DE1728005

1,195,993. Centrifugal pumps. ROLLSROYCE Ltd. Aug.8, 1968 [Aug. 11, 1967], No.36981/67. Heading F1C. A centrifugal pump comprises an impeller and a rotor body mounted coaxially and upstream of the impeller and rotatable with the impeller, the pump being intended to be operated with a low flow inlet angle and at a low suction pressure, the rotor body having a number of circumferentially - spaced, rod-like, bluff projections extending therefrom transversely across the inlet passage of the pump. A centrifugal pump for liquid or a mixture of liquid and gas and operating at a low flow inlet angle, e.g. less than 3 degrees or 4 degrees, and at a low suction pressure e.g. about 1 or 2 pounds per square inch above the vapour pressure of the liquid, comprises pins 14 for preventing the formation under these conditions of vapour-filled regions in the eye 15 of the impeller 1 and consequent instability and decrease in flow. The pins 14 each produce a trailing cavity containing vapour or gas, the cavities being of helical form and merging together downstream of the plane of the pins to form an annular cavity extending downstream into at least the impeller eye 15. The annular cavity enables the pump to be operated stably and forms a region of fluid of lower viscosity than the liquid being pumped tending to prevent transmission of pre-whirl to the liquid upstream of the region. The reduction of pre-whirl assists in preventing instability. The length of the pins 14 is at least 30% of the distance across the flow passage and is preferably between one and six times the diameter of the pins. Instead of extending radially as shown, the pins may be inclined in either or both the axial and circumferential directions. The length and inclination of the pins may be adjustable. The cap 9 carrying the pins may have an annular lip of resilently deformable material which is snapped over an undercut annular rib on the impeller or impeller shaft. The flow through the pump may be varied by means of an iris diaphragm provided in the inlet passage 5. Otherwise, liquid may be returned from the volute 6 through a control valve 22 to a manifold 25 having an annular opening 26 extending around the inlet passage 5. Liquid discharged from the opening 26 forms a boundary layer having a low or reverse velocity and reducing the effective flow area in the inlet passage 5. Instead of an annular opening 26, nozzles facing in an upstream direction may be provided.