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6.22
Irrigation User’s Manual
Because there is no flow, the suction side losses will be nil.
When a vacuum meter is used upstream of the pump, it can be added to the pressure gauge reading
together with height difference between the vacuum meter, to obtain a value for H0. If the vacuum
meter registers a reading of nil with a positive suction height, for example, then the pump is lower than
the water level of the water source, this pressure can also be determined by means of a pressure gauge.
• Open the sluice valve slightly to approximately half of the pumps normal delivery and measure the
pump delivery (Q1), as well as the pressure gauge reading, plus the height difference (Hs), plus the
friction losses and secondary losses upstream of the measuring point for Q1 (Hf1), and add this
together to obtain the value of the total pressure (H1). Therefore, H1 = Hs + pressure gauge reading
+ Hf1. A vacuum meter can also be used here as described in the previous step.
If the friction and secondary losses on the suction side cannot be determined, ignore it. It does
have an influence and if possible, must be determined, but it will not make the test meaningless if
it is not determined. The values for H1 and H2 will be marginally less than the actual values
thereof.
• Now open the sluice valve completely, measure the pump delivery (Q2) and determine a value for
H2, as in the step above. Therefore, H2 = Hs + pressure gauge reading + Hf2. A vacuum meter can
also be used here as described in the previous step. Note that the friction and secondary losses will
now be greater as a result of a greater pump delivery. This point should be the point where the
system curve and pump curve cut each other under normal circumstances and is actually the service
point of the pump.
• Now plot the values for [H0; Q=0}, [H1; Q1} and [H2; Q2] on the original pump curve on which the
system curve is indicated and compare it with the following examples to obtain an indication of the
problem, if any. The system curve is obtained by plotting the system pressure (H in m) against the
system capacity (Q in m3/h) on the pump curve graph.
These three points determined on-site, are the minimum number of points required. It can however be
too few to find the problem and then alternative pump delivery points must also be determined. The
more points, the better. That is to say, the two points, namely valve completely closed and valve
completely open is compulsory, and then at least one in between.
The speed, at which the pump is driven, must also be measured, especially if the motor is not an
electric motor running at a standard speed. Four representative cases of what can be found with the
tests in practice are now discussed.
Case 1: All three the points determined by the tests, fall on the pump curve, but none on the system
curve. The system curve is therefore incorrect and indicates that the design was done
incorrectly or the pump is utilised differently from the purpose for which it was initially
intended.