US3128714A - Centrifugal pumps - Google Patents

Description

April 14, 1964 G. F. ARKLESS ETAL 3,128,714

CENTRIFUGAL PUMPS Filed Feb. 15, 1962 Inventors GIG/P65 fQ fiR/KA 4/94 1555 A Horn e yg United States Patent Office 3,128,714 Patented Apr. 14, 1964 3,128,714 CENTRIFUGAL PUMPS George Frederick Arkiess and Thomas Orr Leith, Cathcart, Glasgow, Scotiand, assignors to G. & J. Weir Limited, Giasgow, Scetiand Filed Feb. 13, 1962, Sex. No. 173,027 Ciaims. ((Il. 103-408) This invention relates to improvements in centrifugal pumps, particularly such pumps which have to operate with low net positive suction head and, at the same time, produce a relatively high differential head.

Typical examples of such pumps are those used for feeding steam boilers.

In order to produce the high discharge pressures required, it is necessary often to operate a centrifugal pump at a high speed. This produces a high stage pressure rise and, therefore, can often permit the use of a centrifugal pump with a single stage having an overhung impeller. The overhung arrangement not only provides an inexpensive design but also provides ideal suction conditions which are conducive to low net positive suction head requirements. Unfortunately, the net positive suction head required by a centrifugal pump increases rapidly as the speed of rotation of the impeller increases.

On the other hand, if more modest rotational speeds of the impeller are used, coupled with an increase in the number of stages in the pump, the shaft on which the multi-stage impellers are mounted must be sufliciently inflexible not to whirl and must in consequence be of relatively large diameter. This, in turn, necessitates the provision of an impeller for each stage having an eye of large diameter, resulting in a high peripheral speed of the eye, which speed increases the net positive suction head requirements. Moreover, the cost of a pump with a large number of stages is high.

A centrifugal pump, according to the invention, has two stages, a driving shaft, an overhung impeller in each stage mounted on the driving shaft, and an eye in each impeller, said driving shaft terminating short of the eye of the impeller of the first stage, whereby the diameter of said eye is maintained at a minimum to reduce the net positive suction head requirements of the pump for a predetermined rotational speed of the shaft and flow of fluid being pumped through the stages.

A bearing, lubricated by fluid being pumped, or a stuffing box or a seal (hereinafter referred to simply as a bearing) is preferably so arranged on the shaft to protrude below the eye of the impeller of the second stage as to maintain the overhang of the impellers at a minimum.

Desirably, the outside diameter of the impeller of the first stage is less than that of the impeller of the second stage, a discharge annulus from said last-mentioned impeller surrounding a discharge passage from said first mentioned impeller.

A wear ring for the eye of the impeller of the first stage has an inner diameter approximately the same as that of the eye of the impeller of the second stage, so that the first stage will be in hydraulic axial balance during opera tion of the pump.

One embodiment of the invention is illustrated by way of example only in the accompanying drawing which is a part axial section of a pump.

Referring to the drawing, 1, 2 denote, respectively, overhung impellers of two stages of the pump which is arranged to operate at a speed intermediate that of a single-stage pump and that of a multi-stage pump. The impellers, 1, 2 are so mounted on a driving shaft 3 that the shaft 3 terminates short of the eye 4 of the impeller 1, whereby the diameter of the eye 4 is maintained at a minimum to reduce the net positive suction head requirements of the pump for a predetermined rotational speed of the shaft 3 and flow of fluid being pumped through the stages.

5 denotes a bearing lubricated by fluid being pumped and so arranged on the shaft 3 as to protrude below the eye 6 of the impeller 2, whereby to maintain the overhang of the impellers 1, 2 at a minimum.

The outside diameter of the impeller 1 is less than that of the impeller 2, a discharge annulus '7 from the impeller 2 surrounding a discharge passage 8 from the impeller 1 to the eye 6 of the impeller 2, whereby to provide a compact construction of pump.

A wear ring 9 for the eye 4 has an inner diameter approximately equal to the inner diameter of the eye 6, so that the first stage is in hydraulic axial balance.

A Wear ring 10 for the eye 6 is smaller in diameter than that of a second wear ring Ill. During pumping of liquid through the stages there will therefore be an axial force arising from the pressure forces in the direction from the impeller 1 towards the impeller 2. Water at approximately discharge pressure from the impeller 2 leaks through the wear ring 11 and passes into a space 12 behind the back of the impeller 2. At a diameter approx imately intermediate the wear ring 11 and the shaft 3 a fiat annular face 13 is formed on the back of the impeller 2, which face 13 corresponds in diameter to a similar fiat face 14 formed on a stationary part of the pump casing and spaced from the face 13 to define an annular passage 15 therewith. The rotating assembly comprising the shaft 3 and the impellers l, 2 fixed thereto is free to move axially as a unit. A space 16 intermediate the annular passage 15 and the impeller 2 communicates with the suction of the pump by way of a passage 17 or, alternatively, to some other low pressure zone, such as the tank from which the pump draws its supply of liquid to be pumped.

Owing to the unbalanced forces arising from the differential diameters of the wear rings 16, 11 in the second stage, the passage 15 will tend to close. This restricts the flow of liquid flowing by way of the large diameter wear ring 11, the space 12 and the passages 15, 17 to the low pressure zone.

Accordingly, the pressure rises in the space 12 and produces an axial force which counteracts the out-of-balance force in the second stage arising from the difference in diameters of the wear rings 10, 11. An increase in the discharge pressure of the Whole pump will cause the faces 13, 14 to close together, and the resulting increase in pressure in the space 12 counteracts what would otherwise be an increase of axial out-of-balance force. A diminution in pressure rise over the Whole pump will cause a reduction in the out-of-balance force of the second stage arising from the difference of diameters of the wear rings 10, 11 and the faces 13, 14 will tend to move apart. This will result in a diminshed pressure in the space 12 which will again allow the assembly of the impellers 1, 2 and the shaft 3 to remain in complete axial balance.

We claim:

1. A centrifugal pump including two stages, a driving shaft, a rotatable assembly including an overhung impeller in each stage mounted on the driving shaft, each impeller having an eye, the driving shaft terminating short of the eye of the impeller of the first stage whereby the diameter of said eye is maintained at a minimum, bearing means on the shaft for the rotatable assembly, the rotatable assembly being cantilevered out from the bearing means, and the bearing means extending underneath the eye of the impeller of the second stage thus to bring the centre of gravity of the rotatable assembly close to the bearing means.

2. A centrifugal pump as claimed in claim 1, in which a wear ring is provided for the eye of the impeller of the first stage, this wear ring having an inner diameter approximately the same as the inner diameter of the eye of the impeller of the second stage.

3. A pump as claimed in claim 1, in which the impellersand the shaft are free to move axially of the pump, a wear ring surrounds the eye of the impeller of the second stage, a second wear ring of a diameter greater than that of the first-mentioned wear ring, is provided and the back of said impeller has a projection rotatable in said second wear ring, said projection defining with the second wear ring a passage for leakage of liquid from said impeller to a low pressure zone.

4. A pump as claimed in claim 3, in which the back of the impeller of the second stage is formed with a flat annular face at a diameter intermediate the driving shaft and the second wear ring, and a stationary part of the pump is formed with a similar fiat face corresponding in diameter to the first-mentioned face on and defining with the first-mentioned face an annular passage communieating with the passage between the projection and the second wear ring and with the low pressure zone.

5. A pump as claimed in claim 4, in which the back of the impeller of the second stage and a stationary part of the pump define a space which provides communication between the annular passage, defined by the flat annular faces, and the passage defined by the projection and the second wear ring.

References Cited in the file of this patent UNITED STATES PATENTS 1,151,964 Peterson 1- Aug. 31, 1915 1,392,090 Tamini Sept. 27, 1921 1,412,105 Eichelcr Apr. 11, 1922 1,473,802 Woock et a1 Nov. 13, 1923 2,159,422 Buchi May 23, 1939 2,691,944 Grosskopf Oct. 19, 1954 2,888,193 Greenwald May 26, 1959 2,923,461 Dallenbach Feb. 2, 1960 2,960,939 Buschhorn et al Nov. 22, 1960 FOREIGN PATENTS 574,006 Germany Apr. 7, 1933 1,091,307 France Oct. 27, 1954 883,007 Great Britain Nov. 22, 1961

Claims (1)

1. A CENTRIFUGAL PUMP INCLUDING TWO STAGES, A DRIVING SHAFT, A ROTATABLE ASSEMBLY INCLUDING AN OVERHUNG IMPELLER IN EACH STAGE MOUNTED ON THE DRIVING SHAFT, EACH IMPELLER HAVING AN EYE, THE DRIVING SHAFT TERMINATING SHORT OF THE EYE OF THE IMPELLER OF THE FIRST STAGE WHEREBY THE DIAMETER OF SAID EYE IS MAINTAINED AT A MINIMUM, BEARING MEANS ON THE SHAFT FOR THE ROTATABLE ASSEMBLY, THE ROTATABLE ASSEMBLY BEING CANTILEVERED OUT FROM THE BEARING MEANS, AND THE BEARING MEANS EXTENDING UNDERNEATH THE EYE OF THE IMPELLER OF THE SECOND STAGE THUS TO BRING THE CENTRE OF GRAVITY OF THE ROTATABLE ASSEMBLY CLOSE TO THE BEARING MEANS.

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