U.S. patent number 3,628,881 [Application Number 05/029,842] was granted by the patent office on 1971-12-21 for low-noise impeller for centrifugal pump.
This patent grant is currently assigned to General Signal Corporation. Invention is credited to Arthur J. Herrmann, Jr..
United States Patent |
3,628,881 |
Herrmann, Jr. |
December 21, 1971 |
LOW-NOISE IMPELLER FOR CENTRIFUGAL PUMP
Abstract
A practical scheme for reducing the amplitude of fluidborne
noise produced by a centrifugal pump which comprises an improved
impeller in which the vanes are arranged in a single row and are
skewed with respect to the shrouds so that the tips of adjacent
vanes overlap in the circumferential direction. The arrangement
results in a substantially continuous interaction between the vanes
and the cutwater.
Inventors: |
Herrmann, Jr.; Arthur J.
(Batavia, IL) |
Assignee: |
General Signal Corporation
(N/A)
|
Family
ID: |
21851173 |
Appl.
No.: |
05/029,842 |
Filed: |
April 20, 1970 |
Current U.S.
Class: |
415/119;
415/211.1; 415/227; 416/186R; 415/204; 415/214.1; 415/228 |
Current CPC
Class: |
F04D
29/2261 (20130101); F04D 29/669 (20130101); F04D
29/2216 (20130101) |
Current International
Class: |
F04D
29/66 (20060101); F04D 29/22 (20060101); F04D
29/18 (20060101); F04b 039/00 (); F04d 017/08 ();
F04d 029/26 () |
Field of
Search: |
;415/204,206,219,212,213,119,98 ;416/186,178,188 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Raduazo; Henry F.
Claims
I claim:
1. In a centrifugal pump for liquids including a casing (12)
containing a pumping chamber (13) provided with an outlet passage
(14) and an adjacent cutwater (15), and a rotary impeller (11) in
the chamber and including at least one supporting shroud (27) which
carries a circumferential series of vanes (28) arranged to guide
fluid outward from a central inlet zone (18) to the periphery of
the impeller, the improvement which comprises impeller vanes (28)
having tip (29) which are skewed with respect to the shroud (27) to
provide each tip with leading and trailing ends (31, 32) referenced
to the direction of impeller rotation, the trailing end (32) of
each tip being so positioned circumferentially relatively to the
leading end (31) of the next succeeding tip that portions of both
of said ends can lie abreast the cutwater (15) in one angular
position of the impeller.
2. The improvement defined in claim 1 in which the trailing end
(32) of each tip (29) overlaps circumferentially the leading end
(31) of the next succeeding tip.
3. The improvement defined in claim 2 in which the area (A.sub. 1
or A.sub.2 +A.sub. 3) of the vane tips abreast the cutwater (15)
remains substantially constant throughout a complete revolution of
the impeller (11).
4. The improvement defined in claim 3 in which the vanes (28) are
formed in one piece with and extend between a front shroud (26),
which encircles said inlet zone (18), and a back shroud (27); and
in which the leading and trailing ends (31, 32) of the vane tips
are joined to the front and rear shrouds, respectively.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
Most conventional centrifugal pumps employ impellers having a small
number of vanes e.g., two to six vanes) whose tips are normal or
slightly skewed with respect to the supporting shroud or shrouds.
As the impeller rotates in its casing, the tips of the vanes pass
by the stationary cutwater in succession and produce pressure
pulsations in the output stream. These pulsations give rise to
fluidborne noise having a frequency equal to the product of the
number of vanes and the rotational speed of the impeller, and an
amplitude which is a function of various design parameters. A
useful theory for predicting noise level in terms of these
parameters was developed by Dr. H. C. Simpson (see "Hydraulic Noise
Generation in Pumps," Report 512, Weir Research Laboratories,
Glasgow, Scotland, June 7, 1963) and is expressed by the
equation:
where p.sub. v is the fluidborne noise level, in pounds per square
inch, root mean square
K is an empirical constant having a value of 2.66.times.
10.sup.-.sup.2
H is the developed head of the pump, in pounds per square inch
N is the operating speed, in radians per second
Z is the number of vanes
p is the fluid density, in pounds-square seconds per square inch
per square inch
R.sub.c is the cutwater radius, in inches
R.sub.v is the maximum radius of the volute, in inches
r.sub.i is the radius of the impeller eye, in inches
r.sub.o is the outer radius of the impeller, in inches.
Inspection of this equation reveals that the factor having the
greatest influence upon noise level is the quantity (r.sub. 0 /
R.sub. c).sup. z.sup.+1. Both the equation and experience teach
that noise level decreases as the ratio r.sub.0 / R.sub.c is made
smaller, and this knowledge has been used for some time in
designing pumps for application, such as installations aboard
surface ship and submarines, where low noise levels are required,
However, since the cutwater-impeller clearance cannot be made too
large without adversely affecting pump performance, this approach
obviously can afford only limited gains.
Since the ratio r.sub.0 / R.sub.c always is less than one, the
exponent z+1 in the quantity (r.sub.0 /R.sub.c).sup.Z.sup.+1
manifestly has a pronounced effect on noise level. However,
although noise level can be reduced by increasing Z, it also is
true that use of a very large number of vanes will impair the
capacity of the pump and make it impractical, it not physically
impossible, to cast the impeller. Therefore, if the noise problem
is to be solved by increasing the exponent Z+1, it is essential to
find some way of giving the quantity Z an effective value greater
than the actual number of vanes in the impeller. U.S. Pat. No.
3,478,691 to John W. Henry IV discloses one possibility. According
to this patent, the impeller employs a relatively large number of
vanes which are arranged in several axially spaced, staggered rows
so that the waterways between vanes in the various rows overlap
each other in the circumferential direction. Tests of a specific
embodiment of this design, which employed 26 vanes in two rows,
show that the scheme is effective to reduce the amplitude of
fluidborne noise below the level predicted by the noise equation.
In spite of this significant reduction in noise level, the patented
impeller has limited utility at the present time because it cannot
be cast reliably in the small sizes normally needed for low-noise
applications using standard foundry techniques.
The object of this invention is to provide a low-noise impeller for
centrifugal pumps which is economical to manufacture. According to
the invention, the impeller employs a single row of vanes which are
skewed relatively to the supporting shroud or shrouds so that the
leading and trailing ends of the tips of each vane
circumferentially overlap the trailing and leading ends,
respectively, of the tips of the adjacent vanes. Preferably, the
degree of overlap is such that there is a substantially constant
vane area abreast the cutwater in all angular positions of the
impeller. Since the blade tips in this scheme move gradually past
the cutwater, and the overlap of the tips produces a substantially
continuous tip-cutwater interaction, the noise level of the
impeller should be lower than that predicted by the noise equation
for a conventional impeller having the same number of vanes. In
other words, the arrangement of the vane tips, in effect, makes the
factor Z in the noise equation somewhat greater than the actual
number of vanes in the impeller. Moreover, since the new impeller
necessarily employs at least a few more vanes than a conventional
impeller intended to provide the same hydraulic characteristics,
and the head produced at a given impeller diameter increases with
the number of vanes, it follows that the new design can produce a
given head with a smaller impeller diameter. This means that the
ratio r.sub. 0 / R.sub.c can be reduced, thereby effecting a
further reduction in noise level. Actual comparison tests conducted
with a 26-vane, dual row henry impeller and an 11-vane version of
the new impeller have shown that the two designs, run in the same
casing and affording substantially the same head and flow
characteristics, had comparable, low noise levels, And, since the
new impeller has a simpler structure, it can be, and in fact was,
cast without difficulty using normal foundry techniques.
BRIEF DESCRIPTION OF THE DRAWINGS
The preferred embodiment of the invention is described herein with
reference to the accompanying drawings in which:
FIG. 1 is an axial sectional view of a portion of a centrifugal
pump incorporating the new impeller.
FIG. 2 is a perspective view of the pump, with the front cover
removed and a portion of the casing broken away.
FIG. 3 is a sectional view, on reduced scale, taken on line 3-3 of
FIG. 1, but showing only six vanes.
FIG. 4 is an enlarged developed view of a portion of the periphery
of the impeller.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
As shown in FIGS. 1 and 2, the improved impeller 11 is incorporated
in an otherwise conventional centrifugal pump which includes a
casing 12 containing a volute-pumping chamber 13 provided with a
discharge passage 14 and a stationary cutwater 15. The front end of
casing 12 is closed by a cover 16 containing a suction passage 17
which leads directly into the eye 18 of impeller 11. The impeller
is driven by a shaft 19 to which it is connected by a lock screw 21
and a key 22, and is provided at its front and rear ends with wear
rings which cooperate with stationary wear rings 24 and 25 to seal
the pumping chamber.
Impeller 11 includes front and rear shrouds 26 and 27,
respectively, and a single row of 11 equiangularly spaced pumping
vanes 28. The impeller is basically of the Francis type, consisting
of an integral, mixed flow inducer portion, and a radially arranged
diffuser. However, in contrast to the conventional Francis
impeller, each vane 28 is skewed with respect to the shrouds 26 and
27 over its entire length so that, as shown in FIGS. 2 and 4, its
tip 29 has leading and trailing ends 31 and 32, respectively, which
overlap the opposite ends of the tips of adjacent blades.
The tip overlap is indicated in FIG. 4 by the dimension X, and its
size is important. Ideally, the overlap is such that, in all
angular positions of the impeller 11 relative to cutwater 15, the
tip area lying abreast the cutwater is the same. This is
illustrated in FIG. 4 wherein the area A.sub.1 of a single tip 29
lying beneath the cutwater in position 15a equals the sum of the
areas A.sub.2 and A.sub.3 of two tips lying beneath the cutwater in
position 15b. This arrangement minimizes noise because the cutwater
15 " sees" a continuous vane. In other words, the arrangement
approximates the tip-cutwater interaction effect of an infinite
number of vanes. As the overlap is progressively increased or
decreased from this ideal, the tip area lying beneath the cutwater
will fluctuate between wider and wider limits during rotation of
the impeller, and, as a result, the fluidborne noise level will
increase.
It should be noted that indiscriminate use of the invention can
produce a marked decrease in the head versus capacity curve of the
pump. The reason for this is that the pronounced skewing of the
vanes 28 required to achieve the desired overlap at the tips
inherently creates very acute corners 33 at the leading and
trailing ends of the waterways between vanes. The boundary layer
buildup in these corners restricts flow and leads to separation
which adversely affects the hydraulic performance of the pump. In
view of this situation, intelligent use of the invention requires
that the exit area of each waterway be made larger than in a
conventional impeller having waterways which are substantially
rectangular in cross section, and that the inlet areas be sized to
insure against detrimental throttling in this region.
While most, if not all, pumps intended for present-day low-noise
applications will use closed impellers, i.e., impellers having two
shrouds, it will be evident that the invention can be incorporated
in the open or single-shroud type of impeller.
* * * * *