Page 353 - Centrifugal Pumps 2E

318 Centrifugal Pumps: Design and Application

the same support head and bearing housing construction as on the metal-
lic pumps. This allows the user to be able to interchange the bearing
housing parts from metallic pumps to nonmetallic pumps. To obtain addi-
tional strength, some manufacturers employ back-up rings that are either
separate pieces bolted to the support head or they have support heads that
include a back-up ring. The nonmetallic pumps were initially designed
with integral nozzles, but there were many molding problems. Some
manufacturers resorted to molding separate nozzles and then either
molded them to the casing or adhered them to the casing. This was found
to be a problem when applying external nozzle loads. With the advance-
ment of materials and dies, many manufacturers now mold the nozzle
integral with the casing without incurring nozzle loading problems. Be-
cause the materials have moduli that are between Vis and Vso that of stan-
dard metallic materials, it is advisable not to put excess nozzle loadings
on. composite casings.

Nozzle Loading

There are no standards for the nozzle loads on ANSI pumps, and the
manufacturer's specifications are usually referred to for the maximum
load. The criterion used by the manufacturer for maximum nozzle loads
is usually the movement on the coupling end of the shaft. This may be
.0050 to .0100 depending on the size of the pump. This deflection can be
caused by:

» The movement of the entire assembly when load is put on.
* Movement of the feet of the casing relative to the bedplate due to the
friction force between the two.
* The movement of the bearing housing relative to the bedplate.
* Internal movements causing rubbing of the impeller against the casing.
» Deflection of the bedplate surface relative to the driver shaft.

With nonmetallic pumps the allowable nozzle loads are much less than
with the metallic pumps because the casing feet move or deflect under a
much lighter load. When nonmetallic beds are employed with either a
metallic or nonmetallic pump, the movement of the top surface of the bed
is the weak member of the assembly resulting in low allowable nozzle
loads. This will occur if the bed is grouted or ungrouted, especially if the
force is along the X axis or a moment around the Z axis.