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.

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.

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.