Pump Pre-rotation and Stop Pieces
Improper entrance conditions and inadequate suction approach shapes may cause the liquid column in the suction pipe to spiral for some distance ahead of the impeller entrance. This phenomenon is called pre-rotation, and it is attributed to various operational and design factors in both vertical and horizontal pumps.
Pre-rotation is usually harmful to pump operation because the liquid enters between the impeller vanes at an angle other than that allowed in the design. This frequently lowers the net effective suction head and the pump efficiency.Various means are used to avoid pre-rotation both in the construction of the pump and in the design of the suction approaches.
Practically all horizontal, single-stage, double-suction pumps and most multistage pumps have a suction volute that guides the liquid in a streamline flow to the impeller eye. The flow comes to the eye at right angles to the shaft and separates unequally on the two sides of the shaft. Moving from the suction nozzle to the impeller eye, the suction waterways are reduced in area, meeting in a projecting section of the sidewall dividing the two sections. This dividing projection is called a stop piece. To minimize pre-rotation in endsuction pumps, a radial-fin stop piece projecting toward the center is sometimes cast into the suction nozzle wall.