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H
Q
Referencekurve
3% kurve
Målt løftehøjde
107107
6.3.2 NPSH
3%
test by increasing the flow
For NPSH
3%
test where the NPSH
3%
curve is steep, this procedure is prefer-
able. This type of NPSH
3%
test is also well suited for cases where it is dicult
to change the inlet pressure e.g. an open test stand.
The NPSH
3%
test is made by keeping a constant inlet pressure, constant wa-
ter level or constant setting of the regulation valve before the pump. Then
the flow can be increased from shuto until the head can be measured be-
low the 3% curve, see figure 6.10. The NPSH
3%
curve is made by repeating the
measurements for dierent inlet pressures.
Procedure for NPSH
3%
test where the flow is gradually increased
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 inlet pressures
4. The test stand is set for the wanted inlet pressure
5. The flow is increased from the shuto and flow, head and inlet
pressure are measured
6. The measurements continue until the head is below the 3% curve
7. Point 4-6 is repeated for each flow point
6.3.3 Test beds
When a closed test bed is used for testing pumps in practise, then the in-
let pressure can be regulated by adjusting the system pressure. The system
pressure is lowered by pumping water out of the circuit. The system pres-
sure can, furthermore, be reduced with a throttle valve or a vacuum pump,
see figure 6.11.
Figure 6.10: NSPH
A
measurement by
increasing flow.
Figure 6.11: Draft of closed
test bed for NPSH measurement.
Reference curve
3% curve
Measured head
Pressure
control
pump
Bae plate
Heating/
cooling coil
Vacuum pump
Shower
Flow valve
Flow meter
Test pump
Throttle valve