U.S. patent application number 14/305316 was filed with the patent office on 2014-12-18 for control of low volumetric flow instabilites in steam turbines.
The applicant listed for this patent is ALSTOM Technology Ltd. Invention is credited to Brian Robert HALLER, Timothy Stephen Rice.
Application Number | 20140369815 14/305316 |
Document ID | / |
Family ID | 48669768 |
Filed Date | 2014-12-18 |
United States Patent
Application |
20140369815 |
Kind Code |
A1 |
HALLER; Brian Robert ; et
al. |
December 18, 2014 |
CONTROL OF LOW VOLUMETRIC FLOW INSTABILITES IN STEAM TURBINES
Abstract
Configuration of the last stage of a steam turbine where rotor
blades rotate encircled by a vane carrier, such that a plurality of
passages are located in the vane carrier, such that a fluid is
blown through these passages forming a flow that impinges onto the
rotor blades, the number of passages, the location of the passages
in the vane carrier and the velocity of the flow impinging onto the
rotor blades, being calculated in such a way that rotating flow
instabilities in the rotor blades when the steam turbine operates
at low volumetric flow conditions are avoided.
Inventors: |
HALLER; Brian Robert;
(Lincolnshire, GB) ; Rice; Timothy Stephen;
(Warwickshire, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ALSTOM Technology Ltd |
Baden |
|
CH |
|
|
Family ID: |
48669768 |
Appl. No.: |
14/305316 |
Filed: |
June 16, 2014 |
Current U.S.
Class: |
415/119 |
Current CPC
Class: |
F01D 21/003 20130101;
F01D 5/00 20130101; F01D 19/00 20130101; F01D 5/16 20130101; F01D
5/10 20130101; F01D 25/04 20130101; F01D 9/04 20130101; F01D 11/10
20130101; F01D 25/06 20130101; F01D 25/24 20130101 |
Class at
Publication: |
415/119 |
International
Class: |
F01D 5/10 20060101
F01D005/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 17, 2013 |
EP |
13172223.3 |
Claims
1. Configuration of a last stage of a steam turbine where rotor
blades rotate encircled by a vane carrier, characterized in that a
plurality of passages are located in the vane carrier, such that a
fluid is blown through these passages upstream of the rotor blades
and arranged to form a flow that impinges onto the rotor blades
with an injection angle of between zero to -90 degrees against the
rotation of the rotor blades such that rotating flow instabilities
in the rotor blades when the steam turbine operates at low
volumetric flow conditions are reduced.
2. Configuration according to claim 1, wherein the passages are
configured and arranged to blow fluid towards a point that is
approximately 80% of a height of the last stage rotor blade taken
from a base to a tip of the rotor blade.
3. Configuration according to claim 1, wherein the plurality of
passages are circumferentially uniformly spaced in the vane
carrier.
4. Configuration according to claim 3, wherein the plurality of
passages circumferentially uniformly spaced in the vane carrier
number eight.
5. Configuration according to claim 3, wherein the plurality of
passages (circumferentially uniformly spaced in the vane carrier
number twelve.
6. Configuration according to claim 1, wherein the passages are
circumferentially shaped in such a way that a circumferential
coverage in the vane carrier is increased.
7. Configuration according to claim 1, wherein the injection angle
is in a range from -45 to -75 degrees.
8. Configuration according to claim 1, wherein the injection angle
is about -60 degrees.
9. Configuration according to claim 1 wherein in that the flow
injected through the passages is up to 10% of the mainstream flow
circulating through the rotor blades and the vane carrier.
10. Configuration according to claim 1 wherein in that the flow
injected through the passages is approximately 10% of the
mainstream flow circulating through the rotor blades and the vane
carrier.
11. Configuration according to claim 1, wherein the fluid blown
through the passages is steam.
12. A steam turbine comprising a last stage configuration according
to claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to European application
13172223.3 filed Jun. 17, 2013, the contents of which are hereby
incorporated in its entirety.
TECHNICAL FIELD
[0002] The present invention relates to a configuration of the last
stage in a steam turbine for controlling rotating flow
instabilities in the last stage rotor blades when the steam turbine
operates at low volumetric flow conditions, particularly during
starting and low load conditions.
BACKGROUND
[0003] Stalling is a known phenomenon based on the sudden decrease
of the load exerted onto a profile subjected to a flow: in steam
turbines, the stalling phenomenon induces rotating flow
instabilities in the rotor blades, particularly in the last stage
rotor blades.
[0004] In steam turbines, during starting and low load conditions
(up to around 10% of the design mass flow), the flow structure is
very disorderly, particularly in the low pressure stage of the
steam turbine: this flow is centrifuged radially outwards in the
rotor blades, the flow being centrifuged radially inwards in the
stator blades. At low load conditions there is high flow incidence
onto the last stage rotor blades, which can cause flow separation
from the rotor blades surface and flow instabilities, these
instabilities are commonly found to rotate at about one half of the
blade rotational speed. At this point the flow field also contains
large toroidal vortex structures are set up. These rotating
instabilities can couple with the natural frequency of the rotor
blades and produce undesirable vibration effects.
[0005] Some solutions known in the prior art minimize this problem
by removing the last stage low pressure channel, which is replaced
by a newly designed part, frequently comprising a perforated plate.
However, this results in a great loss of efficiency in the steam
turbine, also being very costly. Besides, it is possible that the
rotating flow instabilities move upstream and makes that other
stages in the steam turbine fail.
[0006] The invention is oriented towards solving these
problems.
SUMMARY
[0007] The present invention relates to a configuration for
controlling flow instabilities in steam turbines when they operate
at low volumetric conditions, particularly during starting and low
load conditions. The configuration of the invention comprises a
plurality of passages located in the last stage vane carrier of the
low pressure stage of the steam turbine, these passages being
located at specific positions at the vane carrier: through these
passages, a fluid is blown onto the rotating blades to counteract
rotating flow instabilities in them. The number of passages and
their specific positions are defined in such a way that the fluid
blown is directed towards the rotor blades and preventing the
excitation.
[0008] According to one embodiment of the invention, the passages
are shaped circumferentially in order to increase the
circumferential coverage of each passage.
[0009] The fluid blown through the passages into the rotor blades
is such that the swirl injection angle incident on the rotor blades
forms an angle from zero to -90 degrees. The positive angle being
taken in the direction of the turbine rotor rotation, with zero
degrees being axial, wherein in the axial/radial plane the jet is
directed downwards from the outer flow boundary.
[0010] With the configuration of the invention, a near complete
elimination of the rotor blade vibration is achieved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The foregoing objects and many of the attendant advantages
of this invention will become more readily appreciated as the same
becomes better understood by reference to the following detailed
description when taken in conjunction with the accompanying
drawings, wherein:
[0012] FIG. 1 shows the characteristics of low flow in a steam
turbine. Radial movements of the through flow can be seen together
with the recirculation zones created;
[0013] FIG. 2 shows an embodiment of the invention where the
passages are shaped in the circumferential direction to increase
the flow coverage for each passage provided;
[0014] FIG. 3 shows schematically the configuration of the
invention for controlling flow instabilities in the last stage
rotor blades of a steam turbine when the turbine operates at low
volumetric conditions;
[0015] FIGS. 4a,b,c shows the influence of injection swirl angle on
Volumetric flow versus fractional Speed and Vibration amplitude;
and
[0016] FIG. 5 shows the influence of blowing mass flow on dynamic
blade stress.
DETAILED DESCRIPTION
[0017] The present invention relates to a configuration 10 for
controlling flow instabilities in the last stage rotor blades 2 of
a steam turbine when the turbine operates at low volumetric
conditions, particularly during starting and low load conditions.
The configuration 10 is such that a plurality of passages 20 are
located in the vane carrier 1 of the last stage of the steam
turbine, these passages 20 being located at specific positions at
the circumference of the vane carrier 1. Through these passages 20,
a fluid is blown onto the rotor blades 2. The number of passages 20
and their specific positions are defined in such a way that the
fluid blown through the passages 20 is directed towards the rotor
blades 2 avoiding rotating stability problems in these last stage
rotor blades 2 that produce undesired vibration effects on
them.
[0018] FIG. 1 shows the flow pattern in the last stage low pressure
vane carrier 1 during starting and low load conditions (up to
around 10% of the design mass flow), showing that the flow
structure is very disorderly. The through flow in the vane carrier
1 adopts a wavy shape, as shown in FIG. 1, existing large toroidal
vortex structures 30: the last stage low pressure vane carrier 1
actually acts as a radial pump and there is net energy input to the
stage. According to the known prior art, a solution is to use water
sprays injected in the exhaust diffuser to cool the exhaust casing
vane carrier walls and last stage blades, but this solution has not
been found to be reliable.
[0019] The purpose of the configuration 10 of the invention is to
design the passages 20 to eliminate the rotating flow instabilities
in the last stage rotor blades 2 during starting and low load
conditions of the steam turbine.
[0020] The positions of the passages 20 upstream of the last stage
rotor blade 2 is such that the injection flow is directed through
the last stage vane carrier 1 to approximately 80% last stage blade
height, as measured from the blade platform to the tip, so as to
blow into the torodial vortex 30 typically formed upstream of the
rotor blade 2 tip region.
[0021] From the series of FIGS. 4a, 4b and 4c shows a series of
tests that demonstrate the surprising effect that a negative
injection angle results in a more stable and steady separated flow,
decoupled from resonance can be seen. The tests were carried out in
a one third scale model low pressure steam turbine over a range of
mass flow rates and condenser pressure. During the tests
measurement were made of last stage blade stress using a strain
gauge located on the surface of the last stage blade. Results of
these measurements are shown as lines representation vibrational
amplitude in FIGS. 4a, 4b and 4c.
[0022] An additional dynamic pressure sensor, acting as a
microphone, was additional located in the flow to detect the
formation of the rotating events that can give rise to blade
vibration. From the pressure signal it was possible to determine
frequency, which is transformable into fractional speed, and
represent this as spheres in FIGS. 4a, 4b and 4c. The amplitude
from the pressure sensor was then used in FIGS. 4a, 4b and 4c to
define the size of the grey spheres on each of the graphs.
[0023] Plots were then produced of fractional speed and vibrational
amplitude versus volumetric flow for each of the cases of +60
degree injection as shown in FIG. 4a no injection as shown in FIGS.
4b, and -60 degree injection as shown in FIG. 4c, wherein
volumetric flow is defined as the average axial flow velocity
leaving the last stage divided by the blade root speed.
[0024] In each case, measured high vibration amplitude events were
found to coincide with higher dynamic pressure amplitude and loss
of its frequency scatter. With an injection at +60 degrees appeared
to exacerbate vibrational amplitude, as seen in FIG. 4a when
compared with the no injection case shown in FIG. 4b. With an
injection angle of -60 it was possible to eliminate blade
vibration, as can been seen in FIG. 4c. As further shown in FIG. 5,
a negative injection rate has a positive effect on reducing
relative dynamic stress even at very shallow injection angles.
[0025] According to an embodiment, the fluid injected from the
passages 20, which preferably is steam, is such that the injection
angle incident on the rotor blades 2 forms an angle from zero to
-90 degrees, the negative angle being taken in the direction
counter to the turbine rotor rotation. According to a further
embodiment of the invention, the preferred injection angle range is
-45 to -75 degrees, the most preferred injection angle being -60
degrees. According to still a further embodiment of the invention.
The flow injected from the passages 20 is up to 10% of the
mainstream flow.
[0026] The number of passages 20 relative to the number of rotor
blades 2 is set to provide sufficient stabilization of the rotating
events. In the case of the test results given, 12 passages were
used. Other embodiments of this invention may use a different
number of passages to obtain sufficient stabilization.
[0027] In an embodiment of this invention the passages are equally
spaced around the circumference. In an alternative embodiment the
passages are unevenly spaced around the circumference for enhanced
performance or for practical considerations.
[0028] Together with the injection angle the velocity of the fluid
blown onto the rotor blades 2 is also important.
[0029] Therefore, the following parameters influence the
performance of the configuration 10 of the invention maintaining
the trajectory length of the fluid blown from the passages 20 as
small as possible; maintaining the velocity of the fluid injected
as high as possible; and maximizing the circumferential extent of
the passages 20 in the vane carrier 1.
[0030] It is difficult to weight the above-cited parameters and,
therefore, a different optimum absolute injection angle exists and
has to be calculated for each specific case.
[0031] According to one embodiment of the invention, the passages
20 are circumferentially shaped to increase the circumferential
coverage in the vane carrier 1 as shown in FIG. 2.
[0032] With the configuration 10 of the invention, a major
minimization of the rotor blades 2 vibration and of their critical
resonance is achieved. Moreover, the use of passages 20 to control
rotating flow instabilities constitutes a way of controlling the
flow instabilities rotating problem and does not lead to a loss in
the efficiency at design full-load conditions.
[0033] Although the present invention has been fully described in
connection with preferred embodiments, it is evident that
modifications may be introduced within the scope thereof, not
considering this as limited by these embodiments, but by the
contents of the following claims.
* * * * *