Pump Velocities

When calculating the water velocitiy in a centrifugal pump, it is common to calculate the rotation speed of the outer edge of the impeller and quote this as the maximum flow velocity.  However, this gives far too high a velocity than actually occurs in service.  This is because the water in this region is also moving, although not as fast as the impeller.

A more reliable way to calculate the water velocity seen by the pump materials is to recognize that most impellers have vanes that follow a logarithmic spiral (or close to it).  The properties of this curve are that the flow velocity between the vanes is approximately constant, because as the cross sectional area between the vanes increases, the water is accelerating.  Hence, the maximum water velocity can be calculated from the liquid flow from the impeller and the cross sectional area of the impeller at the circumference.

In many water pumps this velocity is 4 to 5 m/s and explains why nickel aluminium bronze works so well in this application, when it would not at the 10 - 20m/s velocity calculated from the rotation speed of the impeller circumference.

Posted on: 7th June 2017

Image (top left) by Agnieszka