Sheet metal interstage casing for a pump

A multistage centrifugal pump has a plurality of series-connected interstage casings of pressed sheet metal housing respective impellers. Each of the interstage casings includes a cylindrical side wall for housing an impeller, a bottom wall joined to the cylindrical side wall and extending around an inlet for the impeller, and a return blade attached to the bottom wall remotely from the impeller. The bottom wall has a central region projecting conically or spherically radially inwardly toward the impeller by a distance corresponding to the extent to which the bottom wall is deformable under an interstage pressure difference. The deformation of the bottom wall that is developed under the pressure generated by the impeller is borne only by the casing body itself without affecting the welded spots, which are prevented from being ripped off under stresses that would otherwise concentrate on the welded spots.

BACKGROUND OF THE INVENTION 
The present invention relates to a sheet metal interstage casing for a pump 
a sheet metal, and more particularly to an interstage casing for a pump 
which is pressed into shape for use in a multistage centrifugal pump. 
Conventionally, there is known an interstage casing for a pump in which the 
casing is formed of sheet metal such as a stainless steel plate and 
manufactured by press work. 
This type of interstage casing is shown in FIG. 2 of the accompanying 
drawings. As shown in FIG. 2, the interstage casing is of a cylindrical 
receptacle-like structure comprising a cylindrical side wall 1 and a 
bottom wall (or casing end wall) 2 on an end thereof (on the right-hand 
side in FIG. 2) which is connected to a subsequent (or the next) 
interstage casing. The axial ends of the cylindrical receptacle-like 
structure are machined into a bottom end surface 3 joined to the bottom 
wall 2 and an open end surface 4. The bottom wall 2 has a radially outer 
cylindrical surface 5 to be fitted in the next interstage casing, 
providing a spigot joint. The open end surface 4 has a radially inner 
cylindrical surface 6 fitted over the radially outer cylindrical surface 5 
of a preceding interstage casing, providing a spigot joint. These surfaces 
5, 6 are also machined to desired dimensional accuracy. 
The interstage casing houses a impeller 7 having an inlet end disposed in 
the opening of the cylindrical side wall 1 which is defined by the open 
end surface 4. That is, the inlet side of the impeller 7 is disposed in 
confronting relation to the bottom wall 2 of the preceding interstage 
casing. The interstage casing also accommodates a return blade 8 with a 
side plate 9 joined thereto, the return blade 8 being welded at plural 
spots 10 to the surface of the bottom wall 2 which faces the impeller 7. 
The impeller 7 can be rotated by a shaft 11. A liner ring 12 is attached to 
the bottom wall 2. A shaft sleeve 14 is fitted over the shaft 11. The side 
plate 9 is mounted on the shaft sleeve 14 through a bearing or bushing 13. 
When the multistage centrifugal pump is in operation, the liquid to be 
pumped is pressurized by the impeller 7, passes through a passage defined 
in the return blade 8 between the side plate 9 and the bottom wall 2, and 
is led to the next impeller by which the liquid is further pressurized. 
The pressure of the liquid is applied to the reverse side of the bottom 
wall 2 as indicated by the arrows P, tending to deform the bottom wall 2 
radially inwardly toward the lower-pressure side (toward the left-hand 
side in FIG. 2). 
If the bottom wall 2 is deformed to a large extent, then welded spots 10 
between the return blade 8 and the bottom wall 2 may be subjected to 
excessive stresses that may rip off the welded spots 10. To prevent the 
bottom wall 2 from being deformed excessively, the interstage casing has a 
stiffener plate 2A welded to the bottom wall 2. The return blade 8, the 
side plate 9, and the bottom wall 2 are also increased in thickness to 
prevent them from being deformed excessively. 
FIG. 3 of the accompanying drawings shows in cross section an interstage 
casing including a spherical bottom wall 2 which has a relatively thin 
wall thickness. Those parts shown in FIG. 3 which are identical or similar 
to those shown in FIG. 2 are denoted by identical or similar reference 
characters. 
FIG. 4 of the accompanying drawings shows in fragmentary cross section a 
vertical-shaft multistage centrifugal pump comprising interstage casings 
each of the structure shown in FIG. 2. The interstage casings are 
assembled together by a fastening band 15. The multistage centrifugal pump 
includes a discharge port 16 and a cable 17. 
When the multistage centrifugal pump is in operation, the liquid to be 
pumped is drawn from a suction port (not shown) and pressurized by the 
successive impellers 7. The pressure head of the liquid is restored as the 
liquid passes through each of the return blades 8. Finally, the liquid is 
discharged out of the pump through the discharge port 16. 
Of various forces induced by the liquid pressure and pressure differences 
applied to the interstage casings, the most problematic would be the force 
imposed by the interstage pressure difference acting on a flat portion 
normal to the shaft between adjacent ones of the interstage casings. 
Heretofore, such force has not caused any substantial problem because the 
interstage casings have been formed by casting. 
As described with reference to FIG. 2, however, the interstage casing 
pressed from sheet metal suffers various drawbacks when the bottom wall 2, 
which corresponds to the flat portion referred to above, is deformed. More 
specifically, the pressure P that has been increased by the impeller 7 is 
applied to the bottom wall 2, thus deforming the bottom wall 2 in the 
direction of the force p radially toward the lower-pressure side. When the 
bottom wall 2 is thus deformed, very large stresses are developed in the 
welded spots 10 between the bottom wall 2 and the return blade 8. 
Consequently, the pressure that can be increased by a single impeller is 
determined by the extent to which the flat portion (bottom wall) is 
deformed. Therefore, the interstage casing cannot be greatly increased in 
size, and should require a considerable wall thickness. 
On the other hand, the spherical bottom wall that has a spherical shape to 
thereby reduce deformation and is employed to make the thickness 
relatively thin, as shown in FIG. 3, is also disadvantageous in that the 
return passage for the liquid is not of a good shape, resulting in a 
reduction in the performance of the pump. 
SUMMARY OF THE INVENTION 
In view of the aforesaid conventional problems, it is an object of the 
present invention to provide an interstage casing made of sheet steel 
which is prevented from suffering problems due to the deformation of a 
bottom wall corresponding to a flat portion between adjacent interstage 
casings, caused by forces induced by the pressure difference generated 
between adjacent interstage casings, and which is particularly prevented 
from welded spots between the bottom wall and a return blade ripped off. 
To achieve the above object, there is provided in accordance with the 
present invention a sheet metal interstage casing for a pump, comprising a 
cylindrical side wall for housing an impeller; and a bottom wall (or 
casing end wall) joined to the cylindrical side wall and extending around 
an inlet for the impeller. The bottom wall supports return blade attached 
thereto at the opposite side of the impeller. The bottom wall has a 
central region projecting conically or spherically axially inwardly toward 
the impeller by a distance corresponding to the extent to which the bottom 
wall is deformable under an interstage pressure difference. 
The interstage casing further includes two side plates, the return blade 
being sandwiched and welded between the side plates and, defining a 
passage between the side plates. One of the side plates is welded to the 
bottom wall near an outermost peripheral portion thereof. The central 
region of the bottom wall is spaced from the one of the side plates by a 
gap. 
With the above structure, during operation of the pump, the interstage 
pressure difference developed by the impeller is applied to the bottom 
wall. However, since the central region of the bottom wall projects 
conically or spherically axially inwardly toward the impeller by a 
distance corresponding to the extent to which said bottom wall is 
deformable under an interstage pressure difference, the distance and the 
extent to which the bottom wall is deformable substantially offset each 
other. 
Consequently, the welded spots between the return blade and the side plate 
welded to the bottom wall are not affected by the deformation of the 
bottom wall. The welded spots are thus prevented from being ripped off 
under stresses which would otherwise concentrate on the welded spots. 
The above and other objects, features, and advantages of the present 
invention will become apparent from the following description when taken 
in conjunction with the accompanying drawings which illustrate a preferred 
embodiment of the present invention by way of example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A sheet metal interstage casing for a pump, according to an embodiment of 
the present invention will be described with reference to FIG. 1. 
FIG. 1 shows in fragmentary cross section one half of an interstage casing 
according to an embodiment of the present invention. The interstage casing 
is formed of sheet metal such as a stainless steel plate and used 
particularly in a multistage centrifugal pump. 
As shown in FIG. 1, an interstage casing is in the form of a deformed 
cylindrical receptacle-like body 20 comprising a cylindrical side wall 21 
having a thickness t. The cylindrical side wall 21 has, around its 
opening, an end surface 22 and an inner surface 23 serving as the female 
member of a spigot joint. Between the cylindrical side wall 21 and a 
bottom wall (or casing end wall) 24, there are provided a cylindrical 
portion 25 joined to the bottom wall 24 and having an outside diameter 
slightly smaller than the inside diameter of the cylindrical side wall 21, 
a relief portion 26 smaller in diameter than the cylindrical portion 25, 
and a flat portion 27 joined to the relief portion 26 and serving as an 
end surface near the bottom wall 24. The cylindrical portion 25 serves as 
a male member of a spigot joint. The flat portion 27 and the cylindrical 
side wall 21 are integrally joined to each other through a projecting 
portion 28 projecting radially outwardly. The projecting portion 28 has an 
outside diameter larger than the outside diameter of the cylindrical side 
wall 21, providing a region to engage the end surface 22 of an adjacent 
interstage casing. The difference, 2h, between the outside diameters of 
the projecting 28 and the cylindrical side wall 21 is approximately twice 
the thickness t. The bottom wall (or casing end wall) 24 has a central 
flange 29 to which a liner ring 50 is attached. The liner ring 30 is 
spaced apart by a small gap from an inlet end of an impeller (not shown). 
The bottom wall 24 of the interstage casing body 20 comprises a conical 
body 31 extending toward a hole 24A in which the liner ring 30 is 
inserted. Specifically, the central region of the bottom wall 24 projects 
toward a casing interior 21A in which an impeller is housed. The 
projecting distance .delta. is a predetermined dimension so that the side 
plate 33 adjacent to the bottom wall 24 is not deformed even if the bottom 
wall 24 is deformed by the pressure generated by the impeller. That is, 
the projecting distance .delta. is predetermined to ensure that the bottom 
wall 24 is caused to contact the side plate 33 adjacent to the bottom wall 
24 either slightly or not at all. More specifically, the projecting 
distance .delta. is about 0.8 or more times the extent to which the bottom 
wall 24 can be deformed under the pressure developed by the impeller, and 
is sufficient to prevent the bottom wall 24 from coming excessively close 
to the impeller. 
The pressure of the liquid that has been generated by a preceding impeller 
(shown on the left-hand side in FIG. 1) is introduced into a passage 32A 
defined in a return blade 32 of a guide vane sandwiched and welded between 
side plates 33, 34. The liquid is then guided from the passage 32A to a 
subsequent (or the next) impeller (not shown). The side plate 33 is welded 
or otherwise fixed to the bottom wall 24 near an outermost peripheral 
portion 35. The side plate 33 is welded to the return blade 32 at plural 
spots 33a through 33d. 
During operation of the pump, the liquid flows through the return passage 
32A in the preceding interstage casing (on the left-hand side in FIG. 1), 
and is introduced into the inlet of the non-illustrated impeller housed in 
the interstage casing 20. The liquid is then pressurized by the impeller, 
flows through the return blade passage adjacent to the impeller, and is 
introduced into the next impeller. At this time, the interstage pressure 
difference developed by the liquid pressure produced by the impeller acts 
o the inner surface of the bottom wall 24, tending to push the bottom wall 
24 toward the return blade that is attached to the reverse side 
(lower-pressure side) of the bottom wall 24. However, the bottom wall 24 
conically projects inwardly toward the impeller by the distance .delta.. 
Consequently, the distance .delta. by which the bottom wall 24 projects 
and the extent to which it is deformed under the interstage pressure 
difference substantially offset each other. 
The welded spots 33a through 33d between the return blade 32 and the side 
plate 33 that is joined to the bottom wall 24 at a position remote from 
the impeller and close to the outermost peripheral portion 35, are not 
subjected to the influence of the deformation of the bottom wall 24. These 
welded spots are thus prevented from being ripped off by stresses that 
would otherwise be developed. 
In the above embodiment, the interstage casing body 20 has the relief 
portion 26 lying between the bottom wall 24 and the cylindrical side wall 
21, joined to the bottom wall 24, and having an outside diameter slightly 
smaller than the inside diameter of the cylindrical side wall 21. However, 
the present invention is also applicable to an interstage casing that 
dispenses with the relief portion 26. 
The same advantages as those described above can be attained if the central 
region of the bottom wall 24 projects spherically. 
Alternatively, the region of the bottom wall 24 radially inward of the 
outermost peripheral portion 35 may be of a concave shape such that it is 
spaced from the side plate by a gap. 
As described above, according to the present invention, the central region 
of the bottom wall projects conically or spherically by a distance that 
corresponds to the extent to which the bottom wall is deformed under the 
interstage pressure difference. Therefore, the deformation of the bottom 
wall that is developed under the pressure generated by the impeller is 
borne by only the casing body itself, without affecting the welded spots. 
Accordingly, the welded spots are prevented from being ripped off. The 
return blade, or the return blade section of a guide vane, is sandwiched 
and welded between the side plates, defining the passage for the liquid to 
be pumped, and one of the side plates is welded to the bottom wall of the 
interstage casing body near the outermost peripheral portion. The welded 
spots between the return blade and the side plate are prevented from being 
ripped off under stresses which would otherwise concentrate on the welded 
spots. 
Since the passage defined in the return blade is not subject to forces 
under the liquid pressure, the return blade and the side plates may be of 
suitable thickness, thus reducing the weight and cost of the interstage 
casing. 
Although a certain preferred embodiment of the present invention has been 
shown and described in detail, it should be understood that various 
changes and modifications may be made therein without departing from the 
scope of the appended claims.