Brochure
105105
6.3 Measurement of the pump’s NPSH
The NPSH test measures the lowest absolute pressure at the inlet before
cavitation occurs for a given flow and a specific fluid with vapour pressure
p
vapour
, see section 2.10 and formula (2.16).
A typical sign of incipient cavitation is a higher noise level than usual. If the
cavitation increases, it aects the pump head and flow which both typically
decrease. Increased cavitation can also be seen as a drop in flow at constant
head. Erosion damage can occur on the hydraulic part at cavitation.
The following pages introduce the NPSH
3%
test which gives information
about cavitation’s influence on the pump’s hydraulic performance. The test
gives no information about the pump’s noise and erosion damage caused
by cavitation.
In practise it is thus not an actual ascertainment of cavitation but a chosen
(3%) reduction of the pump’s head which is used for determination of NPSH
R
- hence the name NPSH
3%
.
To perform a NPSH
3%
test a reference QH curve where the inlet pressure is
sucient enough to avoid cavitation has to be measured first. The 3% curve
is drawn on the basis of the reference curve where the head is 3% lower.
Grundfos uses two procedures to perform an NPSH
3%
test. One is to gradual-
ly lower the inlet pressure and keep the flow constant. The other is to gradu-
ally increase the flow while the inlet pressure is kept constant.