Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

Chemical Pumps Metallic and Nonmetallic 321
because the stresses of the material are distributed throughout its circumference
relative to the shaft surface. The disadvantage of the polygon attachment
is that there are more surfaces that have to be sealed to prevent
external fluid from attacking the shaft.
The sealing of the impellers with either the insert or a polygon fit is
similar to that used in the metallic pumps. Most manufacturers will employ
the same sealing mechanisms for the two types.
Nonmetallic Immersion Sump Pumps
Typical applications include wet pit chemical waste handling, effluent
handling, and liquid transfer operations where broad corrosion resistance
is required. These pumps are made of the same basic materials as horizontal
nonmetallic pumps, either vinyl ester or epoxy. The hydraulics
cover the same basic range as the horizontal pumps and in many cases,
the casing impeller, and casing cover are the same parts as used in the
horizontal pumps.
The shaft material is 316, alloy 20, Hastelloy B or C, or titanium, depending
on the liquid being pumped. Optional shafts of 316 coated with
various materials such as kynar are also available. The use of pultruded
nonmetallic shafts is being investigated to eliminate all metallic parts for
this type of application.
The column supporting the wet end to the mounting plate is a one-piece
construction with inserted bearings or a multi-construction of short columns
with flanges and the bearing support sandwiched between the
flanges of the column. The column material is usually the same base material
as the pump and impeller. The bolting of the casing and the columns
can be of a nonmetallic material compatible with the fluid.
Bearings are made out of teflon or carbon with spiral flutes. The lubrication
is either external or clean product lube. Clean liquid for lubrication
is one that has less than 5 micron particle size. The lubrication to
each bearing should be at least one half GPM at 160°F temperature or
less and at a pressure of approximately 25 psig. Carbon bearings are furnished
when external lubrication or injection pressure is not adequate,
Figure 15-20 shows when to supply carbon or teflon bearings based on
particle size in the fluid and the flush pressure available to these bearings.
It also shows when cyclone separators are required and what flow
for a given flush pressure is obtainable from the separators. The lower
bearings are usually twice as long as the line bearings to absorb the radial
thrust developed by the impeller. A gap or relief hole is placed between
the throat bushing of the casing cover and the bearing itself so that dirty
liquid under pressure will be relieved of pressure and not be forced into
the bearing clearances resulting in short life.