Characteristic Curves Q-p Curve

Power consumed by the pump is plotted against different flow rate, the curve is know as Q-p Curve. Power gradually increases as we move towards right side (Note:- This phenomenon is only with centrifugal pumps). This means that as we open discharge valve, flow increases and so does the Power. This continues up to certain point and after that, there is a sudden drop of flow and power. This is not shown in following graph but same will be dealt within other topic. This point is known as cutoff point.

Please note that at point A, flow is q1 and power consumed by the pump is p1. And at point B, flow is q2 and power is p2. At point B, flow q2>q1 and p2>p1.

Characteristic Curve Q-H Curve

This curve is known as Q-H Curve. When the discharge valve is fully closed, head developed by the pump is maximum. At this point, discharge is zero. This head is known as Shutoff Head. As we slowly open discharge valve, flow starts and flow increasea whereas head developed by the pump decreases. We can see on graph that at point A, flow q1 corresponds to head h1. And at point B, flow q2 corresponds to head h2. We can see that q2>q1 and h1>h2.

On field, if we want to know the head developed by the pump, place a pressure gauge just above pump discharge or at discharge flange (if tapping is provided). We can then take the reading of pressure gauge and convert it to head. Remember the conversion from pressure to head and vice versa?. Sp.gr. is very important. Hence if any doubt is there, pl refer to Head section of this app.