Impeller blade pitch control
Impeller blade pitch control is used in propeller pumps with axial and mixed flowimpellers to change the pitch of the impeller blades during operation. At constant rotational speed, impeller blade pitch control changes the flow rate, head, and power input of the pump, enabling optimal closed-loop control with low losses. Considerable outlay is required for this configuration, however.
The impeller blade pitch control method most frequently used in centrifugal pumps takes the form of an axially displaceable, bearing-supported adjusting rod in a hollow pump shaft. It can absorb adjusting forces of up to 600 kN in large cooling water pumps, for example, or even greater forces in special-purpose applications. The adjusting rod is moved in the axial direction by a mechanical screw actuator or hydraulic piston.
See Fig. 1 Impeller blade pitch control
On smaller propeller pumps, blade pitch is frequently adjusted manually with transmission gearing (also see Adjustment mechanism). See Fig. 2 Impeller blade pitch control
The impeller blade pitch can also be adjusted by an adjustment mechanism which is fitted in the impeller hub, together with the hydraulic or electric actuator, or by adjustment mechanisms which rotate the blade trunnions via rotation of the adjusting rod. Impeller blade pitch control as applied to centrifugal pumps evolved from early designs used for ship propellers and water turbines.
The outer profile (see Flow profile) and inner profile (hub profile) of the adjustable blades must be located on concentric spherical surfaces to avoid efficiency-reducing gaps (see Clearance gap width). This design requirement determines the shape of the pump casing and the impeller hub in the flow area of the impeller. The spherical shape is not always favourable from a hydraulic perspective, however. This is a disadvantage shared by adjustable blades without impeller blade pitch control, which require dismantling of the rotating assembly to change the pitch.