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Monday, December 28, 2009

Pump Impellers

In a single suction impeller, the liquid enters the suction eye on one side only. A double suction impeller is, in effect, two single suction impellers arranged back to back in a single casing. The liquid enters the impeller simultaneously from both sides, while the two casing suction passageways are connected to a common suction passage and a single suction nozzle.

For the general service single stage, axially split casing design, a double suction impeller is favored because it is theoretically in an axial hydraulic balance and because the greater suction head. For small units, the single suction impeller is more practical for manufacturing reasons, as the waterways are not divided into two very narrow passages. It is also sometimes preferred for structural reasons. End suction pumps therefore use single suction impellers. Because an overhung impeller does not require the extension of a shaft into the impeller suction eye, single suction impellers are preferred for pumps handling suspended matter, such as sewage. In multistage pumps, single suction impellers are almost universally used because of the design and first cost complexity that double suction staging introduces.

Impeller are called radial vane or radial flow when the liquid pumped is made to discharge radially to the periphery. Impellers of thus type usually have a specific speed N, below 4200 rpm if single suction and below 6000 rpm if double suction.

Impellers can also be classified by the shape end form of their vanes:
  • The straight-vane impeller
  • The Francis-vane or screw-vane impeller
  • The Mixed flow impeller
  • The Propeller or axial flow impeller
The description of each type those impeller will discussed more detail.

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