U.S. patent application number 11/073708 was filed with the patent office on 2005-09-08 for sealing arrangement in turbine machinery.
This patent application is currently assigned to ALSTOM Technology Ltd.. Invention is credited to Hogg, Simon Ian.
Application Number | 20050194745 11/073708 |
Document ID | / |
Family ID | 32117253 |
Filed Date | 2005-09-08 |
United States Patent
Application |
20050194745 |
Kind Code |
A1 |
Hogg, Simon Ian |
September 8, 2005 |
Sealing arrangement in turbine machinery
Abstract
A sealing arrangement in a steam turbine is deployed in a
position where it can restrict flow of steam along a flowpath in a
gap between two bodies of the turbine rotatable relative each
other, the flowpath proving communication between a space of
relatively high pressure and another of relatively low pressure.
The sealing arrangement comprises a leaf seal mounted on one of the
bodies and sealingly engaged with the other. Two or more leaf seals
can be juxtaposed with each other, without need for labyrinth
seals, in a steam turbine application, without deleterious effects
experienced in use of brush seals in similar circumstances. The
sealing arrangement can include segments displaceable from a
sealing position to increase clearance during start-up of a
turbomachine, thereby taking account of potential vibration
problems as the turbine rotor passes through natural
frequencies.
Inventors: |
Hogg, Simon Ian;
(Warwickshire, GB) |
Correspondence
Address: |
BUCHANAN INGERSOLL PC
(INCLUDING BURNS, DOANE, SWECKER & MATHIS)
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
ALSTOM Technology Ltd.
Baden
CH
|
Family ID: |
32117253 |
Appl. No.: |
11/073708 |
Filed: |
March 8, 2005 |
Current U.S.
Class: |
277/301 |
Current CPC
Class: |
F16J 15/3292
20130101 |
Class at
Publication: |
277/301 |
International
Class: |
F01D 011/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 8, 2004 |
GB |
0405196.7 |
Claims
1. A steam turbo machine comprising a stator and a rotor rotatable
with respect to the stator, for receiving an inlet flow of steam
for driving the rotor, wherein opposing circumferential surfaces of
the stator and rotor define between them a gap capable of enabling
fluid communication between a space of said machine of relatively
high pressure in use to a space of relatively low pressure in use,
the gap being sealed by a sealing arrangement, wherein the sealing
arrangement is a leaf seal.
2. A machine according to claim 1 wherein said rotor includes a
balance piston, for maintaining a compensating force with respect
to the reaction of the rotor against the inlet steam in operation,
said balance piston having a circumferential surface which defines,
with an opposing circumferential surface of the stator, said gap
between a space which, on operation of the turbine, is at a
relatively high pressure and a space which, on operation of the
turbine is at a relatively low pressure, a sealing arrangement
being interposed in said gap and wherein said sealing arrangement
is a leaf seal.
3. A machine according to claim 1 wherein said relatively high
pressure is a steam inlet pressure at a steam inlet of the steam
turbine machine.
4. A machine according to claim 1 wherein said relatively low
pressure is an exhaust pressure at a steam exhaust of said steam
turbine machine.
5. A machine according to claim 1 wherein said rotor has an
external circumferential surface, and said stator has an internal
circumferential surface, developed around the axis of rotation of
said rotor, and a turbine chamber is defined between said surfaces,
said rotor having a plurality of substantially radially extending
turbine blades, configured to present a surface against which said
inlet flow of steam reacts in use to generate a torque acting on
said rotor relative said stator, each of said turbine blades
extending radially to adjacent said inner surface of said stator,
so as to define a gap in which a sealing arrangement is interposed,
said sealing arrangement being a leaf seal.
6. A machine according to claim 5 wherein said stator has a
plurality of inwardly substantially radially extending turbine
blades configured to direct, in operation of said turbine, said
inlet flow onto said turbine blades of said rotor to cause said
torque to be developed in use, wherein each of said stator turbine
blades extend radially to adjacent said external surface of said
rotor and thereby defining a gap in which a sealing arrangement is
interposed, said sealing arrangement comprising a leaf seal.
7. A machine according to claim 1 wherein the rotor comprises
opposing and coaxial mounting means for cooperation with
corresponding formations of said stator, said mounting means and
formations each having opposing adjacent circumferential surfaces
defining a gap in which a sealing arrangement is interposed,
wherein said sealing arrangement is a leaf seal.
8. A machine according to claim 1 wherein said sealing arrangement
comprises two leaf seals, interposed in said gap.
9. A machine according to claim 1 wherein the or each leaf seal
comprises a plurality of leaves extending from one of the rotor and
stator across said gap towards the other of the rotor and
stator.
10. A machine according to claim 1 wherein said sealing arrangement
comprises a plurality of support segments, circumferentially
arranged about the gap, each support segment presenting a leaf seal
in said gap, each segment being radially displaceable in said gap,
wherein said sealing arrangement further comprises resilient means
for urging said segments into an initial position displaced away
from a sealing position, said segments are arranged to be urged
into said sealing position by a reaction force developed in use
through high pressure in said high pressure region of said turbine
machine.
11. A method of sealing a gap in a steam turbomachine between a
region of relatively high pressure and an area of relatively low
pressure, between relatively rotatable members of said
turbomachine, said method comprising interposing a leaf seal in
said gap, mounting said seal on one of said members and directed
sealingly at a surface defined on the other of said members.
12. A steam turbine machine substantially as described herein with
reference to FIGS. 1 to 3 of the accompanying drawings.
13. A steam turbine machine substantially as described herein with
reference to FIG. 4 of the accompanying drawings.
14. A method of sealing a gap in a steam turbomachine substantially
as described herein with reference to FIGS. 1 to 3 of the
accompanying drawings.
15. A method of sealing a gap in a steam turbomachine substantially
as described herein with reference to FIG. 4 of the accompanying
drawings.
Description
[0001] The present invention relates to a sealing arrangement in
turbine machinery, particularly in steam turbine machinery.
[0002] Seals are employed in steam turbine machinery at points
where a surface of a rotating body of a machine (the rotor) opposes
a surface of a stationary part of the machine (the stator). Seals
are used to restrict the passage of fluid from a space of
relatively high pressure to a space of relatively low pressure.
[0003] Seals for steam turbine machinery have historically been in
the form of so-called "labyrinth" seals. A labyrinth seal, formed
in a flow pathway between a rotor and a stator comprises one or
more formations on surfaces of the rotor and/or stator, to present
obstructions to the flow of fluid in the flow pathway. These
obstructions constrict the cross-sectional area of the flow pathway
so that the fluid is forced to accelerate and thus become of
relatively low pressure, and then to decelerate very rapidly as it
passes through the labyrinth seal. This rapid deceleration of the
fluid as it exits the constriction causes uncontrolled expansion of
the fluid, which induces energy losses which are effective at
presenting upstream obstruction to fluid flow. However, it is
appreciated widely that labyrinth seals are only partially
effective, and improvements thereto are desired.
[0004] To provide improved performance, brush seals have been
introduced to act instead of or alongside labyrinth seals. A brush
seal comprises a backing plate into which are embedded a very large
plurality of pliable wire or fibre filaments. A brush seal also
operates to provide a tortuous path to the progress of gas or other
fluid through the seals. In operation, a brush seal is about 90%
more effective than a labyrinth seal in holding a pressure drop
between two spaces in a turbo machine.
[0005] In some circumstances, therefore, brush seals provide an
effective seal in turbo machines where the gap between the rotor
and stator is in the order of 1 mm. However, in gas turbines, the
gap between the rotor and stator components can vary widely. In
particular, thermal, gyroscopic and other mechanical effects can
cause variation in the gap between the rotor and stator, and brush
seals have been found to be effective at dealing with this.
[0006] One observation has been made that the maximum pressure drop
that can be carried by the seal is inversely proportional to the
square of the gap. Thus, by increasing the gap to, say, 3 mm, a
seal with the same bristles as the seal described above would only
be able to accommodate a maximum pressure drop one ninth of that
which can be accommodated by the seal described above with the 1 mm
gap.
[0007] It has been found through experience that the construction
of a brush seal capable of withstanding these gaps would require
fibres so thick and strong that their compliance would be severely
limited. This would raise the risk of damage to the brush, and to
wear within the turbo machine. Thus, the need has been identified
to devise a seal capable of overcoming this problem.
[0008] However, in steam turbines different technical problems are
presented that are of greater significance than the above problems
relating to the operation of gas turbines. Indeed, the above
problems may not be exhibited at all in steam turbines. Steam
turbines are generally much larger than gas turbines and they
operate at relatively lower temperatures. Further, rotational
speeds of rotors in steam turbines can be lower than in a gas
turbine.
[0009] For these reasons steam turbines are perceived to operate in
more controlled conditions than those under which a gas turbine
operates. Because of these less extreme conditions, rotor-stator
gap is not highly variable--temperature expansion of moving parts
in a steam turbine is likely to be less significant and gyroscopic
effects are likely to be greatly reduced. In these circumstances, a
1 mm clearance between the rotor and stator is usually sufficient
and can usually be relied upon to remain relatively constant, at
least to the extent that `hard on hard` contact (contact of the
rotor and stator surfaces themselves) can be realistically assumed
not to be a risk.
[0010] On the other hand, pressure drops experienced in steam
turbines are often significantly higher than in gas turbine
technology. For example, a typical pressure in a high-pressure
cylinder of a steam turbine could be as much as 200 bar, while
exhaust pressure might be in the region of 80 bar. In the past,
labyrinth seals have been used to control this substantial pressure
drop. However, it is appreciated that the length of flow path
presented by such a labyrinth must be significant to provide
significant constriction on the flow path, and this will add
significantly to the axial length of a turbo machine. This could
significantly increase the size of the machine in relation to its
purpose and could impact on the positioning of other machines to
which the turbo machine should be connected in installation.
[0011] Brush seals have been presented as possible solutions to
this problem, in that they can control such a pressure drop over a
shorter axial length than a labyrinth seal. However, in practice,
brush seals have been found to be inappropriate in steam turbine
applications. In particular, it has been found that brush seals are
inappropriate in situations where very high-pressure steam and high
swirl are encountered. The swirl kinetic energy can cause
significant disruption of the brush elements of the brush seal,
which can lead to failure of the seal. Further, this swirling of
the inlet steam can, over time, lead to fatigue related damage to
the filaments of the brush.
[0012] In addition, high-pressure steam, such as is encountered in
steam turbines, can carry significant particulate matter, such as
detritus from the interior of boiler apparatus used for generating
the steam. Such particulate matter can impact with the brush seal
so as to cause substantial damage to the brush filaments.
[0013] Thus, it would be desirable to be able to provide a seal for
a steam turbine machine capable of operating in conditions of
swirling intake and in which the risk of damage to the seal by
steam borne particulates is reduced.
[0014] In accordance with one aspect of the invention, a steam
turbo machine comprises a stator and a rotor rotatable with respect
to the stator, opposing circumferential surfaces of the stator and
rotor defining between them a plurality of gaps capable of fluid
communication, the gaps being sealed by sealing arrangements,
wherein at least one of the sealing arrangements is a leaf
seal.
[0015] By using a leaf seal in a steam turbine in preference to a
labyrinth seal or a brush seal, problems previously exhibited
through the use of a labyrinth seal (such as the extreme length of
such a labyrinth seal) or a brush seal (susceptibility to damage
through swirl and steam borne particles) can be reduced.
[0016] In a balance piston seal environment, pressure differences
between the high-pressure area and the low-pressure area can be
extreme. Even in the case of a leaf seal, it can be difficult to
arrange for a sealing means to support the pressure drop required
for effective operation of a steam turbine. Therefore, it has long
been understood that placing two or more seals in series may be an
effective way of producing an appropriate seal.
[0017] However, it has been found that brush seals, placed in
series are less effective than anticipated, as the flow of steam
through the first seal produces unpredictable flow effects in the
space between the two seals, affecting the sealing effectiveness of
the second seal. This can have a deleterious impact on the
efficiency and operation of brush seals in series.
[0018] In the past, this problem has been solved by inclusion of
labyrinth seals between brush seals in series; as noted above,
labyrinth seals can compromise the compactness of a sealing
arrangement, which can have a negative impact on the effective
arrangement of turbine machinery in a plant. The inclusion of a
labyrinth seal can also sometimes compromise the assembly of a
sealing system. This has led to compromise in the level of
compactness of a sealing arrangement comprising two or more brush
seals in series. Thus it is desirable to provide a sealing
arrangement directed at addressing these problems, such that
sealing effectiveness can be improved without compromising machine
compactness.
[0019] According to a further aspect of the invention, a
turbomachine powered with a flow of steam defines a region of
relatively high pressure and a region of relatively low pressure,
separated by a flow path between two relatively rotatable members
of the turbomachine, and a sealing arrangement operable to restrict
the passage of steam in said flowpath, wherein said sealing
arrangement comprises a series of leaf seals.
[0020] A surprising consequence of providing leaf seals in series
is that the need to provide labyrinth seals to stabilise flow
patterns to maintain seal effectiveness is obviated.
[0021] Further aspects of the invention, advantages thereof, and
problems solved thereby will become apparent from the following
description of specific embodiments of the invention accompanied by
the appended drawings in which:
[0022] FIG. 1 is an axial cross-section through a steam turbine in
accordance with a specific embodiment of the invention;
[0023] FIG. 2 is an axial cross-section through a sealing
arrangement in the steam turbine of FIG. 1;
[0024] FIG. 3 is a cross-section through a line marked A-A in FIG.
2; and
[0025] FIG. 4 is a cross-section, in a similar orientation to that
illustrated in FIG. 2, of a sealing arrangement according to a
second embodiment of the invention.
[0026] FIG. 1 illustrates a steam turbine of substantially known
construction, but fitted with seals in accordance with a preferred
embodiment of the invention. Thus, the turbine 10 comprises a
cylindrical casing 12 and a rotor 14 which includes a balance
piston 16 and trunnions 18, 20 at upstream and downstream ends
respectively.
[0027] An outer cylindrical surface 22 of the rotor 14 and an inner
surface 24 of the casing 12 define a turbine chamber 26. The
turbine chamber is defined such that it is radially narrower at the
upstream end than the downstream end, in accordance with usual
turbine design practices.
[0028] Projecting from the rotor surface 22 and the casing surface
24 are series of rotor blades 30 and stator blades 32
respectively.
[0029] An inlet manifold 34 is defined in the casing 12, in fluid
communication with the upstream end of the turbine chamber 26. As
indicated in FIG. 1, in operation the turbine 10 receives a flow of
steam through this inlet manifold 34 at a pressure of up to 200
bar. An exhaust manifold 36 is defined in the casing 12 and is in
fluid communication with the downstream end of the turbine chamber
26; in operation exhaust steam exits the turbine 10 through the
exhaust manifold 36, as indicated in FIG. 1.
[0030] Upstream end glands 40 are located circumferentially around
the upstream end trunnion 18, to prevent escape of inlet gas from
the gap formed between the trunnion 18 and the casing 12. Balance
piston seals 42 are placed circumferentially around the balance
piston 16, to restrict flow of gas through the gap defined between
the balance piston 16 and the casing 12. The balance piston seals
42 can experience a pressure drop across them of up to 120 bar in
operation. This is because the input high pressure steam introduced
by inlet 34 can be input at a pressure of up to 200 bar, whereas
the pressure in the exhaust manifold 36 (with which the balance
piston is in communication in order to provide balancing thrust to
the rotor) may be at a pressure of the order of magnitude of 80
bar. Rotor blade seals 44 are provided in the gaps between the tips
of the rotor blades 30 and the inner surface 24 of the casing 12.
Corresponding stator blade seals 46 are provided between the tips
of the stator blade 32 and the outer surface 22 of the rotor 14.
Provision of rotor blade seals 44 and stator blade seals 46
restrict flow of gas at the tips of the blades 30, 32, thereby
increasing the tendency of steam in the turbine chamber 26 to be
forced past the reaction surfaces of the turbine blades and thereby
provide torque to the rotor 14.
[0031] FIG. 2 illustrates a balance piston seal 42, which comprises
a support casing 50 of substantially annular construction, mounted
on the interior surface 24 of the casing 12.
[0032] The casing 50 defines a channel in which are loaded a
plurality of leaves 52. The leaves 52 are generally rectangular and
extend longitudinally of the channel and thus radially of the
balance piston 16. Laterally extending lugs 54 correspond with
opposing surfaces 56 of the channel 50, so as to cause the leaves
52 to be retained in the channel. The ends of the leaves 52
directed away from the retaining lugs 54 extend longitudinally from
the casing 50 and protrude beyond the radial extent of casing
50.
[0033] FIG. 3 shows in further detail the leaves 52 in relation to
the direction of rotation of the piston 16. The leaves 52 are
shingled so as to engage against the surface of the balance piston
16 as it moves past. It is anticipated that the compliance of the
leaves 52 will provide a return force to provide the desired
sealing effect, as the seal leaves 52 engage the surface of the
balance piston 16.
[0034] It has been found that a leaf seal in this form is capable
of supporting a pressure drop of 120 bar, such as will be
experienced in the position of a balance piston in a steam turbine.
Further, it will be appreciated that similar leaf seals can be
placed at the point of inlet gland seal 40, exhaust gland seal 48,
and for rotor blade and stator blade seals 44, 46. Further, the
structural rigidity of leaves 52 is superior to a brush seal of
similar size, and is thus able to withstand the impact of a
swirling inlet gas. In addition, leaves 52 will be less susceptible
to damage as a result of impact from particles borne on the inlet
steam.
[0035] FIG. 4 illustrates a sealing arrangement according to a
second embodiment of the invention. This sealing arrangement is
shown as, for the purposes of this example, a balance piston seal
42.
[0036] Two axially spaced, parallel, bearing piston leaf seals 42
are arranged on the casing 12 and directed inwardly of the turbine
casing 12, to engage sealingly with the balance piston 16. No
labyrinth features are interposed between the two leaf seals 42.
The two leaf seals act in tandem to provide adequate separation
between an upstream zone and a downstream zone in the turbine,
thereby affecting the seal required for effective operation of the
turbine 10.
[0037] By arranging leaf seals in this manner, previous problems
related to efficiency of brush seals operated in series, together
with the additional axial length that would be required of the
machine in which two brush seals were placed in series, together
with supposed labyrinth seal, can be avoided.
[0038] In either of the above-described embodiments, it would be
desirable to arrange that the leaf seals 42 are detachable from the
casing 12, to allow worn leaves 52 of the leaf seals to be replaced
from time to time.
[0039] It would be appreciated that the present invention is not
limited to its application to steam turbines. Other machines, in
which similar conditions of temperature and pressure arise, could
be susceptible to the implementation of the present invention. The
present invention essentially addresses problems related to sealing
a flow path defined between two relatively rotatable parts of a
machine, running at speeds comparable to those encountered in a
steam turbine, wherein seal limits flow of steam between a region
of high pressure (a pressure that might be experienced at an inlet
of a steam turbine) and a region of low pressure (a pressure that
might be experienced at an exhaust outlet of a steam turbine).
[0040] It will be appreciated further that other modifications and
adaptations, which provide technical effects complementary to the
technical effect of the present invention, can be provided in a
steam turbo machine. For example, as per the disclosure of U.S.
Pat. No. 4,436,311A, the sealing arrangement may be mounted on
radially movable segments about the rotor, the segments being urged
apart by circumferentially aligned springs to provide for a larger
rotor/stator gap at low rotational speeds and low pressure to avoid
`hard on hard` contact occurring during a start up phase of a
turbine. As described in that document, the turbine is particularly
susceptible to hard on hard contact during start up as turbine
rotor may, as it accelerates rotationally, pass through speeds at
which a natural frequency of the rotor is encountered. This can
lead to vibration of the rotor and consequent risk of fouling
against a stator component that is positioned too close to the
rotor.
[0041] Once the rotor has reached a driving speed, high-pressure
steam is then introduced to the turbine. The segments of the
sealing arrangement are arranged such that the reaction force
applied by the steam to the segments of the sealing arrangements is
operable to urge the segments towards a sealing position against
the rotor. Radial movement of the segments towards the rotor may be
limited by a captive formation with respect to the turbine casing,
to prevent the sealing arrangement segments from fouling the
rotor.
* * * * *