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Pump parts comprise a large part of our inventory at Pump Products and our Product Experts receive a lot of inquiries from customers looking for the right repair and replacement parts from Armstrong, Zoeller, Berkeley, Grundfos, Bell & Gossett and other major manufacturers. In the interest of spreading information on our products as widely as possible, here’s what they had to say in response to a recent inquiry about pump impellers.

Q. Is there a reason I’d pick a silicon bronze impeller over cast iron?
– Kevin, ID

Good question, Kevin. We’ll address it as soon as we explain what an impeller is. An impeller is the rotating component that converts motor energy to fluid movement in a centrifugal pump. Impellers are usually circular in shape and have an open inlet through which incoming fluid passes. They are manufactured in radial, mixed flow, axial and peripheral configurations, and are available in a variety of materials including iron, steel, bronze, brass, aluminum and plastic. An impeller should be selected on the basis of pump design and application.

The size of an impeller depends on the capacity of the pump it will serve. When possible, it is advisable to select a pump that will accommodate an impeller that can be increased in size. This will allow for a future increase in head and capacity.

Now for your question, Kevin.

An essential difference between cast iron and bronze is that cast iron rusts while bronze does not. Cast iron works best in neutral, non-potable water applications, while bronze is a better choice in applications where potable water or sea water are involved.

Silicon bronze has excellent chemical resistance and is widely used for pumps, boilers and marine hardware. It’s low lead content makes it suitable for drinking water applications, but it’s limited machinability inhibits use in high-volume systems. It is also prone to wear in rough or abrasive water conditions.

Additionally, silicon bronze is subject to a corrosion process called “dezincification”  whereby the zinc content of the metal is leached out when exposed to water containing high levels of dissolved oxygen, carbon dioxide or chlorine. The result is a porous mass of copper with very little mechanical strength.

So if you’re looking for a replacement impeller for a low-to-moderate volume, potable water system, go with the silicon bronze. For high volume, rugged water conditions, go with a cast iron or aluminum bronze impeller.