1.1 Using affinity laws:
1.1.1 An important equation is
n1 = original pump speed (usually the standard pump speed – 2900min1)
H1 = head supplied by pump at speed n1
n2 = new pump speed (min-1)
H2 = head supplied by pump at speed n2
Increasing pump speed allows the head supplied by a pump to be
increased. This often leads to a decrease in efficiency however, as
shown in the pump curves.
1.1.2 Another important equation is
Q1 = Flowrate at speed n1
Q2 = Flowrate at speed n2
This equation is normally used simply to obtain new plots of the pump
curve. A change in flow rate only occurs if head is constant. And for a
constant flow rate, it is the head that changes. Observe the pump curve
to understand this.
1.1.3 The above equations can be used to perform minor changes on pump
speed to alter the head it provides.
1.1.4 Example: An 80-315 pump with impeller diameter of 300mm can provide
a head of 114 m when driving a liquid at a flow rate of 130.88 m3/hr at a
standard speed of 2900min-1. Suppose the head loss in a section of pipe
amounts to 116m (incl. satisfying NPSH requirements). At what speed
should the pump operate in order to service this section of pipe and
maintain the flow rate at 130.88?
n2 = n1
H2
116
= 2900
= 2925.33min-1
H1
114
1.1.5 The affinity laws can be used to move the entire pump curve to meet the
operating point. This is illustrated on the next page. In this case the
speed had to be lessened.
Standard Speed
Q (m3/hr)
0
20
40
60
80
100
120
140
160
180
200
2900
H (m)
110
110
108.5
108
105.5
103
95.5
86.5
75
59.5
43
New Speed
Q(m3/hr)
0
19.52
39.05
58.57
78.10
97.62
117.15
136.67
156.20
175.72
195.24
2831.052
H (m)
104.83
104.83
103.40
102.93
100.54
98.16
91.01
82.44
71.48
56.70
40.98