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There are multiple decisions to be made about impeller
selection and not all pump suppliers are qualified to make them:
- The impeller shape or specific
speed number will dictate the shape of the pump curve, the
NPSH required and influence the efficiency of the pump.
- Has the impeller configuration been iterated
in recent years? Impeller design is improving with some of the
newer computer programs that have become available to the design
- The suction specific speed
number of the impeller will often predict if you're going to
experience a cavitation problem.
- The impeller material must be chosen for both chemical
compatibility and wear resistance. You should consider one of
the duplex metals because most
corrosion resistant materials are too soft for the demands of a
- The decision to use a closed impeller, open impeller,
semi-open, or vortex design
is another decision to be made.
- Closed impellers require wear
rings and these wear rings present another maintenance
- Open and semi-open impellers are less likely to clog, but need
manual adjustment to the volute or back-plate to get the proper
impeller setting and prevent internal recirculation.
- Vortex pump impellers are great for solids and "stringy"
materials but they're up to 50% less efficient than conventional
- Investment cast impellers are
usually superior to sand cast versions because you can cast
compound curves with the investment casting process. The compound
curve allows the impeller to pump abrasive fluids with less vane
- If you're going to pump low
specific gravity fluids with an open impeller, a non-sparking
type metal may be needed to prevent a fire or explosion. You'll
be better off choosing a closed impeller design, with soft wear
rings in these applications.
- The affinity laws will predict the affect of changing the
impeller speed or diameter. You'll want to be familiar with
these laws for both centrifugal
and PD pumps.
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