Brochure

H
Q
Referencekurve
3% kurve
Målt løftehøjde
106106
6.3.1 NPSH
3%
test by lowering the inlet pressure
When the NPSH
3%
curve is flat, this type of NPSH
3%
test is the best suited.
The NPSH
3%
test is made by keeping the flow fixed while the inlet pressure
p
stat,in
and thereby NPSH
A
is gradually lowered until the head is reduced with
more than 3%. The resulting NPSH
A
value for the last measuring point before
the head drops below the 3% curve then states a value for NPSH
3%
at the
given flow.
The NPSH
3%
curve is made by repeating the measurement for a number of
dierent flows. Figure 6.9 shows the measuring data for an NPSH
3%
test
where the inlet pressure is gradually lowered and the flow is kept fixed. It is
these NPSH values which are stated as the pump’s NPSH curve.
Procedure for an NPSH
3%
test where the inlet pressure is gradually lowered:
1. A QH test is made and used as reference curve
2. The 3% curve is calculated so that the head is 3% lower than the
reference curve.
3. Selection of 5-10 flow points
4. The test stand is set for the seleted flow point starting with the
largest flow
5. The valve which regulates the counter-pressure is kept fixed
6. The inlet pressure is gradually lowered and flow, head
and inlet pressure are measured
7. The measurements continue until the head drops below the 3% curve
8. Point 4 to 7 is repeated for each flow point
Figure 6.9: NPSH
A
measurement by lowering
the inlet pressure.
Reference curve
3% curve
Measured head
6. Pump tests