Page 145 - Know and Understand Centrifugal Pumps

Shaft

Deflect ion

Introduction

Along with the sounds, evidence and signs of cavitation, there is a
broad range of other information and signals available to the
maintenance mechanic. Almost all mechanics have seen the gouge and
scratch marks, and signs of heat on the pump when disassembled in the
shop. Sadly, most mechanics are never trained to interpret these marks.
This brings us to failure analysis of the pump, or performing an autopsy
on a broken pump. You must stop throwing away used and worn pump
parts, or sending them to the machine shop. This action destroys the
evidence needed to repair and resolve pump problems. There are too
many mechanics wasting their careers changing parts and not really
repairing anything.
Let's begin with a discussion and explanation on how a volute
centrifugal pump works.

60" and 240"
The volute type pump has its impeller mounted eccentrically within the
volute. The degree of eccentricity governs the pressure that the pump
can generate. If the impeller were concentric inside the volute, or
equidistant, the pump would generate flow, but not much pressure or
head (Figure 9-1).
The impeller throws the liquid against the volute wall at a constant
speed, the speed of the electric motor. The internal diameter of the
volute wall converts the velocity into head or pressure (Figure 9-2).

See Table opposite for what is happening inside the pump around the
internal volute wall.