Most manufacturers employ semi-open impellers in their self priming centrifugal sewage and trash pump designs. While other types of impellers—enclosed, open or recessed—are sometimes used, this type of impeller has a proven field record for clog resistance and high efficiency. Unlike a totally enclosed impeller, the semi-open impeller lacks a front shroud. The impeller face operates in close proximity to an adjustable wear plate. The adjustment feature is necessary in order to obtain desirable pump efficiency and minimize recirculation from the high pressure point at the impeller tip(s) back to the impeller eye. A relatively close running tolerance must be maintained between the impeller face and the wear plate—somewhere on the order of .010 to .020 in.
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