System Curves

For a specified impeller diameter and speed, a centrifugal pump has a fixed and predictable performance curve. The point where the pump operates on its curve is dependent upon the characteristics of the system in which it is operating, commonly called the System Head Curve..or, the relationship between flow and hydraulic losses in a system. This representation is in a graphic form and, since friction losses vary as a square of the flow rate, the system curve is parabolic in shape.

sect a9 01
By plotting the system head curve and pump curve together, it can be determined:

  1. Where the pump will operate on its curve.
  2. What changes will occur if the system head curve or the pump performance curve changes.
As the levels in the suction and discharge are the same (Fig. 1), there is no static head and, therefore, the system curve starts at zero flow and zero head and its shape is determined solely from pipeline losses. The point of operation is at the intersection of the system head curve and the pump curve. The flow rate may be reduced by throttling valve.

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Fig.1 No Static Head All Friction
The parabolic shape of the system curve is again determined by the friction losses through the system including all bends and valves. But in this case there is a positive static head involved. This static head does not affect the shape of the system curve or its "steepness", but it does dictate the head of the system curve at zero flow rate.

The operating point is at the intersection of the system curve and pump curve. Again, the flow rate can be reduced by throttling the discharge valve.

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Fig. 2 Positive Suction Head
In the illustration below, a certain flow rate will occur by gravity head alone. But to obtain higher flows, a pump Is required to overcome the pipe friction losses in excess of "H" - the head of the suction above the level of the discharge. In other words, the system curve is plotted exactly as for any other case involving a static head and friction head, except the static head is now negative. The system curve begins at a negative value and shows the limited flow rate obtained by gravity alone. More capacity requires extra work.

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Fig. 3 Negative (Gravity) Head
The system head curve in the illustration below starts at the static head "H" and zero flow. Since the friction losses are relatively small (possibly due to the large diameter pipe), the system curve is "flat". In this case. the pump is required to overcome the comparatively large static head before it will deliver any flow at all.

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Fig. 4 Mostly Lift - Little Fricition Head

*Hydraulic losses in piping systems are composed of pipe friction losses, valves, elbows and other fittings, entrance and exit losses (these to the entrance and exit to and from the pipeline normally at the beginning and end not the pump) and losses from changes in pipe size by enlargement or reduction in diameter.