Page 47 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

Remember from Chapter 2, the NPSHa formula is: NPSHa = Ha + Hs - Hvp - Hf - Hi.
If you want to raise the NPSHa, it will be necessary to increase the elements (Ha, Hs)
that add energy to the fluid, or decrease the elements (Hvp, Hf, Hi) that rob energy
from the fluid. Also remember that the NPSHr reading, printed on a pump curve,
currently represents a point where the pump is already suffering a 3% loss in
function due to cavitation. Some people in the industry are calling for a more precise
definition of NPSHr, and higher safety margins on NPSHa.

With the pump disassembled in the shop, the damage from
vaporization cavitation is seen behind the impeller blades toward the
eye of the impeller as illustrated below (Figure 3-1).
To resolve and prevent this type of cavitation damage:
1. Lower the temperature. This reduces the Hvp

2. Raise the liquid level in the suction vessel. This elevates the Hs.
3. Change the pump.
Reduce the speed. This reduces the Hf.
Increase the diameter of the eye of the impeller. This reduces Hf
and Hi.
Use an impeller inducer. This reduces the Hi, and increases Ha.
Use two lower capacity pumps in parallel. This reduces Hf and
Hi.
Use a booster pump to feed the principal pump. This increases
the Ha.
A typical situation often resulting in vaporization cavitation is a boiler

Fiaure 3-1