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CHEMICAL ENGINEERING
5.4.5. System curve (operating line)
There are two components to the pressure head that has to be supplied by the pump in a
piping system:
1. The static pressure, to overcome the differences in head (height) and pressure.
2. The dynamic loss due to friction in the pipe, the miscellaneous losses, and the
pressure loss through equipment.
The static pressure difference will be independent of the fluid flow-rate. The dynamic
loss will increase as the flow-rate is increased. It will be roughly proportional to the flow-
rate squared, see equation 5.3. The system curve, or operating line, is a plot of the total
pressure head versus the liquid flow-rate. The operating point of a centrifugal pump can be
found by plotting the system curve on the pump’s characteristic curve, see Example 5.3.
When selecting a centrifugal pump for a given duty, it is important to match the pump
characteristic with system curve. The operating point should be as close as is practical to
the point of maximum pump efficiency, allowing for the range of flow-rate over which
the pump may be required to operate.
Most centrifugal pumps are controlled by throttling the flow with a valve on the pump
discharge, see Section 5.8.3. This varies the dynamic pressure loss, and so the position
of the operating point on the pump characteristic curve.
Throttling the flow results in an energy loss, which is acceptable in most applications.
However, when the flow-rates are large, the use of variable speed control on the pump
drive should be considered to conserve energy.
A more detailed discussion of the operating characteristics of centrifugal and other
types of pump is given by Walas (1990) and Karassik et al. (2001).

Example 5.3
A process liquid is pumped from a storage tank to a distillation column, using a centrifugal
pump. The pipeline is 80 mm internal diameter commercial steel pipe, 100 m long.
Miscellaneous losses are equivalent to 600 pipe diameters. The storage tank operates at
atmospheric pressure and the column at 1.7 bara. The lowest liquid level in the tank will be
1.5 m above the pump inlet, and the feed point to the column is 3 m above the pump inlet.
Plot the system curve on the pump characteristic given in Figure A and determine the
operating point and pump efficiency.
3
2
Properties of the fluid: density 900 kg/m , viscosity 1.36 mN m s.
Solution
Static head
Difference in elevation,z D 3.0 1.5 D 1.5m
5
5
Difference in pressure,P D 1.7 1.013 10 D 0.7 ð 10 N/m 2
5
as head of liquid D 0.7 ð 10 / 900 ð 9.8 D 7.9m
Total static ead D 1.5 C 7.9 D 9.4m