I recently received an e-mail asking to explain what the terms “open” or “closed” referred to when discussing impellers in a slurry pump. So here it is.
Although there are numerous styles/designs offered by pump manufacturers, most slurry pump offerings boil down to a variation of one of two basic designs, with a third design, slightly less common, that is becoming more widely used every year.
The first, and arguably the most, rugged design is the closed impeller. In this design, the liquid follows a path down a tunnel formed on the sides by the vanes and is “closed” in on the top and bottom (or front and back depending on shaft orientation) by two shrouds. The two images below illustrate this design.
Semi Open Impeller
The semi open impeller is very similar in its design to the closed impeller. The one major difference is the absence of the front shroud. The vanes of a semi open impeller are supported by a back shroud only and the impeller is completely void of a front shroud. In the absence of a front impeller shroud, the front wear plate completes the tunnel used to accelerate the slurry within the pump.
The third and slightly less common slurry impeller is the recessed impeller. Often called “torque flow” impellers, they create a centrifugal force in a unique manner. Instead of accelerating liquid down the vanes, these impellers use the vanes to create a hydraulic coupling. The coupling then spins the slurry within the casing, thereby creating the centrifugal force required to create discharge pressure. The drawing below illustrates this principle.
With the vanes basically out of the normal flow path, erosion is minimized and the vanes do not need to be as thick as they have to with other impeller styles. In fact, finer blades increase efficiency and are therefore often deemed more desirable.
The picture below is a good representation of the impeller, and the section view beside it illustrates the impeller location within the pump.
I hope these short descriptions help clarify the terms “closed”, “open” and “recessed”. Next month we will look into the advantages and common application of each of these designs.