US1955929A - Impeller - Google Patents

Description

April .24, 1934. H MUELLER 1,955,929

IIMPELLER Filed March 6, 1933 INVENTOR HANS M U E L LE R ATTORNEY ?atented Apr. 24, 1934 TENT IMPELLER Hans Mueller, Heidenheim-on-the-Brenz, Germany, assignor to J. M. Voith, Heid'enheim-onthe-Brenz, Germany, a copartnership composed oi Walther Voith, Hermann Voith, and Harms Voith Application March 6, 1933, Serial No. 659,793 In Germany March 18, 1932 3 Claims.

This invention relates to improvements in the construction of rotary machines which are capable of either propelling. or of being propelled by fluid, and relates more particularly to the construction of the blades of the impellers of pumps,

turbines, and the like.

An object of the invention is to provide an improved impeller for pumps, turbines and the like. Another object of the invention is to provide an improved impeller in which the blades are so constructed as to prevent the formation of cavitation in the water joint between the outer edges of the blades and the casing.

Another object of the invention is to provide an improved bandless type of impeller for hydraulic turbines.

Another object of the invention is to provide an improved impeller of the character mentioned, which is simple in construction, and reliable and exact in function under all conditions of service. The invention also comprises certain new and useful improvements in the construction, arrangement and combination of the several parts of which it is composed, as will be hereinafter more fully described and claimed;

In the accompanying drawing:-- Figure l is a view, partly in section, of a by draulic turbine having a bandless impeller;

Fig. 2 is an enlarged section of the end of one of the blades of the impeller shown in Fig. 1, showing the relation of the same to the casing of the turbine;

Fig. 3 is a view similar to Fig. 2, showing one form of construction of the blades of the impeller according to the present invention;

Fig. i is a similar View of another form of impeller blade;

Fig. 5 is a similar view of a still further form of impeller blade; and

Fig. 6 is a perspective of a portion of one or the impeller blades constructed according to the invention.

There occurs on the outer edge of handles impeller blades, a decrease in crosswise load relative to the inner part of the blades, due to the equalizing of pressure through the water joint between the blades and the surrounding casing.

This decrease in crosswise load effects an induced edge whirl in the immediate vicinity of the vacuum or suction side of the blades. On account of the high rotary speed of the impeller and the prevailing low absolute pressure on the suction side of the impeller blades, cavitation occurs in this whirl or in its vicinity, and due to the well known harmful effects of the caviof the setting below the turbine forms a draft tation, the portions of the impeller blades which are located in proximity to the whirl are easily pitted or corroded.

It has been found that if the impeller blades are so constructed that the dangerous edge whirl is removed from the endangered blade sur-- face on the suction side of an impeller, the effect of the whirl will be harmless, and therefore pitting or corroding of the impeller blades, as the result of the cavitation, will be prevented.

The invention will now be described in connec tion with a hydraulic turbine of well known type having an adjustable blade impeller. However, it will be understood that the invention is also applicable to other types of turbines, pumps, and the like.

Referring to Fig. 1, the turbine comprises a casing 11 which forms a peripheral water inlet in which are mounted an annular series of wicket gates or guide vanes 12, only a portion of which are shown. The lower portion 13 of the turbine casing forms an axially directed chamber in which the runner la operates.

The hub of the runner 14 is fixed to the lower end of a shaft 15 which is journalled in a suitable bearing 16 mounted in the turbine casing.

Any suitable type of setting may be used for the turbine installation, a concrete setting 17 being shown in the present instance. The portion S5 tube 18 into which the water is discharged from the runner.

As shown, the turbine is of the axial i'low type, and a suitable number of blades 19 are supported in the hub of the runner I l and project radially therefrom.

Each blade 19 has an entrance edge 30 and a discharge edge 31, both of which edges extend from the hub of the runner l'; outwardly to the outer edge 21.

As shown in Fig. 2, heretofore in the construc= tion of impellers or runners for hydraulic tur bines, it was customary to form the outer edge 2i oi the blades flat or parallel to the wall of the turbine casing 13. With this construction sharp corners 22 and 23 are, respectively, formed at the junction of the outer edge 21 with the upper or pressure side 24 and the lower or, suction side 25 of the blades.

The water joint 20 thus defined by the outer edge 21 of the runner blades and the wall of the casing 13, consists of a passage having parallel .side walls, as shown in Fig. 2. It is well known that with a water joint of this construction, a.

contraction of the fluid Jet occurs at the inlet at the pressure side 24, as indicated by the line 26, Fig. 2.

P1 and P2 designate the pressure, respectively, on the pressure and suction sides of the turbine blade 19 shown in Fig. 2.

Directly beneath the portion of the water joint 20 in which the water jet is contracted, as indicated by line 26, Fig. 2, there is a mixing zone 27.

On account of the partly regaining of energy in the mixing zone 27, a statical pressure will occur in the contracted section of the fluid jet. This statical pressure is less than the pressure Pa on the suction or vacuum side 25 of the runner blade 19.

Cavitation occurs in the narrowest section of the contracted water jet. That is to say, when the absolute statical pressure P: is small enough relative to the diiference between the statical pressures PlP2, while the average pressure in the contracted jet decreases to the vapor pressure, when the absolute 'statical premure P2 is high relative to the pressure difierence Pl-P2, while high rotation velocities are created in the local whirl centers of the mixing zone 19, the pressure in these local whirl ceniers decreases to the vapor pressure of the water.

Due to the equalizing of pressure through the water joint 20, an induced edge whirl is formed in the water, jet in the immediate vicinity of the suction or vacuum side 25 or the runner blade 19. as shown by the curved line 28, Fig. 2.

On account of the high rotary velocity of the runner and the prevailing low absolute pressure on the suction side 25 of the runner blades, cavitation occurs in the whirl 28, or in the vicinity thereof.

Due to the well known harmful eflects of the cavitation, the parts of the runner blades which are located in proximity to the whirlingfluid will be pitted or corroded, as indicated by the double section lines 29 in Fig. 2.

In order to eliminate the cavitation which occurs as the result of the whirling water in the immediatevicinity. of the suction or vacuum side 25 of the runner blades 19. a rib or ridge 35 is.

formed along the edge 21, as shown in Figs. 3, 4i and 5. This rib or ridge should have a depth or depend downwardly from the suction side 25 of the runner blades such a distance that the whirling water, indicated by the curved line 28 Fig. 3, is removed from the endangered blade surface. The addition of the rib or ridge 35 to the under side of the runner blades will thus render the whirling water harmless.

ribs or ridges 35.

The intended effect is produced by the formation of the rib on the suction side of the blades, and it is immaterial whether the ribs are cast integral with the blades, as shown in Fig. 3, or whether the ribs are attached to the blades by welding or other means, as indicated at 36 in Fig. 4.

In some instances, such for instance as when a turbine is operated under a high head. it is of advantage to provide for the free flowing of the water through the water joint, so that at no place inside of the water joint will the jet depart from the sides of the water joint.

Therefore, in order to eliminate the cavitation in the fluid joint between the blades of the runner and the wall of the turbine casing and at Figs. 3 and 4 illustrate two dill'erent forms or the same time eliminate the cavitation onthe suction side of the blades, the runner blades may be iormed in the manner illustrated in Fig. 5.

As shown in Fig. 5, the outer edge 21 of the blades is so constructed as to form a tapered water joint 37 with the wall 0! the casing 13.

The water joint 3'7 should be wider at the 'inlet than at the outlet, and the inlet corners of the runner blades 19 should be rounded ofi, as

indicated at 38.

Obviously, the configuration of the outer edge 21 can be varied from that illustrated to suit other conditions. However, it has been determined by computations and tests that the shape or term imparted to the outer edge 21', in order to producethe tapered water joint 37 relative to the vertical direction of flow, should not be less than 3, as indicated in Fig.v 5, and that the radius of curvature of the rounded corner 38.

slightly, as shown, so as to provide a stream line surface for the water.

By constructing a tapered water joint between the blades of the runner and the wall of the casing 13, as shown in Fig. 5, all particles of the water jet inside of the water joint are accelerated steadily in such a way that the water remains in full flowing contact with the sides of the water joint. Furthermore, the rib 35 will prevent whirling water discharged through the water joint from acting upon the suction side of the runner blades sumciently to seriously pit or corrode the surfaces of the blades adjacent the water joint.

Having thus described my invention what I claim and desire to secure by Letters Patent is:-

1. A device of the class described comprising an impeller having a plurality of blades, a casing surrounding the outer edges of the blades and defining a water joint with the blades through which the whirling water flows, anda single rib on the edge of each blade which forms the water joint, said rib extending into the low pressure area for preventing the whirling water from acting on the blade sufllciently to cause pitting or corroding thereof.

2. A hydraulic machine comprising an impeller having a plurality of blades, a casing surrounding the outer edges of the blades and defining a water joint with the blades, and a single rib on the suction side of the edge of each blade which forms side and extending into the low pressure area for preventing the whirling water from acting on theimpeller sufllciently to cause pitting or corroding thereof.

HANS MUELLER.

Images