Page 92 - Centrifugal Pumps Design and Application

74 Centrifugal Pumps: Design and Application

Casing Mismatch

On horizontally split pump casings mismatching between the upper and
lower casing halves is quite common. This mismatch is normally cor-
rected by hand filing using a template (Figure 6-7). The same template is
used by the machine shop to define bolt locations and also by the foun-
dry. It is quite important that the fluid passages in both halves of the cas-
ing match in both the horizontal and vertical planes since any mismatch-
ing will adversely affect pump performance.

Check for Volute interference

After the pump casing is completely finished to satisfy quality specifi-
cation, the assembled rotating element should be installed in the casing
(as shown in Figure 6-8) to check for interference against volute walls.
Also check the element for total end float, which should be no less than
one-half inch total or one-quarter inch on each end.

* It is important to have the volute symmetrical about shaft centeriine.
* A Vie-in, deep relief should be machined around each stud at the split.
At each stud at assembly the metal will rise. The relief will allow the
gasket to lay flat, reduce gasket area, and increase gasket unit pressure.
* It is very important to install splitters at each impeller suction entrance.
These can be installed recessed at the casing split or cast on casing hub
rings.
« Horizontally split centrifugal pumps are designed for relatively high
pressures; 3,000 psi hydrotest is very much standard by many manu-
facturers. Pressure up to 6,000 psi hydro can be obtained by special
design of bolting and case.

It is essential (to prevent stage-to-stage bypassing) to have all bolts lo-
cated as close as possible to the open area. The high pressure differential
in opposed impeller design multi-stage pumps is between two center
stages and at the high pressure end. Particular attention to bolting should
be paid in these two areas. It is sometimes advantageous to provide ele-
vated bosses to bring bolting closer to open area.