US3411451A

US3411451A - Centrifugal pump inlet elbow

Info

Publication number: US3411451A
Application number: US615625A
Authority: US
Inventors: Matthias Heinz-Bernd; Offenhauser Hans; Schramek Walter; Strscheletzky Michael
Original assignee: JM Voith GmbH
Current assignee: JM Voith GmbH
Priority date: 1966-03-05
Filing date: 1967-02-13
Publication date: 1968-11-19
Anticipated expiration: 1985-11-19
Also published as: AT272846B; FR1526007A; DE1528691A1; ES337516A1; CH451712A

Description

NOV. 19, 1968 E| MATITH|AS ET AL 3,411,451

CENTRIFUGAL PUMP INLET ELBOW 2 Sheets-Sheet 1 Filed Feb. 13, 1967 INVENTORS HElNZ-BERND MATTHIAS HANS OFFENHZX'USER WALTER SCHRAMEK MICHAEL STRSCHELETZKY N ,1968 HElNZ-BERND MATTHIAS ET 3,411,451

CENTRIFUGAL PUMP INLET ELBQW Filed Feb. 13, 1967 2 Sheets-Sheet 2 Fig. 5

Fig. 4

MP; 02L 2 IN VE/V TORS HElNZ-BERND MATTHIAS HANS OFFENHKUSER WALTER SCHRAMEK MICHAEL STRSCHELETZKY United States Patent Claims. cl. 103-103) ABSTRACT OF THE DISCLOSURE A 90 degree centrifugal pump inlet elbow wherein cross-sectional area of the fluid passageway progressively decreases from inlet to outlet with the middle portion of the elbow having an elliptical cross-section whose minor axis is in the central plane of the elbow and corresponds to the shortest distance in the central plane of the elbow between the inner and outer walls.

The present invention relates to an inlet eblow for a centrifugal pump, more particularly, to such an elbow having a cross-section which will reduce considerably the turbulence of the fluid discharged from the outlet of the elbow to the pump impeller.

The fluid which is to be pumped by a centrifugal pump is introduced to the impeller through an inlet conduit. In the construction of such an inlet conduit every effort is made to minimize the hydraulic losses therein and the deviations from radial'symmetry of the flowing fluid just before entering the impeller. If the incoming fluid must be turned through an angle of about 90 degrees just prior to entering the impeller a number of relatively complicated paths will be followed by the fluids. In such a conduit the streamlines not only will change their direction but also their average velocity. Fluid velocity in the supply conduit upstream from the curved inlet conduit or elbow is generally smaller than the average velocity of the fluid as it enters the impeller.

As fluid flows through an elbow, the normally uniform velocity will undergo deformation. This is caused by secondary flows produced by the centrifugal forces of the several diverted fluid masses. These secondary flows will influence the fluid velocity field so that, e.g., the fluid particle's having a maximum velocity and ordinarily occurring along the center line region of a straight conduit will be thrown toward the inner surface of the outermost elbow wall because of the greater centrifugal force which they will exert as compared with the more slowly moving fluid particles adjacent the boundary layer of the conduit. The radial symmetry of the flowing fluid will be considerably distorted before entering the impeller and as a result the impeller will receive a radially unsymmetrical load.

The unsymmetrical loads thus thrown upon the impeller greatly increase the danger of cavitation by the impeller blades and considerably increase the likelihood of losses because of internal friction.

Some forms of inlet elbows have the pump shaft passing therethrough with the saft generally being enclosed in a non-rotating protective sleeve. The boundary layers at the surface of the pump shaft or its protective sleeve will separate from these components and will usually form vortices which will separate the intermediate distorted streamlines from the main streamlines. As a result, some of the vortices in this vortex-containing separated fluid flow will be entrained with the principal flow of fluid and which is distorted thereby, and some of the vortices will be carried into the impeller. These vortices will be broken down by the individual impeller blades thereby causing cavitation and unsymmetrical loading of the impeller, both of which may be injurious to the pump.

It is therefore the principal object of the present invention to provide a novel and improved inlet elbow for a centrifugal pump.

It is another object of the present invention to provide a centrifugal pump inlet elbow in which the formation of secondary fluid currents and vortices on the propeller shaft or its surrounding sleeve are significantly decreased while keeping straight axial portions of the elbow to a minimum.

It is a further object of the present invention to provide a centrifugal pump inlet elbow which supplies a substantially balanced load of fluid to the pump impeller.

In one aspect of the present invention there is provided a centrifugal pump inlet elbow which changes the direction of fluid flow about 90 degrees to introduce the fluid to the impeller of the pump in a direction which is substantially parallel to the pump shaft. The elbow has an inlet and an outlet with at least the outlet being preferably circular, and a fluid passageway therebetween. The outlet is positioned in front of the pump impeller and is generally concentric therewith. The cross-sectional area within the passageway normal to the direction of flow therethrough becomes progressively smaller from the inlet to the outlet of the elbow. The middle third portion of the fluid passageway has a substantially elliptical crosssection whose minor axis lies in the central plane of the fluid passageway and corresponds to the shortest distance of the central plane between the inner and outer walls of the fluid passageway.

The cross-sectional area decreases at varying rates throughout the fluid passageway. In the first third of the passageway adjacent the inlet the cross-sectional area decreases relatively slowly. In the middle third of the passageway the area decreases at considerably greater rate. In about the last third of the passageway adjacent the outlet, and preferably less than a third of the total length of the passageway, the area again decreases slowly. As a result of this relationship, the velocity distribution within the fluid at the outlet of the elbow is considerably more uniform. The decrease in cross-sectional area in the middle third of the elbow is about 50 to percent of the total decrease in cross-sectional area within the elbow.

In an inlet elbow through which passes a pump shaft surrounded by a protecting sleeve at least one radial rib is mounted on the sleeve surface away from the elbow inlet. The thickness of the rib is gradually increased in a direction toward the outer wall of the elbow.

A second radial rib may also be provided on the shaft sleeve surface directed toward the elbow inlet. This rib also may have a gradually increasing thickness in a direction toward the outer wall of the elbow. The shaft sleeve may have a somewhat elliptical cross-section whose major axis extends in the direction of flow.

Other objects and advantages of the present invention will be apparent upon reference to the accompanying description when taken in conjunction with the following drawings wherein:

FIGURE 1 is a longitudinal sectional view through an inlet elbow according to the present invention and also through a portion of the pump impeller;

FIGURE 2 is a sectional view taken along the line 22 of FIGURE 1;

FIGURE 3 is a sectional view taken along the line 3--3 of FIGURE 2;

FIGURE 4 is a diagram of the hydraulic cross-sectional .area of the fluid passageway through the elbow;

FIGURE 5 is a half-sectional view showing several transverse hydraulic half-sections at different places along the elbow; and

I FIGURE 6 is a sectional view similar to that of FIG URE 2 but showing a modification.

Proceeding next to the drawings wherein like reference symbols indicate the same parts throughout the various views, a specific embodiment and modifications of the present invention will be described in detail.

As may be seen in FIGURE 1 the inlet elbow of the present invention is indicated at 1 and has a pump shaft 2 passing therethrough with a pump impeller 3 mounted on the shaft. Within the elbow, pump shaft 2 is surrounded by a protecting sleeve 4 which is sealed in a known manner, not shown, against the pump shaft 2.

The elbow is provided with an inlet 5 which is preferably circular to facilitate connection to conventional piping and conduits. The elbow is also provided with an outlet which is preferably circular in shape and is positioned in front of the impeller 3 as shown in FIGURE 1. The inlet and outlet define a fluid passageway therebetween and are positioned at an angle of about 90 degrees apart so as to change the direction of flow of fluid passing therethrough about 90 degrees.

The central plane of the pump is indicated at 6 and is shown in FIGURE 2. The central plane 6 is substantially parallel to the radius of curvature of the elbow 1. Further reference to FIGURE 2 reveals

radial ribs

7 and 8 positioned in the central plane 6 of the elbow on the shaft sleeve 4 on the sides thereof away from and directed toward the inlet respectively. The outlet edge of rib 7 is indicated at 9 and, as is shown in FIGURE 3, the thickness of the rib 7 gradually increases from edge 9 to the region 10 where the sleeve 4 passes through the outer wall 11 of the elbow on that side of the sleeve that faces away from the inlet. Therefore, in a plane parallel to the central longitudinal axis of pump shaft 2 the rib 7 will be triangular in cross-section.

The radial rib 8 is provided with a uniformly thin leading edge 12 and is joined at its edge 13 with the inner wall 14 of the elbow. In a similar manner, the thickness of the rib 8 gradually increases from leading edge 12 and junction edge 13 and along the outer wall 11 toward the region 16 where the sleeve connects to the outer wall 11 of the elbow on the side of the sleeve that is directed toward the inlet of the elbow. The outlet or trailing edge of rib 8 is also of uniform thickness with the thickness of the rib gradually increasing from outlet edge 15 toward the region 16.

The outlet of the elbow 1 is provided with a converging funnel-shaped outlet portion 17 to direct the fluid to the impeller 3. Instead of the funnel-shaped outlet portion, the outlet portion may be cylindrical.

The shaft protecting sleeve 4 is substantially cylindrical in shape but may have a slight conical taper as indicated by the dot-dash lines 4. This taper is outwardly toward the outer wall 11 of the elbow. The angle between the generatrix of the protective sleeve 4' and the central longitudinal axis of pump shaft 2 is preferably about 8 degrees and not greater than 15 degrees.

As a further modification the sleeve 4 may have an elliptical cross-section whose major axis extends in the direction of'flow of fluid through the elbow.

The changes in the cross-section along the fluid passageway of the elbow are illustrated in FIGURES 4 and 5. As shown in FIGURE 4, the hydraulic cross-sectional area has a maximum at the plane A which is at the inlet 5 as shown in FIGURE 1. The cross-sectional area then decreases gradually throughout about the first third of the developed axial length L of the elbow to about plane B located as shown in FIGURE 1. The cross-sectional area then decreases considerably more rapidly in the middle third of the elbow to about plane C located as shown in FIGURE 1. From plane C to the outlet of the elbow at plane D the cross-sectional area decreases more gradually. As graphically illustrated in FIGURE 4, the cross-sectional area F decreases from 0.35 m? to 0.21 m? which is a total decrease in area of 0.14 m.

4 It is pointed out that in the middle third of the elbow, between planes B and C, the cross-sectional area decreases 0.11 m? or about 79 percent of the total decrease in cross-sectional area throughout the elbow.

As illustrated by the dotted curved line in FIGURE 4, the greatest decrease in cross-sectional area can occur in the second half of the middle third of the elbow.

The cross-section of the elbow fluid passageway along the several planes A to D are illustrated in FIGURE 5. While the inlet plane A is illustrated as circular, it is pointed out that an elliptical cross-section such as shown at A in FIGURE 5 would provide the fluid a more convenient flow path.

The cross-sections on planes B and C are approximately elliptical while the outlet opening at plane D is circular.

As shown in FIGURE 1, the shortest distance between the inner and outer walls of the fluid passageway is indicated at a which corresponds to the central plane 6 of the elbow. This shortest distance is positioned between transverse planes B and C. The minor axes of the elliptical sections of the elbow will thus correspond with the central plane 6 of the elbow.

The smallest radius of curvature r of the inner wall of the elbow is approximately equal to the smallest distance a between the inner and outer walls of the elbow. The ratio between this radius of curvature and the shortest distance is greater than 0.8 and preferably 1.0. The ratio of the smallest radius of curvature r to the diameter of the inlet A is not smaller than 0.4 and preferably 0.7. In this embodiment this ratio is about 0.66.

It is pointed out that the width b of the elbow normal to central plane 6 decreases progressively from its average value between planes B and C to its outlet D.

It is pointed out that the apparent discrepancy between the cross-sectional areas illustrated in FIGURE 5 and FIGURE 1 is due to the showing in FIGURE 5 of only the purely hydraulic cross-sections.

In FIGURE 6 there is illustrated a modification of the radial ri-b structure shown in FIGURE 2. In this modification

radial ribs

18 and 19 are provided which correspond to

radial ribs

7 and 8 respectively of FIGURE 2. In this modification, however, the upper rib 18 is positioned at a slight angle to central plane 6 of the elbow and the rib 19 is positioned in the central plane so as to impart an initial rotating movement to the fluid flowing through the elbow.

It will therefore be apparent that in the elbow of the present invention where the cross-section of the fluid passageway progressively decreases from the inlet to the outlet the flow of the fluid therethrough will be continually accelerated as it passes through the elbow from the inlet to the outlet. Further, at the outlet of the elbow just before the fluid enters the impeller the fluid currents will have approximately the same Velocities and directions. In those elbows which are provided with a protecting sleeve for the pump shaft, the formation of vortices on, the sleeve or on the shaft where no sleeve is present will be virtually eliminated. The gradual decrease in cross-section adjacent the outlet of the elbow will resuit in a more uniform velocity distribution of the fluid at the elbow outlet. In the second half of the middle third of the elbow where there is a rapid decrease in the cross-section of the fluid passageway, there will be a resulting stronger acceleration of the fluid velocity accompanied by a considerable thinning of the boundary layers of fluid. In proportion to this thinning of the boundary fluid layers, the velocity distribution in this region of the elbow will become more irregular.

The radius of curvature of the inner wall of the elbow also has a considerable effect on the formation of secondary fluid currents and vortices in the region of the pump shaft passing through the elbow. The present invention therefore provides for minimizing these effects by making the radius of curvature as great as possible at the narrowest place in the central plane of the elbow. The relationship of the radius of curvature to the shortest distance between the inner and outer walls of the central plane of the elbow and to the inlet diameter of the impeller as described above produce this advantageous effect.

The present invention additionally provides for improving the flow of fluid toward the pump impeller by gradually decreasing the width of the elbow normal to the central plane of the elbow from the middle portion of the elbow to its outlet. This structure is particularly advantageous in those elbows traversed by a pump shaft or shaft sleeve. The tendency to form vortices in the fluid flow on the side of the shaft or sleeve away from the pump inlet will be greatly decreased.

The position and shape of radial ribs extending from the pump shaft sleeve also contributes largely to decreasing the formation of vortices within the elbow. In some pump arrangements it may be advantageous to impart a rotating motion to the fluid just before the fluid enters the pump impeller. This can be readily accomplished by positioning the radial ribs so that their central planes are at a slight angle to each other.

It will be understood that this invention is susceptible to modification in order to adapt it to different usages and conditions, and accordingly, it is desired to comprehend such modifications within this invention as may fall within the scope of the appended claims.

What is claimed is:

1. An inlet elbow for changing the direction of fluid flow about 90 degrees and then introducing the fluid to the impeller of a centrifugal pump in a direction parallel to the pump shaft and comprising an elbow of substantially 90 degrees having an inlet and a circular outlet and a fluid passageway therebetween, said outlet being positionable in front of the pump impeller, the cross-sectional area of said fluid passageway decreasing from the inlet to the outlet, the cross-section of the central portion of said fluid passageway being substantially elliptical, the minor axis of said elliptical cross-section corresponding to the shortest distance in the central plane of said passageway between the inner and outer walls thereof.

2. An inlet elbow as claimed in claim 1 wherein the cross-sectional area of said fluid passageway decreases slowly throughout about the first third of its developed axial length from its inlet, more rapidly throughout the middle third, and then again slowly in about the last third adjacent said outlet.

3. An inlet elbow as claimed in claim 2 wherein said cross-sectional area decreases more rapidly in the second half than in the first half of said middle third portion.

4. An inlet elbow as claimed in claim 1 wherein the decrease in cross-sectional area in the middle third of said passageway is about 50 to 80 percent of the total decrease of cross-sectional area between said inlet and outlet thereof.

5. An inlet elbow as claimed in claim 1 wherein the ratio of the smallest radius of curvature of the inner wall of the elbow to the shortest distance between the inner and outer walls in the central plane of the passageway is greater than 0.8, the ratio of said radius to the diameter of said inlet being greater than 0.4.

6. An inlet elbow as claimed in claim 1 wherein the Width of said passageway normal to said central plane thereof decreases from the middle portion of said passageway to said outlet.

7. An inlet elbow as claimed in claim 1 and further comprising a converging funnel-shaped outlet portion on said elbow outlet.

8. An inlet elbow as claimed in claim 1 and further comprising a cylindrical outlet portion on said elbow outlet.

9. An inlet elbow as claimed in claim 1 and further comprising a tubular sleeve within said elbow between the outlet and outer wall thereof for enclosing a pump shaft, and a radial rib on the outer surface of said shaft sleeve away from the inlet of the elbow.

10. An inlet elbow as claimed in claim 9 wherein the thickness of said rib increases toward the outer wall of said elbow so that said rib has a substantially triangular cross-section in a plane parallel to the longitudinal axis of said shaft sleeve.

11. An inlet elbow as claimed in claim 1 and further comprising a tubular sleeve within said elbow between the outlet and outer wall thereof for enclosing a pump shaft, a second radial rib on the outer surface of said shaft sleeve directed toward said inlet, said second rib having a first edge facing said inlet, a second edge extending toward said outlet and a further edge contacting the inner wall of said passageway, the thickness of said second rib increasing from its first, second and inner wall edges toward the intersection of said shaft sleeve and the elbow outer wall.

12. An inlet elbow as claimed in claim 1 and further comprising a tubular sleeve within said lbow between the outlet and outer wall thereof for enclosing a pump shaft, a radial rib on the outer surface of said shaft sleeve away from the inlet of the elbow, a second radial rib on the outer surface of said shaft sleeve directed toward said inlet, the central planes of said ribs being at an angle to each other so as to impart a prerotating motion to fluid passing through said passageway.

13. An inlet elbow as claimed in claim 1 and further com rising a tubular sleeve within said elbow between the outlet and outer wall thereof for enclosing a pump shaft, said sleeve having a substantially elliptical crosssection with its major axis extending in the direction of flow of fluid through said passageway.

14. An inlet elbow as claimed in claim 1 and further comprising a tubular sleeve within said elbow between the outlet and outer wall thereof for enclosing a pump shaft, said sleeve being substantially conical and taperingoutwardly from the outlet end of said elbow to the elbow outer wall.

15. An inlet elbow as claimed in claim 14 wherein the angle between the central longitudinal axis of said sleeve and the generatrix of the conical sleeve is not greater than 15 degrees.

References Cited UNITED STATES PATENTS 2,542,902 2/1951 Chubbuck 103-103 2,743,120 4/1956 Haentjens et al 103-111 3,283,737 11/1966 Gongwer 103-115 FOREIGN PATENTS 217,075 8/ 1908 Germany.

HENRY F. RADUAZO, Primary Examiner.