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

320 Centrifugal Pumps: Design and Application
reinforcing with either a metallic back up or extra strength reinforcing
cloth. When designing the stuffing box area, the heat transfer of the injection
fluid around the seal should be considered. The larger this area,
the better the life of the mechanical seal.
The shaft sleeve can be a separate piece that is usually made by injection
molding or it can be made integral with the impeller. There are advantages
and disadvantages to both. When integral with the impeller, the
entire impeller sleeve mechanism needs to be replaced if something goes
wrong with the sleeve. When a separate shaft sleeve is used, there is an
additional sealing surface between the impeller and the sleeve to prevent
fluid from coming in contact with the shaft. When using nonmetallk
sleeves, mechanical seals with teflon wedges should not be employed because
of the excess fretting. Also, the designer has to be concerned with
the extrusion from holding force of set screws on soft nonmetallic
sleeves. Split clamping rings using a radial type of set screw are sometimes
used to prevent damage to the shaft sleeve.
Mechanical Seals
Because of the corrosive properties of the fluids being used within
nonmetallic pumps, many pumps use outside mechanical seals. As a result,
the only wetted pieces are the stationary seat and the compatible rotating
surface. The remaining springs and secondary seals are external to
the stuffing box. However, care should be taken that if an outside seal
fails it could be catastrophic. It is recommended that a seal guard be employed
when outside mechanical seals are used. This subject is discussed
in detail in Chapter 17.
Impellers
Many of the materials that have to be used for the liquids being pumped
cannot be readily adhered or mechanically attached to themselves.
Therefore, it is difficult to obtain closed impellers and consequently,
most impellers are open vane design. Another basic problem with the
nonmetallic pumps is the attachment of the impeller to the shaft. Depending
on the speed and horsepower, most nonmetallic pumps use more than
one key for attachment due to the stress levels of the material. Many impellers
are attached by using threaded inserts that are molded within the
impeller. The problem here is that care has to be taken that excess stress
doesn't occur around the surface of the molded insert that would result in
a weak surface between the two materials. Another method of attachment
is a multi-keyed or polygon shape that does not require an internal insert