Centrifugal pump with double volute casing

A centrifugal pump comprises a casing which has two volutes disposed substantially diametrically opposite each other in such a way that the end of the first volute is adjacent to the start of the second volute. The outer diameter of the casing approximates or equals the diameter of the first volute in the region of the end of the first volute. This contributes to compactness of the casing. The volutes surround a rotary impeller or a guide wheel which is installed in the casing downstream of the impeller. Each volute extends along an arc of at most 180 degrees. The casing can have one or more elongated channels extending from the end of the first volute, along the second volute, and to a common outlet nozzle for both volutes.

BACKGROUND OF THE INVENTION 
The present invention relates to centrifugal pumps in general, and more 
particularly to improvements in centrifugal pumps having volute casings. 
Still more particularly, the invention relates to improvements in 
centrifugal pumps with double volute casings. 
The casing or housing of a pump serves to seal the conveyed fluid from the 
surrounding area. This can be achieved independently of the exact shape of 
the casing. However, and in order to be capable of carrying out certain 
specific tasks, the casings of centrifugal pumps normally assume one of a 
large number of different shapes which enable them to perform given 
functions more satisfactorily than centrifugal pumps having casings of a 
different configuration. Double volute casings are one of a large category 
of pump casings which can be used in centrifugal pumps, and their function 
is to balance the radial thrust. The two volutes are disposed opposite and 
are normally similar to one another. As a rule, the starts of the two 
volutes are offset 180 degrees in relation to each other. Such design is 
believed to ensure that the radial thrust is not only constant but also 
that the radial thrust is small or negligible within the entire operating 
range of the centrifugal pump. Depending on the structural design of the 
centrifugal pump, the volutes surround the impeller which is rotatably 
mounted in the casing, or they surround a guide wheel or diffuser which is 
installed in the pump casing downstream of the impeller. 
German Offenlegungsschrift No. 26 40 866 discloses a centrifugal pump whose 
casing has two volutes which are offset relative to one another. The first 
volute extends beyond the start of the second volute and the casing has 
channels which connect the end of the first volute with the outlet nozzle. 
The latter is common to both volutes. A drawback of such centrifugal pumps 
is that the casing is relatively large owing to the dimensions of the 
volutes, as considered radially of the casing. This entails greater 
pressures and results in greatly increased bulk of the casing, especially 
when compared with a single volute casing. 
OBJECTS AND SUMMARY OF THE INVENTION 
An object of the present invention is to provide a novel and improved 
double volute casing for use in centrifugal pumps. 
Another object of the invention is to provide a double volute casing whose 
dimensions are a fraction of the dimensions of equally rated conventional 
double volute casings. 
A further object of the invention is to provide a centrifugal pump which 
embodies a double volute casing of the above outlined character. 
An additional object of the inventon is to provide a compact and 
lightweight double volute casing which is just as satisfactory as 
heretofore known more expensive, bulkier and heavier double volute 
casings. 
A further object of the invention is to provide a double volute pump casing 
which is constructed and assembled in such a way that its compactness 
cannot adversely influence the operation of the centrifugal pump wherein 
the casing is put to use. 
Still another object of the invention is to provide a double volute casing 
whose outer diameter is smaller than the outer diameters of similarly 
rated double volute casings of presently known design. 
The invention is embodied in a centrifugal pump which comprises a casing 
having first and second volutes. In accordance with a feature of the 
invention, each of the volutes extends along an arc of maximally 180 
degrees, the start of the second volute is adjacent to the end of the 
first volute (i.e., the volutes are located substantially diametrically 
opposite each other if each extends along an arc of approximately 180 
degrees), and the outer diameter of the casing at least approximates 
(i.e., it need not appreciably exceed) the outer diameter of the first 
volute in the region of the end of such first volute. 
The volutes can surround an impeller which is rotatably mounted in the 
casing, or a guide wheel or diffuser which is mounted in the casing 
downstream of the impeller. The dimensions of the second volute may but 
need not approximate or equal the dimensions of the first volute. 
The casing is further provided with an outlet nozzle and such outlet nozzle 
may be common to both volutes. In such pumps, the casing may be provided 
with at least one channel which extends from the end of the first volute, 
along the second volute and to the outlet nozzle, i.e., both volutes can 
discharge into one and the same outlet nozzle. The channel or channels 
between the end of the first volute and the outlet nozzle can have a 
substantially constant cross section. Alternatively, the cross section of 
at least one such channel can vary (either along the full length of the 
channel or along a certain portion of the channel) in a direction from the 
end of the first volute toward the outlet nozzle. If the casing has two 
channels, they may be mirror symmetrical to each other. The two channels 
(which need not be mirror symmetrical to each other) can flank the second 
volute, i.e., the latter can be disposed between the two channels. 
If the impeller of the centrifugal pump is installed downstream of the 
volutes, the casing may be provided with channel means for delivery of 
fluid from the ends of the volutes to the impeller. The just mentioned 
channel means may comprise a plurality of channels and at least one of 
these channels may be an arcuate channel having a radius of curvature 
which is less (e.g., appreciably less) than the radius of the periphery of 
the casing. 
The novel features which are considered as characteristic of the invention 
are set forth in particular in the appended claims. The improved 
centrifugal pump itself, however, both as to its construction and its mode 
of operation, together with additional features and advantages thereof, 
will be best understood upon perusal of the following detailed description 
of certain specific embodiments with reference to the accompanying 
drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The centrifugal pump which is shown in FIGS. 1 to 4 comprises a double 
volute pump casing or housing 1 and an impeller 2 which is rotatably 
mounted in the casing and is surrounded by two arcuate volutes 3 and 5. 
These volutes are disposed diametrically opposite each other and are of 
similar or identical size and shape. Each of the volutes 3 and 5 extends 
along an arc of not more than 180 degrees (such arc can be less and even 
substantially less than 180 degrees). The end 4 of the first volute 3 is 
adjacent to the start 5a of the second volute 5, and the start of the 
first volute 3 is adjacent to the end 4a of the second volute 5. The 
illustrated casing 1 has a common outlet nozzle or discharge nozzle 6 for 
both volutes. The nozzle 6 contains a stationary guide rib 7 serving to 
enhance the outflow of pressurized fluid from the casing 1. 
FIG. 2 shows that the fluid which issues from the first volute 3 flows into 
two arcuate channels 8 and 9 of the casing 1. These channels extend from 
the end 4 of the first volute 3, along the second volute 5, and to the 
outlet nozzle 6 of the casing 1. The means for connecting the outlet 
nozzle 6 with the fluid discharging nipple or with a next stage of the 
centrifugal pump is not shown in the drawing. The two channels 8 and 9 are 
mirror symmetrical to each other with reference to a vertical plane (as 
viewed in FIG. 2) which is disposed midway between such channels. It will 
be noted that the cross-sectional area of each of these channels varies in 
a direction from the end 4 of the first volute 1 toward the outlet nozzle 
6. The variation may be constant from the inlet end and all the way to the 
outlet end of each channel, or such variation of the cross-sectional area 
can take place only along a portion of the channel 8 or 9. It is equally 
possible to utilize channels having uniform cross-sectional areas from end 
to end. Furthermore, the casing 1 can be formed with a single channel 8 or 
9, or with more than two channels. The cross-sectional areas of the 
channels between the end 4 of the first volute 3 and the outlet nozzle 6 
will be selected with a view to stabilize or uniformize hydraulic 
conditions in the volutes and/or in the outlet nozzle 6. 
As shown in FIG. 3, the first volute 3 of the pump casing 1 receives fluid 
in response to rotation of the impeller 2. FIG. 4 shows that the median 
(developing) portion of the second volute 5 is flanked by the two arcuate 
channels 8 and 9. These two channels are at least substantially parallel 
to the volute 5. The fluid which leaves the end 4 of the first volute 3 
flows through the channels 8, 9 and thence into the outlet nozzle 6 of the 
casing 1. The fluid which issues from the second volute 5 flows directly 
into the nozzle 6. The number of channels between the end 4 of the first 
volute 3 and the outlet nozzle 6 depends on the amount of fluid which is 
to be circulated by the centrifugal pump. 
FIG. 5 illustrates a guide wheel 10 mounted in the casing 1 downstream of 
the impeller 2 and surrounded by the volutes 3 and 5. 
The outer diameter of the casing 1 is identical with or closely 
approximates the outer diameter of the first volute 3 in the region of the 
end 4. This contributes to compactness, lower weight and lower cost of the 
casing 1. Moreover, it is simpler to produce the improved compact casing 
in the form of a casting, and testing of the casing is simpler than the 
testing of conventional (bulkier) casings. Still further, the improved 
double volute casing is subjected to less pronounced stresses than the 
heretofore known double volute casings. 
If desired, the centrifugal pump which embodies the present invention can 
be designed to have an impeller downstream of the volutes. The casing is 
then provided with one or more arcuate channels for delivery of fluid from 
the ends of the volutes to the impeller. The radii of curvature of such 
channels are smaller than the radius of the peripheral surface of the 
improved casing. Such construction can be resorted to when the centrifugal 
pump includes a second stage downstream of a double suction impeller. 
The reduction of dimensions of the improved double volute casing is 
attributable to the feature that the outer diameter of the casing need not 
exceed the outer diameter of the end 4 of the first volute 3. Such 
reduction of dimensions entails a surprisingly large reduction of the 
weight of the casing 1. 
The improved pump is susceptible of many further modifications without 
departing from the spirit of the invention. For example, the casing 1 or a 
similar double volute casing can have discrete outlet nozzles for the two 
volutes. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic and specific aspects of our contribution to 
the art and, therefore, such adaptations should and are intended to be 
comprehended within the meaning and range of equivalence of the appended 
claims.