[0001] This invention relates generally to a system for cooling axial flow turbines, particularly
low-pressure steam turbines. More specifically it relates to a system for cooling
last stage blades in low-pressure steam turbines, in particular where such last stage
blades are made from composite materials.
Background
[0002] The rotating blades of low-pressure steam turbines induce tremendous centrifugal
forces into the rotor. This can be a limiting factor in designing the turbine for
maximum efficiency. A solution is to use lower density blade materials as such blades
exert less force into the rotor. This solution can, however, only be applied if the
low-density material has adequate mechanical properties. While using titanium is presently
regarded as the method of choice, future alternatives may have even better strength
to weight ratios. Among the possible alternatives are blades of composite materials,
examples of which is disclosed the published United States patent application
US 2008/0152506 A1 and the published international patent applications
WO 2011/039075 A1 and
WO 2010/066648 and the Swiss Patent Number
CH 547943.
[0003] Composite materials are typically less temperature resistant than metals. This can
be a problem, in particular during low volume flow operation and full speed conditions.
Under such conditions not enough heat is carried by the volume flow through the turbine
and particularly the last stage blades become susceptible to windage heating of the
blade tip area. Normal blade temperatures typically do not exceed 65° C. However,
last stage blade tip temperatures can exceed 250° C under windage conditions without
corrective means. At such temperatures, the mechanical properties of composite material
are significantly impacted and they may suffer permanent degradation.
[0004] A solution to windage heating is provided by
Patent application No. US2007/292265 A1. The solution comprises injecting a cooling medium in the vicinity of the last stage
tip region. The medium, which includes either steam or water, may be injected from
the casing either fore or aft of the blade tip. As an alternative, or in addition,
a small extraction groove for extracting flow through the outer sidewall may be provided
near the blade tip just forward of the blade.
[0005] In view of the prior art it is seen as an object of the present invention to provide
more efficient means and methods of cooling the tips of turbine blades, in particular
last stage blades of composite materials.
Summary
[0006] According to an aspect of the present invention, there is provided an axial flow
turbine having a casing defining a flow path for a working fluid therein, a rotor
co -axial to the casing, a plurality of stages, each including a stationary row of
vanes circumferentially mounted on the casing a rotating row of blades, circumferentially
mounted on the rotor, with an inner face of the casing exposed to the working fluid
having one or more essentially circumferential grooves of increasing depth each ending
in an extraction port with a bore.
[0007] The grooves follow typically a circumferential line around the inner face of the
casing. However the may also deviate by preferably only up to 10 degrees from the
circumferential line. If deviating, the grooves deviate preferably in general flow
direction through the turbine.
[0008] The inner face of the casing in this invention can be the inner face of any part
mounted onto the actual inner face of the casing such as diaphragms, vane carriers,
heat shield etc. The grooves are machined into the face of the part which is exposed
to the flow of the working fluid.
[0009] Preferably, the depth of the groove start at zero depth. The depth best increases
smoothly to avoid the formation of vortices or other obstacles to a smooth extraction
of working fluid.
[0010] The bore of the extraction port is preferably oriented tangentially to the groove
to take advantage of the flow direction of the steam at low volume flow conditions
in the turbine.
[0011] In a preferred variant of the invention there are two grooves in diametrically opposing
positions along essentially the same circumference. For ease of manufacture, it is
best to design the one or more grooves such that they end at a joint line of the casing
and bores for the extraction ports at the opposite side of the joint line. In this
manner the bore can be implemented by drilling through the face of the joint.
[0012] The one or more grooves in conjunction with the extraction port are best adapted
to remove working fluid from a volume in the vicinity of the tip of the blades for
the purpose of cooling the tips of rotating blades, particularly blades of composite
material, for which heating is a more severe problem than for metal blades. Hence
the preferred position of the grooves is located between vanes and blades of the last
stage of the turbine.
[0013] The above and further aspects of the invention will be apparent from the following
detailed description and drawings as listed below.
Brief Description of the Drawings
[0014] Exemplary embodiments of the invention will now be described, with reference to the
accompanying drawings, in which:
FIG. 1 is a schematic axial cross-section of a turbine;
FIG. 2A shows an enlarged view of the last stage of the turbine of FIG. 1;
FIG. 2B is a circumferential cross-section along line A-A' of FIG. 2A;
FIGs. 2C and 2D are axial cross-sections along line B-B' and C-C' of FIG. 2B, respectively;
and
FIGs. 3A and 3B illustrate the flow through the turbine at full volume flow and at
low volume flow, respectively.
Detailed Description
[0015] Aspects and details of examples of the present invention are described in further
details in the following description. Exemplary embodiments of the present invention
are described with references to the drawings, wherein like reference numerals are
used to refer to like elements throughout. In the following description, for purposes
of explanation, numerous specific details are set forth to provide a thorough understanding
of the invention. However, the present invention may be practiced without these specific
details, and is not limited to the exemplary embodiments disclosed herein
[0016] Fig. 1 shows an exemplary multiple stage axial flow turbine
10. The turbine
10 comprises a casing
11 enclosing stationary vanes
12 that are circumferentially mounted thereon and rotating blades
13 that are circumferentially mounted on a rotor
14 with the rotor resting in bearings (not shown). The casing
11, vanes
12 and blades
13 define a flow path for a working fluid such as steam therein. Each blade
12 has an airfoil extending into the flow path from the rotor
14 to a tip region
131 wherein the tip region
131 is defined as the top one third of the airfoil part of the blade
13. The blade
13 can be made of metal, including metal alloys, composites including layered composites
that comprise layered carbon fibre bonded by resins or a mixture of both metal and
composites. The multiple stages of the turbine
10 are defined as a pair of stationary vane and a moving blade rows wherein the last
stage of the turbine
10 is located towards the downstream end of the turbine
10 as defined by the normal flow direction (as indicated by arrows) through the turbine
10. The turbine
10 can be a steam turbine and in particularly a low pressure (LP) steam turbine. As
LP turbine, it is followed typically by a condenser unit (not shown), in which the
steam condensates.
[0017] The last stage of the turbine 10 with the last row of vanes
12 and blades
13 is shown enlarged in the following figures. The FIG. 2B shows a cross-section of
part of the turbine along the line A-A' of FIG. 2A. Before the last blades 14 a pair
of shallow grooves
111 are machined into the inner face of the casing
11 (or of a vane carrier, if the vanes are not mounted directly onto the casing). The
depth of each groove
111 increases gradually in direction of the rotation of the blades
13 from zero to a final depth
d after approximately one half turn. At the final depth
d the groove enters into an extraction hole or channel
112.
[0018] The extraction hole
112 is tangentially to the groove
111 such that the opening of the channel is essentially perpendicular to groove. The
extraction hole releases the steam into a water cooled mixing chamber or directly
into a condenser.
[0019] The extraction hole or channel
112 can be shut using a valve
113 or other suitable means. In normal operations the extraction channels is closed and
opened only when the extraction is required, i.e under low flow volumes or when the
temperature of the blades is rising beyond their operational limits.
[0020] In FIG. 2C, which shows a cross-section along line B-B' of FIG. 2B, the groove
111 has approached close to half its final depth
d. In FIG. 2D, which shows a cross-section along line C-C' of FIG. 2B, the groove
111 is shown at the point of entering the extraction hole or channel
112.
[0021] The groove
111 and the extraction hole
112 are oriented such that hot steam having a circumferential velocity component due
to the rotation of the turbine is diverted from a volume close to the tip of the last
stage blades
13 and guide by the grooves into the tangential extraction hole.
[0022] The groove
111 and the extraction hole
112 are preferably located between the axial positions of the row of vanes
12 and blades
13 as volumes of hot steam are found to circulate in that volume. The width of the groove
and the and the extraction hole
112 are design parameter and can in an extreme case take up most of the inner surface
of the casing between the blades and vanes but are likely to be much smaller for typical
turbines as in actual use today.
[0023] As shown by the comparison of FIGs. 3A and 3B the flow through the turbine can changes
significantly as the mass flow volume drops from its operational level to a lower
level such as less than 50 per cent of the normal mass flow, or even less than 30
per cent of the normal mass flow. It is found that under such low volume operations
the flow through the turbine, which is usually optimized for the operation mass flow
levels, changes to leave pockets where the flow has only a small axial component.
[0024] As shown in FIG. 3A the turbine has a smooth flow field as indicated by the stream
lines under normal flow volumes. The flow has a predominant axial velocity component
in direction to the exit of the turbine. When the flow volume through the turbine
is reduced as is the case for example during start-up, run-out, load change or emergency
situations the flow pattern changes to a more complex picture as illustrated in FIG.
3B.
[0025] Under reduced flow conditions, there are steam volumes with a small axial components.
The volumes tend to have a much larger circumferential component as for example the
volume
31 in FIG. 3B, which circulates predominantly into and out of the paper plane while
have only a small circulation in axial direction. Thus wet film scraping bores which
are used in turbines are rendered inefficient under low loads, as these devices typically
depend on a axial flow velocity to catch the film.
[0026] By making use of the circumferential velocity hot steam can be extracted even with
an adverse back pressure from the condenser unit of the turbine.
[0027] Estimates show that by extracting about 1% of the mass flow using a groove of 300
mm width and a maximum depth d of 20mm the temperature of a last stage blade can be
reduced from 178 degrees C to 166 degrees C. This value can be further increased by
extracting more albeit at the expense of reducing the overall efficiency of the turbine.
[0028] It is advantageous from a manufacturing point of view to have the bores for holes
112 start at the split between the upper and lower half of the turbine casing
11. However the bores can be placed in principle at any point along the circumference
of the casing or vane carrier. It is also possible to increase the number of grooves
from 2 to 3, 4 or more along the same circumferential line. In such a variant of the
invention, the gradient of the grooves is steeper to achieve the same target depth
d after less than a half turn.
[0029] It can be further advantageous to place the extraction grooves and channels at locations
other than between the last stage vanes and blades or to have extraction grooves and
channels at more than just one location. It is further possible to place the two grooves
and extraction channels as described above not along a single circumferential line
but slightly staggered along the axial length of the turbine.
[0030] The present invention has been described above purely by way of example, and modifications
can be made within the scope of the invention, particularly as relating to the shape,
number and design of the extraction grooves and channels. The invention also consists
in any individual features described or implicit herein or shown or implicit in the
drawings or any combination of any such features or any generalization of any such
features or combination, which extends to equivalents thereof. Thus, the breadth and
scope of the present invention should not be limited by any of the above-described
exemplary embodiments.
[0031] Each feature disclosed in the specification, including the drawings, may be replaced
by alternative features serving the same, equivalent or similar purposes, unless expressly
stated otherwise.
[0032] Unless explicitly stated herein, any discussion of the prior art throughout the specification
is not an admission that such prior art is widely known or forms part of the common
general knowledge in the field.
LIST OF REFERENCE SIGNS AND NUMERALS
[0033]
turbine 10
casing 11
vanes 12
blades 13
rotor 14
tip region 131
groove 111
extraction hole 112
final depth d
valve 113
volume 31
1. An axial flow turbine comprising:
a casing defining a flow path for a working fluid therein;
a rotor co -axial to the casing;
a plurality of stages, each comprising:
a stationary row of vanes circumferentially mounted on the casing; and
a rotating row of blades circumferentially mounted on the rotor,
wherein an inner face of the casing exposed to the working fluid includes one or more
essentially circumferential grooves of increasing depth in the rotational direction
of the rotating blades with each groove ending in an extraction port with a bore.
2. The turbine of claim 1 wherein the bore of the extraction port is oriented tangentially
to the groove.
3. The turbine of claim 1 having two grooves in diametrically opposing positions along
essentially the same circumference.
4. The turbine of claim 1 wherein the one or more grooves end at a joint line of the
casing and bores for the extraction ports are at the opposite side of the joint line.
5. The turbine of claim 1 wherein the one or more grooves in conjunction with the extraction
ports are adapted to remove working fluid from a volume in the vicinity of the tip
of the blades for the purpose of cooling the tips of rotating blades.
6. The turbine of claim 1 wherein the extraction ports have valves designed to open or
shut depending on the mass flow flowing through the turbine.
7. The turbine of claim 1 wherein the one or more grooves in conjunction with the extraction
port are located between vanes and blades of the last stage of the turbine.
8. The turbine of claim 1 wherein the one or more grooves in conjunction with the extraction
port are located in the vicinity of a row of blades at least partly made of composite
material.
9. A method of cooling blades in an axial flow turbine having a casing defining a flow
path for a working fluid therein;a rotor co -axial to the casing;a plurality of stages,
each comprising: a stationary row of vanes circumferentially mounted on the casing;
and a rotating row of blades, circumferentially mounted on the rotor, with an inner
face of the casing exposed to the working fluid including one or more essentially
circumferential grooves of increasing depth in the rotational direction of the rotating
blades with each groove ending in an extraction port with a bore, the method including
the steps of opening the extraction port and extracting working fluid from a volume
close to the tips of the blades through the grooves and the extraction port.
10. The method of claim 10, wherein the extraction port is closed during normal operation
of the turbine and opened when the turbine operates under low mass flow conditions.