IMPELLER OF CENTRIFUGAL PUMPS

Centrifugal pumps are often referred to as kinetic energy machines. Rotation of the impeller causes fluid within the impeller to rotate at a high velocity, imparting kinetic energy to the fluid.

 

This concept is described mathematically by the equation:

Hi = ( u2 × cu2 ) ⁄ g

where:

Hi = theoretical head developed by the centrifugal pump, in ft
u2 = rotational velocity of the impeller at the outer diameter, in ft/sec
cu2 = rotational velocity of the fluid as it leaves the impeller, in ft/sec
g = gravitational constant, in ft/sec².

There are three basic impeller designs:

• a closed impeller that has a shroud (rotating wall) on both the front and the back of the impeller.
• a semi-open impeller that has a shroud on one side and is closely fitted to the stationary wall of the casing on the other side.
• an open impeller (see picture 1.) that may or may not have part of a shroud on one side and is closely fitted to the casing wall on the other side (Figure picture 2.).

As fluid approaches the pump suction, it is assumed to have very little to no rotational velocity. Note: Prerotation of fluid in suction piping can and often does exist, but will be disregarded in this discussion. When fluid enters rotating passages of the impeller, it begins to spin at the rotating velocity of the impeller. Fluid is forced outward from the center of the impeller, and its rotating velocity increases in direct proportion to the increasing impeller diameter. The rotating velocity of the impeller can be calculated at any diameter by the equation:

u = (D)(N)⁄229

where

u = rotational velocity, in ft⁄sec
D = diameter at which the velocity is being calculated, in.
N = impeller rotating speed, in rpm
1/229 = constant to convert rpm×in. to ft⁄sec.

The exit velocity of the fluid (cu2) approaches the rotating velocity of the impeller (u2) at D2 but does not equal u2 in normal operation. The main reason cu2 < u2. The exit velocity of the fluid, cu2, can be calculated from the design parameters of the impeller, but that derivation is beyond the intended scope of this discussion.

The discussion thus far has been of theoretical head (Hi), which does not account for losses that occur as fluid moves through the impeller during normal operation. Losses in the impeller that normally occur are friction, eddy currents, fluid recirculation, entrance losses, and exit losses. Additional losses will occur in the casing.

It should be noted that head produced by a centrifugal pump is a function of fluid velocity and is not dependent (normally) on the fluid being pumped. For example, a pump that will produce 100 feet of head on water (8.34 lb/gal) will also produce 100 feet of head on gasoline (6.33 lb/gal). Note: Fluids with viscosity greater than 20 centipoises will decrease output head produced by a centrifugal pump. For viscous fluids, corrections can be made to predict actual pump performance.