2 Explain why point 3 on Fig 32 curve B is the boundary betw

2. Explain why point 3 on Fig. 3.2, curve B, is the boundary between stable and unstable pump operation.

Solution

The given figure is the pump rating diagram for centrifugal pumps. There are three parts to it:

Curve A: It is called a normal rising or stable pump characteristic. It is because, with the reduction in the discharge, the head increases continuously.

Curve B: It is called a drooping or an unstable pump characteristic curve because below a certain discharge, the head reduces with the decrease in the discharge. At low discharges, the flow will be frequently going to and fro between points 1 and 2 shown in the graph. When the pump operates at a discharge lower than that of point 3 on curve B, the flow becomes unstable and vice versa. Point 3 is the point where this transition between stable and unstable flow occurs and hence it is the boundary.

Curve C: It is known as a steeply rising pump characteristic curve. It is used when the pressure in the pump is changing hugely.