CHAPTER 6
VELOCIS SOLUTIONS @2018
2
Figure 7.1 indicates the chart used to define type of pump for particular head and flow.
Figure 7.1: Pump selection guide
There are two calculations made for all types of pumps: net positive suction head(NPSH) and
specific speed.
NPSH
NPSH is an essential factor to be considered for proper pumping performance. The pressure at the
inlet to a pump must be high enough to prevent cavitation occurring in the pump. Cavitation occurs
when bubbles of vapour, or gas, form in the pump casing. Vapour bubbles will form if the pressure
falls below the vapour pressure of the liquid.
The net positive suction head available (NPSH
avail
) is the pressure at the pump suction, above the
vapour pressure of the liquid, expressed as head of liquid.
Pump manufacturers test their pumps for cavitation in a pump test facility by varying the volume
flow rate and inlet pressure in a controlled manner. Specifically, at a given flow rate and liquid
temperature, the pressure at the pump inlet is slowly lowered until cavitation occurs somewhere
inside the pump. The value of NPSH is calculated and recorded at this operating condition. The
process is repeated at several other flow rates, and the pump manufacturer then publishes a
performance parameter called the required net positive suction head (NPSH
requir
), defined as the
minimum NPSH necessary to avoid cavitation in the pump. The measured value of NPSH
requir
varies
with volume flow rate, and therefore NPSH
requir
is often plotted on the same pump performance