Characteristic Curve NPSH curves

NPSH Curves(Fig-1) shows both, NPSHa and NPSHr. NPSH stands for Net Positive Suction Head. Now what is this?. Pl refer NPSH chapter for details.

Now, Please refer this graphs just for illustration. Here Y axis is not to scale. Means, you can consider two graph overlapped on one just to show the diff. As the flow from the pump increases, NPSH required by the pump keeps on increasing. At the same time, NPSH available in the system keeps on decreasing. When the flow is q1, NPSH available is na1 at point A which is more then na2 at point B where flow q2>q1. Similarly, NPSH required at point B is more then NPSH required at point A.

Curve-2  This graph shows NPSH available and NPSH required in same graph in with Y axis showing head in meters. As flow increases, the margin between NPAHa and NPSHr reduces. The point A, at which flow is q, the margin is known as Safe pressure margin at impeller eye over liquid VP. The safe margin goes on reducing from point A to point B(where it is zero). For proper functioning of the pump, NPSHa must be always more then NPSHr by at least 0.5m.Note:- For every pump, a particular force (Head) in terms of mlc is required for every speed and flow rate to prevent the liquid from vaporization.

Characteristic Curve Q-Efficiency Curve

This curve is Q-Efficiency Curve. Note that this curve increases to the highest point and then drops to certain level and stops.

As we move from point A to B then to C, we can see that efficiency increases to max at point B (this is known as Best Efficiency Point) and then drops. At point A, Efficiency corresponding to q1 is n2 which is less then at point B, where flow q2>q1 but efficiency n1>n2. Also note that efficiency corresponding to point B is maximum. There after, if flow increases, efficiency starts dropping. At point C, flow q3>q2 and efficiency n1>n2. Also we can see that point A and C have same efficiency. Will discuss more about this in later posts.