The present invention generally relates to high speed centrifugal compressor
used in the processing plants and pipe line application for gas transportation.
More particularly, the present invention relates to a method for producing 3D
impeller of high speed centrifugal compressor from cheaper impeller material
with moderate mechanical strength.
BACKGROUND OF THE INVENTION
In a high speed centrifugal compressor, energy is added to the working fluid in
an impeller of the compressor and is diffused in a diffuser including a return
channel for converting kinetic energy into static pressure. The impeller used in
the multistage centrifugal compressor comprises two rotary discs (a disc and a
shroud), and a plurality of vanes disposed between the disc & shroud and
substantially equidistantly in a circumferential direction to define passages by
means of the disc & the shroud and the vanes. The disc, the shroud and the
vanes are manufactured by machining from a single piece or separately
machined and welded together to make impeller.
In general, multi stage centrifugal compressors are provided with interstage
ducting for providing flow communication between multiple stages of centrifugal
compressors that are arranged in series. The cross over duct assembly includes
inner and outer wall sections of compressor diaphragms forming a continuous
annular flow path between compressor stages for turning radially outward gas
flow leaving the impeller to a radially inward direction. The duct includes a vaned
or vaneless diffuser passage, a vaneless turning 180 deg. bend and deswirl vane
section. Typical intermediate stage of multi stage compressor is shown in Fig.1.

Front and rear labyrinth seals are provided to minimize the flow leakage from
impeller up stream to downstream or stage inlet.
The impeller used in the centrifugal compressor, comprises two rotary discs (a
hub and a shroud), and a plurality of vanes disposed between the hub & shroud
and substantially equidistantly in a circumferential direction to form passages by
means of the hub & shroud and the vanes. Cross-sectional view of conventional
3D centrifugal compressor impeller geometry is shown in the Fig.2.
Impeller shroud disc is provided with seal steps near the impeller eye for front
seal seating to reduce the leakage of working fluid from impeller exit to stage
inlet. Stepped labyrinth seals are more effective than the plain labyrinth seals.
This seal steps demands minimum thickness (H) of shroud at inlet and the
thickness is proportional to impeller diameter. Minimum thickness (I & C) is
provided at impeller tip in order to avoid deformations during manufacturing and
under operating conditions.
Centrifugal compressors for various industrial applications like Process industries,
Fertilizers, Cement, Sugar, Gas transportation, Urea, etc., handling various gases
are designed to operate at very high speeds in the order of 10000-15000 rpm.
The induced stresses in the impeller are very high due to higher operating
speeds and it is essential to ensure the mechanical strength of impeller for
trouble free operation of compressor and there by industries. Higher operating
speeds of compressor also yields higher efficiency and higher pressure ratio
resulting in minimized total number of stages and desired overall pressure ratio
of multistage compressor.

Hence, stress analysis of centrifugal compressor is carried out for every new
design or development for estimating maximum induced stresses in order to
avoid mechanical failure of impeller, during compressors operation. According to
the established industrial practice, this is achieved by ensuring the induced
stresses in impeller at maximum continuous speed (MCS) which is 5% higher
than the operating speed, within 85% of the yield strength of the impeller
material.
Mechanical strength analysis of impeller with conventional geometry has shown
that the maximum induced stresses are in the order of 850 MPa at higher
impeller speeds. Typical common alloy steels can give maximum yield strength of
1000 - 1100 MPa with suitable heat treatment and titanium alloys steel can give
up to 1300 - 1400 MPa. Higher operating speeds of compressors requires
superior impeller materials like Titanium alloy steels.
Prior art patents related to the return channel vane is given below,
[1] US 5,158,435 27/10/1992 Edward P. Eardley, Amherst, N.Y.
[2] US 5,277,541 11/01/1994 Donald L Palmer, Cave Creek, Ariz.
[3] US 7,189,059 B2 13/03/2007 Michael T. Barton, Scottsdale, AZ
[4] US 8,425,186 B2 23/04/2013 Hirotaka Higashimori, Nagasaki-Ken
Impeller induced stress reduction through over speed is reported in Ref. [1]
wherein the capability of an impeller to withstand induced stress during rotation
is achieved by inducing a residual compressive stresses in the impeller, which
opposes the steady tensile stress produced by rotation. The method comprises
rotating the impeller at a succession of increasing peak speeds in excess of the
design speed to induce tolerable yielding and residual compressive stress at the
selected locations.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method for producing 3D
impeller of high speed centrifugal compressor from cheaper impeller material, in
which the induced stresses are reduced to acceptable limits with respect to
maximum yield strength.
Another object of the invention is to propose a method for producing 3D impeller
of high speed centrifugal compressor from cheaper impeller material, in which
moderate impeller material like alloy steel is used.
A still another object of the invention is to propose a method for producing 3D
impeller of high speed centrifugal compressor from cheaper impeller material,
which modifies impeller geometry to overcome constraints under maximum
continuous speed (MCS) of the compressor.
A further object of the invention is to propose a method for producing 3D
impeller of high speed centrifugal compressor from cheaper impeller material,
which allows manufacture of the impeller with conventional facilities.
SUMMARY OF THE INVENTION
According to the present invention, the impeller geometry of hub disc, shroud
disc and hub & shroud discs together are modified to reduce the stress in the
impeller that are induced during operation of compressor. In case of prior art,
induced stress is reduced by inducing residual stresses by spinning impeller at
much higher speeds than compressor operating speed.

The following steps will be followed in achieving the desired induced stress levels
in the impeller.
a) Stress analysis of impeller with conventional geometry for estimation of
induced stresses at operating speed (OS) and maximum continuous speed
(MCS) of compressor.
b) Stress analysis of impeller with modified geometry of hub disc for
estimation of induced stresses at OS and MCS of compressor.
c) Stress analysis of impeller with modified geometry of shroud disc for
estimation of induced stresses at OS and MCS of compressor.
d) Stress analysis of impeller with modified geometry of hub disc and shroud
disc for estimation of induced stresses at OS and MCS of compressor.
e) Impeller assembly requirements and sealing requirements (front & rear
seal) are considered during impeller geometry modification.
f) Mechanical design validation of all above cases by ensuring the induced
stress levels within 85% impeller material yield strength at MCS which is
1.05 times the operating speed of compressor.
Maximum thickness of shroud disc, shroud disc thickness distribution and hub dfe
disc thickness distribution are the basic factors that contribute induced stress.
Modifying the shroud disc thickness distribution along the meridional length of
impeller, whereby minimum thickness requirements of stepped front seal are
maintained and modifying the hub disc thickness distribution helps in reducing
the induced stress.
Shifting the location of maximum thickness towards inlet of the shroud
considering the thickness requirement of front seal [H] as per established
engineering practice and thinning towards impeller tip [I] by non-uniform

variable thickness unlike conventional impellers helps in reducing the impeller
shroud thickness at higher radius which ultimately results in reduction of induced
stress levels to acceptable limits. This entire process is carried out by using UG-
NX and ANSYS ADPL software tools with macros that are developed for intended
use.
By following the above procedure in the impeller geometry modification, it is
possible to reduce the induced stress in the impeller to acceptable levels with
respect to yield strength of common impeller materials. This enables the
compressor to operate at much higher speeds which in turn results in higher
efficiency and savings in the material cost.
The salient feature of the present invention vis-a-vis the prior art is described
below.
1) Method adopted by prior art is totally different form the present invention.
Prior invention involves spinning the impeller at much higher speeds than
the normal operating speed. Whereas in the present patent, impeller
geometry is modified to reduce the peak induced stress.
2) Prior art [1] requires the spinning of impeller at very high speeds in the
order of 45000 rpm in order to induce residual compressive stress,
demanding superior quality material which can withstand higher induced
stress during spinning at that speeds. Whereas in the present invention,
impeller is not subjected to any such higher speeds.

3) In the prior art [1], it may not be possible to use material not based on
the estimated stress that are induced during normal operation of impeller
all the time to spin the impeller at such high speed, as it may eventually
lead to impeller failure during high speed spinning. Whereas, in the
present invention, original intended material can be used without any
problem as impeller is not subject to such higher speeds.
4) Prior art [1] does not disclose the root cause, whereas in the present
invention, the impeller shroud geometry thickness distribution which is the
main root cause for the higher induced stress is focused on and solution is
arrived by geometry modification.
5) Prior art [2] caters to the centrifugal compressor stage of gas turbine
engines that are provided with non-moving plurality of vanes in the
leakage path to preferentially augment secondary air flow into the
impeller inlet at maximum speed of operation. This invention addresses
flow improvement.
6) Prior art [3] caters to the centrifugal compressor impeller of a gas turbine
engine includes an enhanced vaned shroud disc and is configured such
that, the flow area ratio is equivalent to that of a conventional, non-vaned
shroud. The vaned shroud includes a plurality of airfoils that vary in
thickness to obtain desired vibrational mode shapes and natural
frequencies for rotordynamic stability of machine.
7) Prior art [4] caters to centrifugal compressor impeller of turbo charger
having higher pressure ratio, which can achieve a large flow rate or an
increase in a flow rate while suppressing a decrease in efficiency.

8) The present invention caters to reducing the stresses in the impeller that
are induced during operation and this clearly differs in the technical filed
from Prior art inventions as prior invention [2] caters to augmentation of
secondary air at inlet, prior art [3] caters to vibrational mode shapes of
compressor and prior art invention [4] caters to increasing the compressor
flow rate.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
Figure. 1: Meridional view of intermediate stage of multistage compressor
Figure.2 : Cross-sectional geometry of conventional impeller of compressor
Figure.3 : Finite element model generated for FE analysis
Figure.4 : Thickness distribution of impeller shroud of conventional & modified
geometry
Figure.5 : Comparison of conventional and modified impeller geometries
Figure.6: Von-Mises stress contours for impeller at operating speed of
conventional geometry
Figure.7 : Von-Mises stress contours for impeller at operating speed of modified
geometry
Figure.8 : Von-Mises stress contours for impeller at MCS of conventional
geometry
Figure.9 : Von-Mises stress contours for impeller at MCS of modified geometry
DETAILED DESCRIPTION OF THE INVENTION
Conventional geometry of centrifugal compressor impellers is shown in Fig.2.
Modification of impeller geometry is carried with an objective of reducing the
maximum induced stress as described below.

Case-1: Modification of shroud disc geometry by non-uniform shroud thickness
distribution. That is shifting the maximum thickness [HJ that is required for
accommodating the stepped front seal towards impeller eye which is at lower
radius and thinning towards impeller tip along the meridional length of
impeller.
Case-2: Modification of hub disc geometry for reducing the hub thickness along
the radius.
Case-3 Modification of hub disc and shroud disc geometry together as
described above in (a) & (b).
In all the above cases, it is ensured that the geometry of impeller is a
combination of lines & arcs for better manufacturability of the impeller using
conventional manufacturing facilities.
The finite element analysis of all three cases is carried out for estimation of
maximum induced stress levels in the impeller at operating and maximum
continuous speed of compressor. The finite element model was created using 21-
noded brick element in ANSYS software. Modeling parameters are specified with
respect to impeller outer diameter. Impeller hub geometry, Impeller shroud
geometry and blade geometry are made for trouble free operation of compressor
by considering past experience and established engineering practices. All the
dimensions are normalized tip diameter.
Modeling of Back face: Impeller hub / back face is modelled using the
following parameters.
• Shoulder Thickness (A) - 3.33%
• Shoulder radius (B) -21.7%
• Web thickness at OD (C) - 0.67%
• Zero thickness radius (R2) - 50%

Modeling of impeller bore: Impeller bore is modelled using the following
parameters considering straight bore for simplicity purpose.
• Bore radius at nose (F) - 8.8%
• Bore radius at back face (G)- 8.8%
• Nose radius (D) - 19.5%
• Nose length (E) - 1.6%
Modeling of impeller shroud: Impeller shroud is modelled considering
variable thickness with the following parameters.
• Shroud inlet thickness (H) - 2.2%
• Shroud exit thickness (I) - 0.66%
• Axial length at inlet (J) - 2.2%
• Seal runner radius (L) - 33.3%
• Inlet radius (K) - 30.8%
Modeling of impeller blade: Impeller blade is modelled considering uniform
thickness of blade from hub to shroud for better manufacturability. Fillet radius
of 1.33% at both hub & shroud is considered.
Application of loads and boundary conditions: FE analysis is carried out by
considering only centrifugal forces and aero-thermal loads are not applied as the
earlier studies have shown the insignificant contribution for the range of pressure
and temperature encountered for such application Boundary conditions are
applied as per the practical physical constraints on the impeller during operating
conditions. Impeller bore, nose and shoulder are encased by spacer rings that
are shrink fitted on both sides which will restrain deformation of impeller in axial
and tangential direction but allows to deform in radial direction. Hence, Impeller
bore, nose and shoulder are constrained in both axial and tangential directions.

Finite element analysis:
To minimize solver time, a l/17th "pie slice" cyclic sector model was used. Grid
size is finalized based the grid sensitivity study where the FEA results consistent.
Grid quality is maintained as per standard engineering practice for better results
without any singularities in the final FE analysis results. The finite element model
generated is shown Fig.9. The FE analysis for the impeller is carried out at
required speeds using ANSYS Mechanical APDL.
Analysis of results: Original geometry of impeller.
Important mechanical strength parameters are extracted with post processing
options available in ANSYS APDL and the induced stresses are specified with
respect to maximum yield strength of impeller material for all three cases. The
results show that the modification of the hub is does not reduce the induced
stresses and a modification of shroud geometry alone can achieve the desired
results. The results of case-1 are presented.
ADVANTAGES OF THE INVENTION
Present invention has reduced the maximum induced stress in the impeller up to
24.8% and the axial & radial deformations of impeller are also with the
acceptable limits meeting the compressor assembly requirements. This enables
the designer to use common impeller materials even at higher speeds and also
the compressor can be operated at much higher speeds which in turn results in
higher efficiency.

A reduction of 24.8% in the peak induced stress enable to operate the
compressor at 11.8% higher speed resulting in stage efficiency improvement of
1.8% and stage pressure ratio of 9.4%. However, issues related to rotodynamic
stability, bearings and surge & chock margins needs to be addressed separately.

WE CLAIM :
1. A method for producing 3D impeller of high speed centrifugal compressor
from cheaper impeller material, the method comprising :-
modifying the impeller geometry in particular the shroud disc geometry by
shifting the maximum thickness (H) required to accommodate the stepped
front seal towards the impeller eye, wherein the impeller eye located at a
lower radius and reducing in thickness towards the impeller tip along the
meridonal length of the impeller;
reducing the hub thickness along the radius;
wherein the method achieves reduction of maximum induced stress in the
impeller about 25% to allow the high speed compressor operate around
12% higher speed resulting in improvement in stage efficiency and stage
pressure ratio respectively by about 2% and 9%.