Page 347 - Centrifugal Pumps 2E

312 Centrifugal Pumps: Design and Application

parts produced from carbon, Teflon, CPVS, Kynar, Ryton, etc. These
were protected by outside metal plates to hold the required pressure. The
various resins were produced with fillers that improved moldability but
did not substantially improve strength.

Reinforced Composite Material Pumps

The next development in nonmetallic pumps led to improved manufac-
turing techniques using thermo resin without armor. Successful resins in-
clude glass-reinforced thermoset composites. These have strengths
equivalent to the metallic chemical pumps and are suitable for applica-
tions of acids, alkalies, oxidizing agents, solvents, and salts with temper-
ature ranges up to 250°F as normal with peak temperatures up to 400°F
(Figure 15-19). These pumps were originally called FRP (fiber rein-
forced polymer) pumps but the term composites has basically replaced
that label.
Proper selection of composite materials offers many combinations to
improve corrosion resistance, lightweight, flame retardation, low-cost
magnetic transparency, and complexity of art design. The terms rein-
farced plastics or composites generally include two large groups of or-
ganic compounds that differ in their make-up. These are thermosetting
polymers and thermoplastics.
How does a designer choose between thermoplastics and thermosets?
With the present state of the art, the chemical compatibility, maximum
applicable temperatures, and consistent quality are about the same for
both processes. The differences are listed in Table 15-2.

Thermosetting Polymers

Thermosetting polymers for pump use are reinforced with fiberglass or
carbon fibers. During the molding cycle, these materials undergo a
chemical change that is irreversible. The resulting material will not
soften or become pliable with heat. They have four basic chemistries;
polyesters, phenolics, vinyl esters, and epoxies. Each has its own set of
advantages, manufacturing processes, and mechanical and chemical
properties. The fibers are either continuous or short fibers and are the
key in developing the temperature range and corrosion resistance of the
final part. There are many manufacturing processes for thermosets and
they are often every bit as critical to the final part performance as the
selection of the proper polymer and reinforcement combination. Com-
pression molding, transfer molding, resin transfer molding, cold mold-
ing, and extrusions are among the most commonly used processes.