U.S. patent application number 11/394489 was filed with the patent office on 2007-10-11 for offset blade tip chord sealing system and method for rotary machines.
Invention is credited to Ronald Scott Bunker.
Application Number | 20070237627 11/394489 |
Document ID | / |
Family ID | 38575477 |
Filed Date | 2007-10-11 |
United States Patent
Application |
20070237627 |
Kind Code |
A1 |
Bunker; Ronald Scott |
October 11, 2007 |
Offset blade tip chord sealing system and method for rotary
machines
Abstract
A rotary machine includes a rotary member disposed inside the
stationary member, wherein the rotary member includes at least one
airfoil having an upstream side wall, a downstream side wall, and a
tip portion disposed between the upstream and downstream side
walls. A sealing system is disposed on the tip portion. The sealing
system consists essentially of at least one seal strip disposed on
the tip portion at an off-center position only between the
downstream side wall and a central position between the upstream
and downstream sidewalls.
Inventors: |
Bunker; Ronald Scott;
(Niskayuna, NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY (PCPI);C/O FLETCHER YODER
P. O. BOX 692289
HOUSTON
TX
77269-2289
US
|
Family ID: |
38575477 |
Appl. No.: |
11/394489 |
Filed: |
March 31, 2006 |
Current U.S.
Class: |
415/170.1 ;
415/182.1 |
Current CPC
Class: |
F01D 5/20 20130101; Y02T
50/673 20130101; F01D 5/145 20130101; Y02T 50/60 20130101; F01D
11/08 20130101 |
Class at
Publication: |
415/170.1 ;
415/182.1 |
International
Class: |
F01D 1/00 20060101
F01D001/00; F01D 25/24 20060101 F01D025/24 |
Claims
1. A rotary machine, comprising: a stationary member; a rotary
member disposed inside the stationary member, wherein the rotary
member comprises at least one airfoil having an upstream side wall,
a downstream side wall, and a tip portion disposed between the
upstream and downstream side walls; and a sealing system disposed
on the tip portion, wherein the sealing system consists essentially
of: at least one seal strip disposed on the tip portion at an
off-center position substantially between the downstream side wall
and a central position between the upstream and downstream
sidewalls.
2. The machine of claim 1, wherein the rotary machine comprises a
turbine.
3. The machine of claim 1, wherein the off-center position of the
at least one seal strip is an offset distance from the downstream
side wall at least equal to a wall thickness of the at least one
airfoil.
4. The machine of claim 1, wherein the off-center position of the
at least one seal strip is an offset distance from the downstream
side wall at least equal to a radius of curvature of a tip edge of
the at least one airfoil.
5. The machine of claim 1, wherein the at least one seal strip
includes a first portion positioned between the downstream side
wall and the central position and a second portion disposed between
the upstream side wall and the central position, wherein the first
portion is substantially greater than the second portion.
6. The machine of claim 1, wherein the seal strip comprises a base
portion having fillets.
7. The machine of claim 1, wherein the seal strip comprises a
rounded top portion.
8. A turbine, comprising: a stationary outer casing; a rotor
disposed inside the stationary outer casing, wherein the rotor
comprises at least one rotor blade having an upstream side wall, a
downstream side wall, and a tip portion disposed between the
upstream and downstream side walls; and at least one seal strip
disposed on the tip portion at an off-center position substantially
between the downstream side wall, and a central position between
the upstream and downstream side walls.
9. The turbine of claim 8, wherein a width of the at least one seal
strip is in range of 0.05 to 0.125 inches.
10. The turbine of claim 8, wherein the off-center position of the
at least one seal strip is an offset distance from the downstream
side wall at least equal to a wall thickness of the at least one
rotor blade.
11. The turbine of claim 8, wherein the off-center position of the
at least one seal strip is an offset distance from the downstream
side wall at least equal to a radius of curvature of a tip edge of
the at least one rotor blade.
12. The turbine of claim 8, wherein a height of the at least one
seal strip is in the range of 0.150 to 0.250 inches.
13. The turbine of claim 12, wherein the off-center position of the
at least one seal strip is an offset distance from the downstream
side wall equal to twice the height of the at least one seal
strip.
14. The turbine of claim 8, wherein the at least one seal strip
comprises a base portion having fillets.
15. The turbine of claim 8, wherein the at least one seal strip
comprises a rounded top portion.
16. A sealing system, comprising: at least one airfoil having an
upstream side wall, a downstream side wall, and a tip portion
disposed between the upstream and downstream side walls; and at
least one seal strip disposed on the tip portion at an off-center
position only between the downstream side wall and a central
position between the upstream and downstream side walls.
17. The sealing system of claim 16, wherein the off-center position
of the at least one seal strip is an offset distance from the
downstream side wall at least equal to a wall thickness of the at
least one airfoil.
18. The sealing system of claim 16, wherein the off-center position
of the at least one seal strip is an offset distance from the
downstream side wall at least equal to a radius of curvature of a
tip edge of the at least one airfoil.
19. The sealing system of claim 16, wherein the off-center position
of the at least one seal strip is an offset distance from the
downstream side wall equal to twice the height of the at least one
seal strip.
20. The sealing system of claim 16, wherein the at least one seal
strip comprises a base portion having fillets.
21. The sealing system of claim 16, wherein the at least one seal
strip comprises a rounded top portion.
22. A method, comprising: rotating a rotary member having at least
one airfoil disposed inside a stationary member; and reducing
leakage of flow of a working fluid between the rotary member and
stationary member, comprising: creating a flow resistance to the
flow of the working fluid between a tip portion of the airfoil and
the stationary member via at least one seal strip disposed on the
tip portion at an off-center position only between a downstream
side wall and a central position between an upstream side wall and
the downstream side wall of the airfoil.
23. The method of claim 22, further comprising internally cooling
the tip portion of the airfoil.
24. The method of 22, wherein the off-center position of the at
least one seal strip is an offset distance from the downstream side
wall at least equal to a wall thickness of the at least one
airfoil.
25. The method of 22, wherein the off-center position of the at
least one seal strip is an offset distance from the downstream side
wall at least equal to a radius of curvature of a tip edge of the
at least one airfoil.
26. The method of 22, wherein the at least one seal strip comprises
a base portion having fillets.
27. The method of 22, wherein the at least one seal strip comprises
a rounded top portion.
Description
BACKGROUND
[0001] The invention relates generally to a rotary machine and,
more particularly, a sealing system for an interface between
rotating and stationary components. In certain aspects, the sealing
system includes a sealing system between a rotating blade and a
surrounding structure of a turbine engine.
[0002] Performance and efficiency of rotary machines, e.g., turbine
engines, are dependent on a clearance gap between rotating and
stationary components within the turbine engine. For example, the
clearance gap between the tip of rotor blades and the adjacent
stationary shrouds provides a narrow flow passage between the
pressure and suction sides of the rotor blade, resulting in hot gas
flow leakage that can reduce the blade aerodynamic performance. As
the clearance gap between the rotating and the stationary
components increases, the efficiency of the turbine decreases. In
addition, the hot gas flow leakage in the area of the blade tip can
increase thermal loading on the rotor blade.
[0003] Several blade tip designs exist for maintaining proper
pressure between the pressure side and the suction side flow
surfaces of the rotor blade as well as for providing minimum
clearances with the stator shroud. Existing blade tip treatments
for sealing have cost implications, involving additional amounts of
material to form the tip treatment for sealing. Moreover, existing
blade tip designs lead to re-attachment of fluid flow at the blade
tip surface causing increased tip heat load at the tip surface.
BRIEF DESCRIPTION
[0004] In accordance with one aspect of the present invention, a
rotary machine includes a rotary member disposed inside the
stationary member, wherein the rotary member includes at least one
airfoil having an upstream side wall, a downstream side wall, and a
tip portion disposed between the upstream and downstream side
walls. A sealing system is disposed on the tip portion. The sealing
system consists essentially of at least one seal strip disposed on
the tip portion at an off-center position substantially between the
downstream side wall and a central position between the upstream
and downstream sidewalls.
[0005] In accordance with another aspect of the present invention,
a turbine includes a rotor disposed inside the stationary outer
casing, wherein the rotor comprises at least one rotor blade having
an upstream side wall, a downstream side wall, and a tip portion
disposed between the upstream and downstream side walls. At least
one seal strip is disposed on the tip portion at an off-center
position substantially between the downstream side wall, and a
central position between the upstream and downstream side
walls.
[0006] In accordance with another aspect of the present invention,
a sealing system includes at least one airfoil having an upstream
side wall, a downstream side wall, and a tip portion disposed
between the upstream and downstream side walls. At least one seal
strip is disposed on the tip portion at an off-center position only
between the downstream side wall and a central position between the
upstream and downstream side walls.
[0007] In accordance with another aspect of the present invention,
a method includes rotating a rotary member having at least one
airfoil disposed inside a stationary member. A flow resistance to
the flow of the working fluid between a tip portion of the airfoil
and the stationary member is created via at least one seal strip
disposed on the tip portion at an off-center position only between
a downstream side wall and a central position between an upstream
side wall and the downstream side wall of the airfoil. Leakage of
flow of a working fluid between the rotary member and stationary
member is reduced.
DRAWINGS
[0008] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed description is read with reference to the accompanying
drawings in which like characters represent like parts throughout
the drawings, wherein:
[0009] FIG. 1 is a diagrammatical view of a rotary machine, e.g.,
gas turbine engine system, having an offset blade tip chord sealing
system in accordance with an exemplary embodiment of the present
invention;
[0010] FIG. 2 is a diagrammatical view of an offset blade tip chord
sealing system for a rotary machine as illustrated in FIG. 1 in
accordance with an exemplary embodiment of the present
invention;
[0011] FIGS. 3-6 are diagrammatical views of an offset blade tip
chord sealing system for a rotary machine in accordance with
various embodiments of the present invention;
[0012] FIG. 7 is a graph representing variation between blade tip
averaged heat transfer with respect to percentage axial chord in
accordance with certain embodiments of the present invention;
and
[0013] FIG. 8 is a flow chart illustrating exemplary steps involved
in a method of operating a rotary machine in accordance with an
exemplary embodiment of the present invention.
DETAILED DESCRIPTION
[0014] As discussed in detail below, embodiments of the present
invention provide a rotary machine in which an offset blade tip
chord sealing system is disposed on a tip portion of at least one
airfoil of a rotary member. In accordance with some embodiments of
the present invention, the sealing system includes one or more seal
strips (e.g., a single strip) disposed on the tip portion only at a
substantially or generally off-center position between a downstream
side wall and a central position between the downstream side wall
and an upstream side wall of the airfoil, for example, a seal strip
at or near the downstream side wall. In other words, at least a
substantial portion, or most, or all of the one or more seal strips
may extend along the tip portion between the central position and
the downstream side wall. Thus, the sealing system may exclude seal
strips disposed on the tip portion entirely or mostly between the
central position and the upstream side wall, for example, a seal
strip at or near the upstream side wall. The exemplary sealing
system is configured to maintain minimal clearances between the
rotary member and a surrounding stationary member resulting in
reduced fluid leakage and increased efficiency of the rotary
machine. The sealing system also facilitates usage of less exposed
material on the tip portion of the airfoil to provide a tip load
heat reduction since a seal strip (e.g., a single strip) is
disposed on the tip portion only at an off-center position between
the downstream side wall and the central position between the
downstream side wall and the upstream side wall of the airfoil.
Specific embodiments of the present invention are discussed below
referring generally to FIGS. 1-9.
[0015] Referring to FIG. 1, an exemplary rotary machine, such as a
turbine assembly 10, is illustrated in accordance with aspects of
the present invention. The turbine assembly 10 includes a plurality
of rotary members or rotors 12 and a stationary member 14, such as
a stationary outer casing, concentrically disposed about the rotary
members 12. As discussed below, the turbine assembly 10 includes an
offset blade tip chord sealing system 32 between the rotary and
stationary members 12 and 14. Each rotary member 12 includes an
inner base portion 16, an airfoil or rotor blade 18, and an outer
tip portion 20. Although the aspects of the present invention are
described herein with respect to turbine assembly 10, in certain
other exemplary embodiments the sealing system may be used in other
rotary machines in which leakage of working fluid is a concern. For
example, exemplary rotary machines may include compressors, pumps,
motors, or the like. Moreover, exemplary systems utilizing these
rotary machines may include power generation systems, for example,
industrial machine, watercraft, aircraft, and other vehicles. In
the illustrated embodiment, the turbine assembly 10 may further
include a steam turbine or a gas turbine. Moreover, the turbine
assembly 10 may include a compressor coupled to a turbine via a
shaft, one or more gas turbine combustors disposed between the
compressor and the turbine, a fuel injection system coupled to the
one or more gas turbine combustors, and so forth.
[0016] The airfoil 18 extends outwardly into a working fluid flow
path of the turbine assembly 10 where the working medium gases
exert motive forces on a plurality of surfaces thereof. The airfoil
18 includes an upstream sidewall 22 and an opposite downstream side
wall 24 (see FIG. 2) joined together at a leading edge 26 and a
trailing edge 28. The stationary member 14 is spaced apart from the
tip portion 20 so as to define a clearance gap 30 therebetween. The
performance and efficiency of the turbine assembly 10 is affected
by the clearance gap 30. As the amount of leakage flow through the
clearance gap increases, the efficiency of the turbine is reduced,
since the leakage flow does not exert motive forces on the airfoil
surfaces and accordingly does not provide work. The offset blade
tip chord sealing system 32 is disposed on the tip portion 20 of
the airfoil 18 and configured to reduce leakage of fluid along the
tip portion 20 from the upstream side to the downstream side of the
airfoil 18, while also reducing the heat load on a tip surface of
the airfoil. The sealing system 32 is explained in greater detail
with respect to subsequent figures.
[0017] Referring to FIG. 2, the offset blade tip chord sealing
system 32 is illustrated in accordance with certain embodiments of
the present invention. In the illustrated embodiment, the tip
portion 20 is defined by the upstream side wall 22, the downstream
side wall 24, the leading edge 26, and the trailing edge 28. The
tip portion 20 also comprises a tip surface 34. The direction of
rotation of the airfoil 18 is represented by the arrow 36. The
sealing system 32 is disposed on the tip surface 34. The sealing
system 32 includes a seal strip 38 disposed on the tip surface 34
only at a generally off-center position between the downstream side
wall 24 and a central position 40 between the upstream side wall 22
and the downstream side wall 24. For example, the seal strip 38 may
extend along the downstream side wall 24 at a slight offset near
the downstream side wall 24. In addition, the seal strip 38 is at
least substantially or mostly between the central position 40 and
the downstream side wall 24, although some portion of the seal
strip 38 may be positioned across the central position 40. However,
the sealing system 32 generally excludes any seal strips extending
along the upstream side wall 22 at a position near or directly at
the upstream side wall 22. Moreover, the sealing system 32
generally excludes any seal strips disposed between the upstream
side wall 22 and the central position 40. Again, the sealing system
32 may include some portion of the seal strip 38 extending across
the central position 40 toward the upstream side wall 22, while at
least a substantial portion or most of the seal strip 38 remains
positioned between the central position 40 and the downstream side
wall 24.
[0018] In one embodiment, the entire seal strip 38 is disposed on
the tip surface 34 only at a generally off-center position between
the downstream side wall 24 and a central line 41 extending between
the leading edge 26 and the trailing edge 28. In another
embodiment, a substantial portion of the seal strip 38 is
positioned between the central line 41 and the downstream side wall
24, while either ends 43,45 of the seal strip 38 extend across the
central line 42 towards the upstream side wall 22.
[0019] In certain embodiments, the sealing system 32 may further
include one or more additional seal strips between the central
position 40 and the downstream side wall 24. The seal strip 38
enhances the flow resistance through the clearance gap 30 and thus
reduces the flow of hot leakage gas for a predetermined pressure
differential so as to improve the overall turbine efficiency. The
width and height of the seal strip 38 may be varied for better
performance, typically depending upon the size of the overall
turbine assembly. Examples of configurations of the seal strip are
illustrated in subsequent figures.
[0020] The offset of the seal strip 38 inward from the downstream
side wall 24 creates a step 42 along the downstream side wall 24.
The leakage flow is directed along the tip portion 20 from the
upstream side to the downstream side of the airfoil in such a way
that the fluid flows over the seal strip 38 to the recessed portion
between the protruded seal strip 38 and the downstream side wall
24) that contains a separated flow region (low heat transfer
region) at the downstream side. Additionally, the seal strip 38
also serves as a flow resistance element. The offset of the seal
strip 38 from the downstream side wall 24 may also be varied for
better performance, depending upon the size of the turbine
assembly.
[0021] The offset of the seal strip 38 from the downstream side
wall 24 also serves to reduce the tip cavity width to depth ratio,
thereby providing lower cavity floor heat transfer. The seal strip
38 may also include at least one slot 44 proximate to the trailing
edge 28 for exiting any tip cavity coolant flow. In other words,
the seal strip 38 may include a series of sequential strips or
longitudinally aligned strips, rather than a single continuous
strip. Location, size, and orientation of the slot 44 may be varied
depending upon the requirement. In accordance with aspects of the
present invention, the seal strip 38 is located on the tip portion
20 in such a way so as to receive a cooling flow from the internal
cooling passages of the airfoil 18. The seal strip 38 may be
formed, for example, by integral casting with the airfoil tip
portion, by electron-beam welding of flow discouragers to the
airfoil tip portion, by physical vapor deposition of material to
the airfoil tip portion, or by brazing, or a combination thereof.
Alternately, the airfoil tip portion which has been cast to
oversized dimensions, may have material removed by various methods,
for example laser ablation, or the like, thereby forming the seal
strip. In certain embodiments, the provision of the seal strip 38
only proximate the downstream side wall 24 of the tip portion 20 of
the airfoil 18 reduces the amount of material used for tip
treatment of an airfoil for sealing. Moreover, the seal strip 38
reduces the likelihood for cracking of the tip portion of the
airfoil.
[0022] Referring to FIG. 3, a diagrammatical sectional view of the
offset blade tip chord sealing system 32 is illustrated in
accordance with certain embodiments of the present invention. The
stationary member 14 is spaced apart from the tip portion 20 so as
to define the clearance gap 30 therebetween. As discussed above,
the offset blade tip chord sealing system 32 is disposed on the tip
portion 20 of the airfoil 18 and configured to reduce leakage of
fluid along the tip portion 20 from the upstream side to the
downstream side of the airfoil 18, while also reducing the heat
load on a tip surface of the airfoil. In certain embodiments, width
"W" of the seal strip 38 ranges from about 0.015 to about 0.040
inches for aero engines, and about 0.050 to about 0.125 inches for
heavy frame turbines. In certain embodiments, seal height "H"
ranges from about 0.060 to about 0.125 inches for aero engines, and
about 0.150 to about 0.250 inches for heavy frame turbines. In
certain embodiments, seal offset "D" is at least equal to a radius
of curvature "R" of a tip edge of the airfoil 18, or a wall
thickness "T" of the airfoil. In certain exemplary embodiments, the
seal offset "D" is less than or equal to about 4 times the seal
height "H". In certain other exemplary embodiments, the wall
thickness "T" is in the range of about 0.020 to about 0.040 inches
for aero engines, and about 0.040 to about 0.150 inches for heavy
frame turbines.
[0023] Referring to FIG. 4, a diagrammatical sectional view of the
offset blade tip chord sealing system 32 is illustrated in
accordance with certain embodiments of the present invention. The
offset blade tip chord sealing system 32 is disposed on the tip
portion 20 of the airfoil 18 and configured to reduce leakage of
fluid along the tip portion 20 from the upstream side wall 22 to
the downstream side wall 24 of the airfoil 18, while also reducing
the heat load on a tip surface of the airfoil 18. In the
illustrated exemplary embodiment, radius of curvature "R" of a tip
edge of the airfoil 18 is equal to zero. In other words, the tip
edge at the upstream and downstream side walls 22 and 24 is not
curved, but rather has a sharp or 90 degree edge. In the
illustrated embodiment, seal offset "D" is at least equal to a wall
thickness "T" of the airfoil. In certain exemplary embodiments, the
seal offset "D" is less than or equal to about 4 times the seal
height "H". In one exemplary embodiment, the seal offset "D" is
equal to about 2 times the seal height "H".
[0024] Referring to FIG. 5, a diagrammatical sectional view of the
offset blade tip chord sealing system 32 is illustrated in
accordance with certain embodiments of the present invention. The
offset blade tip chord sealing system 32 is disposed on the tip
portion 20 of the airfoil 18 and configured to reduce leakage of
fluid along the tip portion 20 from the upstream side wall 22 to
the downstream side wall 24 of the airfoil 18, while also reducing
the heat load on the tip surface of the airfoil 18. In the
illustrated embodiment, the seal strip 38 has a base portion 46
having fillets. In other words, the seal strip 38 gradually curves
outwardly in a downward direction onto the tip portion 20. The
provision of base portion 46 having fillets enhances creation of a
separation zone for leakage fluid with lower heat transfer and also
placement of the seal feature over an internally cooled region of
the airfoil tip portion 20.
[0025] Referring to FIG. 6, a diagrammatical sectional view of the
offset blade tip chord sealing system 32 is illustrated in
accordance with certain embodiments of the present invention. The
offset blade tip chord sealing system 32 is disposed on the tip
portion 20 of the airfoil 18. In the illustrated embodiment, the
seal strip 38 has a rounded top portion 48. In certain embodiments,
the illustrated sealing system 32 reduces the likelihood or amount
of leakage of working fluid along the tip portion 20 from an
upstream side wall 22 to a downstream side wall 24 of the airfoil
18, while also reducing the heat load on the tip surface of the
airfoil 18.
[0026] Referring to FIG. 7, a graph representing variation of
airfoil tip averaged heat transfer (y-axis) represented by
reference numeral 50 versus percentage axial chord of the airfoil
(x-axis) represented by reference numeral 52 is illustrated. Axial
chord refers to the length from the leading edge 26 to the trailing
edge 28 of the tip portion 20 of the airfoil 18 as represented in
FIG. 2. Curves 54, 56, and 58 represent variation of airfoil tip
averaged heat transfer with respect to percentage axial chord for a
40 mils squealer seal strip, a 100 mils squealer seal strip, and a
40 mils seal strip disposed on the tip portion only at an
off-center position between the downstream side wall and the
central position between the downstream side wall and the upstream
side wall, respectively of the airfoil. The squealer seal strips
are located along the entire outer periphery of the tip portion of
the airfoil. With respect to curves 54, 56, and 58, the airfoil tip
averaged heat transfer decreases gradually from the leading edge up
to a predetermined point and then increases gradually towards the
trailing edge of the tip portion of the airfoil. The tip averaged
heat transfer for the 100 mils squealer seal strip and the 40 mils
seal strip disposed on the tip portion at an off-center position
between the downstream side wall and the central position between
the downstream side wall and the upstream side wall, relative to
percentage axial chord is comparatively less than the tip averaged
heat transfer for 40 mils squealer seal strip. As illustrated by
the curves 54, 56, and 58, the tip averaged heat transfer decreases
from the leading edge to the trailing edge upto a predetermined
point, and then increases gradually towards the trailing edge. The
tip averaged heat transfer of tip portion of the 40 mils seal strip
disposed on the tip portion only at an off-center position between
the downstream side wall and the central position between the
downstream side wall and the upstream side wall of the airfoil, is
less compared to that of the 40 mils squealer seal strip, and the
100 mils squealer seal strip.
[0027] Referring to FIG. 8, a flow chart illustrating exemplary
steps involved in method of operating a rotary machine is
illustrated. In accordance with the illustrated exemplary
embodiment, the method includes rotating the rotary member disposed
inside the stationary member as represented by step 60. For
example, as illustrated in FIG. 1, the method involves rotating the
rotor disposed inside the stationary outer casing. During operation
of the machine, the tip portion of the airfoil is internally cooled
as represented by step 62. The tip portion allows cooling flow to
be released from the interior of the blade through holes (not
illustrated) in the tip portion of the airfoil. In accordance with
certain embodiments, the seal strip is located directly over the
internally cooled portion of the tip portion of the airfoil.
[0028] The sealing system includes the seal strip disposed on the
tip surface only at an off-center position between the downstream
side wall and a central position between the upstream side wall and
the downstream side wall. The seal strip enhances the flow
resistance of leakage fluid through the clearance gap between the
tip portion of the airfoil and the stationary member as represented
by step 64. The offset blade tip chord sealing system disposed on
the tip portion of the airfoil also reduces leakage of fluid along
the tip portion from the upstream side to the downstream side of
the airfoil for a predetermined pressure differential so as to
improve the overall turbine efficiency as represented by step
66.
[0029] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
* * * * *