The impellers inside your centrifugal pump may look very similar, but their performance differs due to their design. Besides the number of impellers stacked in a pump, the width of the impeller, the number, angle, and thickness of the vanes inside the impeller all play a role in performance.
There are 3 basic types of pump impellers:
Radial Flow indicates that water exits the impeller perpendicular to the inlet and these are used to build pressure in lower volume pumps, say under 100 GPM.
Axial Flow impellers indicate that water exits the impeller somewhat parallel to the inlet and these are used for high volume low pressure.
Mixed Flow impellers exit water at an angle between perpendicular and parallel to create a flow between 120 GPM and 5000 GPM.
The objective then is to design an impeller that can create pressure in a desired range of flow. How does an impeller achieve a certain performance? Let’s look at the basics using the radial flow as an example.
On a radial flow impeller, the inlet, or eye, of the impeller diameter must be large enough to pass the volume of water needed. Yet small enough for water to be acted on by vanes after it enters the impeller. The width of the impeller between the shrouds also provides a given volume that, when rotating at 3450 or 1725 RPM, will empty and fill to create the given volume. The number of vanes inside the impeller, usually 5 to 8, also control the volume by creating more time to fill and empty the cavity.
Pressure is controlled by diameter and speed at which the impeller rotates as well reacting to the resistance in the system to which the pump discharges. The pressure is determined by calculating the velocity in feet per second that a point on the circumference of the impeller turns, then by converting the velocity units to pressure.
Finally, creating flow and pressure within the pump is a way of adding energy to the liquid being pumped. This energy is what makes the liquid flow and it has pressure added to it. To do this, the impeller needs energy from a source. This energy is referred to as horsepower required. So, the more we add flow and pressure energy to the impeller design, we also require greater horsepower to rotate it.
When the impeller is produced, it has a performance curve that goes from the maximum amount of water that can pass through it with no pressure, down to no flow and maximum pressure. The appropriate size impeller is then selected for installation in a pump requiring this specific performance.
The Webtrol WS Series submersible well pumps use stainless steel impellers as shown in the image.