 (dashed lines)
and on the outer face 41 (continuous line) when the radial width of the clearance
gap 33 is large. With the expression "large" it is intended that the radial width
is greater than a threshold.
[0073] In this situation, on the inner face 40 there is approximately a linear pressure
increase from value P1 to value P2 from the leading edge LE to the trailing edge TE.
[0074] The pressure trend on the inner face 40 depends on several factors, i.e. the shape
of the airfoil of the blade 17.
[0075] On the outer face 41, the pressure in the first chamber 38a is P1 and the pressure
in the second chamber 38b is P2.
[0076] Looking at the diagram in figure 3B, the resulting net force acting on the movable
device 30 is greater towards the tip 29.
[0077] Therefore, if the clearance gap 33 is large, the movable element 30 is moved radially
inward by the pressure field in order to reduce the radial width of the clearance
gap 33.
[0078] The resulting net force is calculated according the following formula:

wherein:
Pouter is the pressure on the outer face 41;
Pinner is the pressure on the inner face 40;
A is the area on which the pressure field acts.
[0079] In figure 4A it is represented the pressure trend on the inner face 40 and on the
outer face 41 when the radial width of the clearance gap 33 is small. With the expression
"small" it is intended that the radial width is lower than a further threshold.
[0080] In this situation, on the inner face 40 the pressure starts from value P1 and increases
strongly close to the leading edge LE and then the pressure trend tends to value P2,
which is reached at the trailing edge TE.
[0081] On the outer face 41, the pressure in the first chamber 38a is P1 and the pressure
in the second chamber 38b is P2.
[0082] Looking at the diagram in figure 4B, the resulting net force acting on the movable
device 30 is greater radially outward (away from the tip 29).
[0083] Therefore, if the clearance gap 33 is small, the movable element 30 is moved radially
outward by the pressure field in order to increase the radial width of the clearance
gap 33.
[0084] If the net force is zero, the clearance gap 33 has a radial width corresponding to
the operational one and is not changed by the pressure field acting on the movable
element.
[0085] The positioning of the sealing element 37 is, therefore, designed specifically in
order to obtain the above behaviour in use.
[0086] The positioning of the sealing element 37, in fact, determines the areas on which
the outer pressures act in the chambers 38.
[0087] In the non-limiting example here disclosed and illustrated, the axial position of
the sealing element 37 is designed in order to obtain the proper adjustment of the
radial width Wr of the clearance gap 33.
[0088] In particular, the positioning of the sealing element 37 is defined so that the net
force acting on the movable element 30 and determined by the pressure fields is zero
at a target radial width Wr of the clearance gap.
[0089] In the non-limiting example here disclosed and illustrated, the axial position of
sealing element is designed so as the second chamber 38b insists on an area of the
outer face 41 greater than the area on which insists the first chamber 38a.
[0090] Figure 5 shows a variant of the present invention, wherein the movable element 30
is provided with a further sealing element 47 arranged at a distance from the sealing
element 37 so as to create an intermediate chamber 48 between the sealing element
37 and the sealing element 47.
[0091] The intermediate chamber 48 is arranged between the first chamber 38a and the second
chamber 38b.
[0092] In the non-limiting example here disclosed and illustrated, the intermediate chamber
48 is in fluidic communication with the clearance gap 33, preferably by means of a
through hole 50 made in the movable element.
[0093] According to a variant not shown, the intermediate chamber 48 can be in fluidic communication
with the first chamber 38a and/or the second chamber 38b through at least one passage
at the sealing element 37 and/or at the sealing element 47.
[0094] Preferably, the sealing element 37 and the sealing element 47 are substantially identical.
[0095] Similarly, the sealing element 47 is housed in a recess 53 having axial dimensions
greater that the axial dimensions of the sealing element 47.
[0096] In figure 6A it is represented the pressure trend on the inner face 40 (dashed lines)
and on the outer face 41 (continuous line) when the radial width of the clearance
gap 33 is large. With the expression "large" it is intended that the radial width
is greater than a threshold.
[0097] In this situation, on the inner face 40 there is approximately a linear pressure
increase from value P1 to value P2 from the leading edge LE to the trailing edge TE.
[0098] On the outer face 41, the pressure in the first chamber 38a is P1 and the pressure
in the second chamber 38b is P2, the pressure in the intermediate chamber 48 is P3.
[0099] The resulting net force acting on the movable device 30 is greater towards the tip
29.
[0100] Therefore, if the clearance gap 33 is large, the movable element 30 is moved radially
inward by the pressure field in order to reduce the radial width Wr of the clearance
gap 33.
[0101] In figure 6B it is shown the pressure trend on the inner face 40 and on the outer
face 41 when the radial width of the clearance gap 33 is small. With the expression
"small" it is intended that the radial width Wr is lower than a further threshold.
[0102] In this situation, on the inner face 40 the pressure starts from value P1 and increases
strongly close to the trailing edge TE and then the pressure trend tends to value
P2, which is reached at the trailing edge TE. As already stated, the pressure trend
on the inner face 40 depends on several factors, i.e. the shape of the airfoil of
the blade 17.
[0103] On the outer face 41, the pressure in the first chamber 38a is P1, the pressure in
the second chamber 38b is P2 and the pressure in the intermediate chamber 48 is P3.
[0104] The resulting net force acting on the movable device 30 is greater radially outward
(away from the tip 29).
[0105] Therefore, if the clearance gap 33 is small, the movable element 30 is moved radially
outward by the pressure field in order to increase the radial width Wr of the clearance
gap 33.
[0106] If the net force is zero, the clearance gap 33 has a radial width Wr corresponding
to the operational one and is not changed by the pressure field acting on the movable
element.
[0107] In order to obtain a proper clearance control of the radial width Wr of the clearance
gap 33, in this embodiment, both the positioning of the sealing element 37 and the
sealing element 47 must be designed properly depending on the pressure field.
[0108] Moreover, also the positioning of the through hole 50 should be properly placed in
order to have a desired pressure field in the intermediate chamber 48. As we will
see later, the pressure in the intermediate chamber 48 is equal to the pressure in
the clearance gap 33 substantially at the axial position of the through hole 50. Therefore,
the positioning of the through hole 50 determines the pressure in the intermediate
chamber 48.
[0109] In particular, the positioning of the sealing elements 37, 47 and the through hole
50 is defined so that the net force acting on the movable element 30 and determined
by the pressure fields is zero at a target radial width Wr of the clearance gap.
[0110] Other solutions not shown may comprise a plurality of sealing elements in order to
create several chambers whose pressure can be adjusted properly in order to obtain
the desired clearance control.
[0111] Finally, it is clear that modifications and variants can be made to the gas turbine
assembly and to the method described herein without departing from the scope of the
present invention, as defined in the appended claims.
1. Gas turbine assembly extending along a longitudinal axis (A) and comprising:
a stator (6) defining a working channel (8, 9);
a rotor (11) comprising a plurality of rotor stages (13);
each rotor stage (13) comprising a plurality of blades (17) rotatable in the working
channel (8, 9) about the longitudinal axis (A); each blade (17) being provided with
a tip (29);
at least one movable element (30) which is coupled to the stator (6) by means of at
least one retaining device (32) and moves radially between the stator (6) and at least
one tip (29) of the blades (17) for regulating a clearance gap (33) between the at
least one tip (29) and the movable element (30);
wherein the movable element (30) is coupled to the stator (6) by means of a retaining
device (32) so as to create a regulating gap (35) between the stator (6) and the movable
element (30) which is in fluidic communication with the working channel (8, 9); the
movable element (30) being provided with at least one first sealing element (37) dividing
the regulating gap (35) in at least two chambers (38); one first chamber (38a) of
the at least two chambers (38) being in fluidic communication with the working channel
(8, 9) upstream the rotor stage (13) and one second chamber (38b) of the at least
two chambers (38) being in fluidic communication with the working channel (8, 9) downstream
the rotor stage (13).
2. Gas turbine assembly according to claim 1, wherein the movable element (30) is provided
with an inner face (40), facing, in use, the at least one tip (29) of the blade (17)
and with an outer face (41), facing, in use, the regulating gap (35); on the outer
face (41) the movable element (30) being provided with at least one first recess (43)
housing the first sealing element (37) .
3. Gas turbine assembly according to claim 2, wherein the axial dimension of the first
recess (43) is greater than the axial dimension of the sealing element (37).
4. Gas turbine assembly according to anyone of the foregoing claims, wherein the first
sealing element (37) is transversal to a longitudinal plane containing the longitudinal
axis (A).
5. Gas turbine assembly according to anyone of the foregoing claims, wherein the first
sealing element (37) is orthogonal to a longitudinal plane containing the longitudinal
axis (A).
6. Gas turbine assembly according to anyone of the foregoing claims, wherein the first
sealing element (37) is a brush seal.
7. Gas turbine assembly according to anyone of the claims 1-6, wherein the first sealing
element (37) comprises a metal sheet.
8. Gas turbine assembly according to anyone of the foregoing claims, wherein the movable
element (30) is provided with a second sealing element (47) arranged at a distance
from the first sealing element (37) so as to create an intermediate chamber (48) between
the first sealing element (37) and the second sealing element (47); the intermediate
chamber (48) being arranged between the first chamber (38a) and the second chamber
(38b) .
9. Gas turbine assembly according to claim 8, wherein the intermediate chamber (48) is
in fluidic communication with the clearance gap (33).
10. Gas turbine assembly according to claim 9, wherein the intermediate chamber (48) is
in fluidic communication with the clearance gap (33) by means of a through hole (50)
made in the movable element (30).
11. Gas turbine assembly according to claim 8, wherein the intermediate chamber (48) is
in fluidic communication with the first chamber (38a) and/or the second chamber (38b)
through at least one passage at the first sealing element (37) and/or at the second
sealing element (47).
12. Gas turbine assembly according to anyone of the foregoing claims, wherein the retaining
device (32) is configured to reduce the regulating gap (35) when the turbine assembly
(1) is not operating.
13. Gas turbine assembly according to anyone of the foregoing claims, wherein the retaining
device (32) is configured to limit the radial movement of the movable element (30)
between a minimum displacement and a maximum displacement.
14. Gas turbine assembly according to anyone of the foregoing claims, wherein the retaining
device (32) can comprises at least one spring (45), or at least one bimetallic element,
or at least one bellow filled with heat-sensitive material or a magnet, or a combination
thereof.
15. Method for operating a gas turbine assembly (1) extending along a longitudinal axis
(A); the gas turbine assembly (1) comprising:
a stator (6) defining a working channel (8, 9);
a rotor (11) comprising a plurality of rotor stages (13);
each rotor stage (13) comprising a plurality of blades (17) rotatable in the working
channel (8, 9) about the longitudinal axis (A); each blade (17) being provided with
a tip (29);
at least one movable element (30) which is coupled to the stator (6) and moves radially
between the stator (6) and at least one tip (29) of the blades (17) for regulating
a clearance gap (33) between the at least one tip (29) and the movable element (30);
wherein the movable element (30) is coupled to the stator (6) so as to create a regulating
gap (35) between the stator (6) and the movable element (30) which is in fluidic communication
with the working channel (8, 9); the movable element (30) being provided with at least
one first sealing element (37) dividing the regulating gap (35) in at least two chambers
(38); one first chamber (38a) of the at least two chambers (38) being in fluidic communication
with the working channel (8, 9) upstream of the rotor stage (13) and one second chamber
(38b) of the at least two chambers (38) being in fluidic communication with the working
channel (38b) downstream of the rotor stage (13);
the method comprising positioning the at least one first sealing element (37) so that
the net force acting on the movable element (30) and determined by the pressure fields
is zero at a target radial width (Wr) of the clearance gap (33) .