The impeller is the rotating part of a centrifugal pump that transfers energy from the motor driving the pump to the fluid being pumped by accelerating the fluid outward from the center of rotation. The velocity achieved by the impeller is converted into pressure when the outward movement of the fluid is restricted by the pump casing. The impeller is usually a short cylinder with an open inlet (called an eye) to accept incoming fluid, blades to push the fluid radially, and a splined, keyed or threaded hole to accept the drive shaft.
In many cases, an impeller made of cast stainless steel can also be called a rotor. It is cheaper to cast the radial impeller directly on the support it is mounted on, which is driven by an electric motor, internal combustion engine or the gearbox of a steam driven turbine. When the rotor is bolted, it is usually named both the main shaft and the impeller.
Impeller type
Impellers of centrifugal pumps can be divided into three basic types:
Open the impeller. The blades on both sides of the open impeller are free. The open impeller structure is weak. They are commonly used in small diameter, inexpensive pumps and pumps that handle suspended solids.
Semi-open impeller. The blades are free on one side and closed on the other. The shroud adds mechanical strength. They also offer higher efficiency than open impellers. They can be used in medium diameter pumps and liquids with small amounts of suspended solids. It is very important that there is a small clearance between the impeller blades and the casing due to minimization of recirculation and other losses.
Closed Impeller. The vanes are located between two discs, all in one casting. They are used in larger pumps with high efficiency and low net positive suction requirements. Centrifugal pumps with closed impellers are the most widely used pumps for handling clear liquids. They rely on tight clearance wear rings on the impeller and pump casing. Enclosed impellers are a more complex and expensive design, not only because of the impeller, but also because of the need for additional wear rings.
Why use stainless steel castings for impellers?
The basic alloying elements of any nickel-based or stainless steel casting are chromium, nickel, and molybdenum (or "molybdenum"). These three components will determine the grain structure and mechanical properties of the casting and will contribute to the casting's resistance to heat and/or corrosion.
Stainless steel has a minimum chromium content of 10.5%, making it more resistant to corrosive liquid environments and oxidation. However, this is not absolute. Stainless steel investment castings are "corrosion resistant" when used in liquid environments and steam below 1200°F (650°C) and "heat resistant" above this temperature.
There is a difference between wrought stainless steel and cast stainless steel. This is due to their chemical composition and micrograin structure, influenced by how the material is produced, cast or extruded, which must be considered when selecting alloys for your stainless cast metal project.
Advantages of Impeller Stainless Steel Castings
Investment cast stainless steel impellers are manufactured using a silica sol investment casting process that provides significantly improved surface and part quality compared to the more common die casting process or the sand casting process most commonly used for bronze casting. The end result is greater efficiency, consistency between impellers, greater durability and more sustainable hydraulic performance.
When followed by the manufacturer's instructions, neither material is more likely to form cracks or have a shortened life expectancy than the other. However, stainless steel materials are generally stronger and may have a longer life expectancy in most cases.
Stainless steel offers improved surface properties, resulting in improved sustainable hydraulic performance and energy efficiency, improved chemical resistance/application flexibility, reduced potential for corrosion or degradation, and reduced maintenance costs. Stainless steel may not be suitable for extreme applications with very high chemical concentrations, but so far we have not seen extreme applications where pumps can also be applied.
