Centrifugal Pump Basics - Impeller

The impeller is the key component that transfers the power generated by the driver (usually an electric motor) into the movement of the fluid.

While all impellers have the common task of moving fluid, they come in different shapes, sizes, and materials to meet the requirements of specific applications. 

It may seem difficult to distinguish among the many options, but understanding the basic principles of impellers will go a long way in selecting the right pump.

Impeller Geometry

It is important to understand the cross-section of a typical centrifugal impeller. The center of the impeller is called the impeller eye. Extending outward from the center are the impeller's blades. The impeller's blades are curved (usually back-curved) and catch and move the fluid as the impeller rotates. The large circular section behind the blades is the impeller's hub. The hub provides support for the blades while connecting all of the blades to the impeller assembly. The outside diameter of the hub is where the impeller shrouds can be found. Semi-open impellers have a shroud that covers the top of the impeller blades. A closed design is an impeller that has shrouds that cover the top and bottom of the impeller blades. Alternatively, the impeller may have no shrouds, and this type is called an open impeller.

The design of the impeller may vary, but some basic principles remain constant in all impellers. The outside diameter of the blades determines the pressure that the pump can produce. A smaller outside diameter will produce a lower pressure than a similar impeller with a larger diameter. The height of the impeller blades determines the flow rate that the pump can produce. A lower impeller blade will produce less flow than an impeller with taller blades. These impeller geometry features provide the background for impeller style design.

Physical representation of an impeller

The impeller is the link between the power input (driver) and the power output (fluid movement) of a pump. The pumping process begins when the driver rotates the impeller through the shaft. As the impeller turns, water is pushed outward from the center along the edges of the blades, increasing pressure. The high-pressure water is released from the ends of the blades into the pump's volute (or guide vanes). Following its path through the volute/guide vanes, the water is discharged at a rate and pressure determined by the impeller geometry. The water accelerates along the impeller blades, creating a low pressure at the impeller suction hole. The atmospheric pressure, which is greater than the low pressure at the impeller suction hole, exerts a force on the surface of the pumped fluid, causing the fluid to continue to flow toward the impeller suction hole.

Solids handling

Many applications require the pumping of fluids containing solids. This is common for submersible pumps, which are located low in a reservoir used to collect runoff and waste. One type of impeller known for its solids handling capabilities is the channel impeller. Channel impellers allow solids to pass efficiently between the impeller blades. This impeller design is often used in sewage and wastewater applications.

Another common solids handling impeller is the vortex type, also known as a recessed impeller. This impeller is located within a volute, creating a large open space between the impeller and the suction inlet. Unlike other impellers that rely on the blades to propel the water, this impeller creates a vortex in the open space of the volute. The vortex created by the rapid rotation of the impeller moves the fluid and solids with minimal contact with the impeller. There is no contact between the pumped fluid and the impeller, which is very beneficial for applications with abrasive or large solids.

Cutter and chopper pumps

For applications that are prone to clogging, there are impellers designed to handle these troublesome solids. A cutter pump is one such design. The impeller of a cutter pump has a sharp leading edge, usually made of a hard material such as tungsten carbide. This sharp edge is designed to have a close clearance with the matching suction plate with a sharp serrated edge. As the impeller picks up solids, a portion of the solids are caught by the stationary serrated edge on the suction plate, allowing the rotating cutter impeller to cut the solids. Chopper pumps reduce clogging caused by a variety of solids and debris, such as rags, "flushable" wipes, and even clothing, which reduces pump downtime. These cut solids can be filtered downstream of the pump.

Another impeller design for clogging-prone applications is the chopper pump. The mechanical structure of a chopper pump is similar to that of a chopper pump. Solid material is torn between the sharp leading edge of the impeller and the sharp stationary edge of the suction port.

Unlike chopper pumps, which use a serrated edge on the suction plate, chopper pumps have a cutter mechanism mounted outside the impeller suction hole. Chopper pump blades maintain close tolerances to the impeller suction hole and the blades to the suction plate. These tight tolerances provide cutting of the entire blade, preventing clogging of the entire volute.

High-Pressure Applications

Some applications require higher pressures, such as pumping to high altitudes or when the system design requires small diameter pipes. To achieve this high pressure, the impeller design can be altered in several ways. One way to increase pressure is to increase the outer diameter of the impeller.

Another way to increase pressure is to add cover plates above and below the impeller blades. The area between the blades of a closed impeller reduces flow recirculation (internal backflow), resulting in a more efficient high pressure output. While closed impellers are effective at increasing pressure, they limit the impeller's ability to handle solids.

High-pressure pumps are often equipped with filters to prevent plugging of closed impellers. However, if a large amount of solids is present, a grinder pump may be more suitable for this application. A grinder pump has a grinding blade on the outside of the suction port. As this sharp blade rotates, it breaks the solids into fine fragments that flow through the suction screen into the pump. The ground solids are then usually transported out of the pump by a high-lift impeller. Grinding pumps are often used in municipal wastewater collection in areas with uneven terrain. These areas rely on small-diameter, high-pressure pipes to transport wastewater. Grinding pumps not only provide the required high pressure, but also reduce the amount of solid matter, thereby reducing the clogging of smaller pipes.

Impeller Material

Choosing the right material of construction is just as important as choosing the impeller style. Impellers are available in a variety of materials, such as different grades of cast iron, stainless steel, bronze, and resin plastics. Cast iron offers good wear resistance and is economical. However, cast iron may not withstand highly abrasive or corrosive fluids. To prevent premature impeller failure due to corrosion, a stainless steel impeller may be required. While stainless steel is more expensive than cast iron, it generally offers better resistance to corrosive chemicals.

Bronze is another corrosion-resistant material that is often used in coastal areas. In offshore applications or processes that transfer salt water, bronze provides protection against salt water attack. Some smaller pumps may use plastic impellers. These molded impellers offer excellent chemical resistance, high wear resistance, and are cost-effective compared to stainless steel or bronze alternatives.

The fluid being pumped, as well as the system conditions, will determine the type of impeller required. Be sure to share this information with your pump supplier to ensure the proper selection of impeller and pump.