12
5 — Explanation of Calculations
In this chapter, the formulas behind PumpSave calculations are presented. The
formulas use metric units. If the user enters the data in US units, PumpSave
converts it to metric units first. The conversion factors are given in the end of
this manual.
Pump operating point
The formula for the pump curve estimate used in calculations is:
n
n
i
pi
H
H
Q
Q
H
H
max
2
max
This might be somewhat erroneous to submersible and high characteristic
speed pumps etc.
Respectively, the system curve used in calculations is
st
n
n
i
st
si
H
H
Q
Q
H
H
2
A pump will always operate where these curves intersect, as Graph 1 shows.
Please notice that the point defined by H
n
and Q
n
is assumed to be known and
valid for pump and system curve.
Graph.1. Typical pump and system curves.
When adjusting the pump speed to control the volume flow, the process moves
via system curve Hsi. Respectively, when throttling the pipeline the pump
operates continuously at the same speed, and the movement of the process is
via pump curve Hpi.
Pump drive calculations
The nominal shaft power of a pump is calculated from the formula
PUMP
SYSTEM
H
max
H
N
H
st
H
dyn
Q
N
pump curve, H
pi
=H
max
-(Q
i
/Q
n
)
2 .
(H
max
-H
n
)
system curve, H
si
=H
st
+(Q
i
/Q
n
)
2.
(H
n
-H
st
)
