Brochure

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In an open test bed, see figure 6.12, it is possible to adjust the inlet pressure
in two ways: Either the water level in the well can be changed, or a valve can
be inserted before the pump. The flow can be controlled by changing the
pump’s counter-pressure by means of a valve mounted after the pump.
6.3.4 Water quality
If there is dissolved air in the water, this aects the pump performance which
can be mistaken for cavitation. Therefore you must make sure that the air con-
tent in the water is below an acceptable level before the NPSH test is made. In
practise this can be done by extracting air out of the water for several hours.
The process is called degasification.
In a closed test bed the water can be degased by lowering the pressure in
the tank and shower the water hard down towards a plate, see figure 6.11,
forcing the air bubbles out of the fluid. When a certain air volume is gath-
ered in the tank, a part of the air is removed with a vacuum pump and the
procedure is repeated at an even lower system pressure.
6.3.5 Vapour pressure and density
The vapour pressure and the density for water depend on the temperature
and can be found by table look-up in ”Physical properties of water” in the
back of the book. The fluid temperature is therefore measured during the
execution of an NPSH test.
6.3.6 Reference plane
NPSH is an absolute size which is defined relative to a reference plane. In
this case reference is made to the center of the circle on the impeller shroud
which goes through the front edge of the blades, see figure 6.13.
Figure 6.12: Drafts of open test beds for
NPSH measurement.
Figure 6.13: Reference planes at
NPSH measurement.
Adjustable water level
Pump
for flow valve
and flow meter
Throttle valve
6. Pump tests
Reference plan