Page 334 - Centrifugal Pumps 2E

Chemical Pumps Metallic and Nonmetallic 299

plied with a glass or plastic bubble; glass is specified in a refinery type of
service.

Shaft

The shaft has to be designed to take the radial, axial, cyclic, and tor-
sional loads. The axial load, depending on suction pressure, can be in
either direction. Shaft diameters are rated by horsepower per 100 RPM.
The shaft material can be 1020, heat treated 4140 or 316. In each case the
material should be reviewed for imposed stresses. Refer to Chapter 16
for methods of calculations.

impeller Attachment

On chemical service pumps, impellers are usually attached with a male
or female threaded connection. Threaded connections are used between
the impeller and shaft because they can be more readily sealed than when
a key design is used, which inherently has an additional joint. Specifica-
tions do allow both types of connection.

Bearings

Specifications require that when the maximum hydraulic load from the
largest impeller at a given speed is applied to the bearings, they will have
a minimum L10 life of 17,500 hours. The designer and user should care-
fully select original or replacement bearings because the interchangeable
dimensional envelope of an AFBMA bearing does not ensure that the size
or number of balls are the same from one manufacturer to another,
thereby resulting in a possible change in the rating of the bearing. It
should also be remembered that the life of a particular design will change
with suction pressure.
Most chemical pumps use semi-open impellers, and operate with a
.010 to .020 inch gap between the impeller and casing wall. To maintain
this clearance, the bearings should have .0015 to .002 of an inch axial
input end play. Double-row bearings have an assembly end play of .002
to .003 of an inch. Back-to-back bearings have an assembly end play of
.005. However, the back-to-back bearings are more forgiving to mis-
alignment when radial loads are applied to the bearings. The life of a
back-to-back bearing is about twice that of the comparable double-row
bearing. Back-to-back bearings are usually offered as an optional feature
on chemical pumps. This applies to both horizontal and vertical pumps.