In this article, we will see the Centrifugal Pump System Curve and Pump Operating Point. The system curve is a very basic element to finalizing the pump operating point ie., flow and head requirement.
Pump System Curve
Pump system curve represents the resistance offered by the system ie., the pressure developed by the pump is high enough to overcome the resistance offered by the system. This pressure is also called as “Total Head”. The total head is the sum of two components. They are
- Static Head
- Friction Head
Static head
Static head is the difference in height between the starting elevation of pumping liquid to the destination of pumping liquid to be pumped. The Static head as two components. They are the Static suction head (Hs) and Static discharge head (Hd)
Static suction head (Hs)
The Static suction head refers the lifting the liquid relative to the pump centre line. The Static suction head is positive if the liquid level is above pump centerline, and negative if the liquid level is below pump centerline.
Static discharge head (Hd)
The Static discharge head refers to the vertical distance between the pump centre line and the surface of the liquid in the destination tank or vessel. As we have seen from the figure and curve, the Static Head is independent of flow.
The below formula used to calculate the static head
Friction head (hf)
The friction head is the frictional loss due to the pipes, valves and pipe fittings. Friction head is also called “Dynamic head loss”.
Unlike the static head, the friction head depends on the flow rate and property of the liquid. Normally the friction head is proportional to the square of the flow rate.
The total system head is a sum of static head and friction head as shown the final curve.
Pump performance curve
The pump performance curve is represented the total head in the y-axis and the flow in the x-axis, which is graphically shown in the below pump characteristic curve. As per the curve, the head gradually decreases with increasing flow. To read about pump performance curve Click Here
Pump operating point
When we merge both the system curve and pump curve as shown in the below figure. The intersection of the system curve and pump curve called the Pump Operating Point. The corresponding head and flow refer to “Rated Head” and “Rated Flow”.
As the resistance of a system increases, the head will also increase. This, in turn, causes the flow rate to decrease and will eventually reach zero. A zero flow rate is only acceptable for a short period without causing to the pump to burn out.
Refer below figure, the change in system curve (represented in dotted line). The intersection of the System Curve with Pump curve respects new Operating point.
Important of System Curve
An error in the system curve calculation leads to a wrong selection of centrifugal pump. On the other hand, if we adding more safety margin in the calculation of System Curve, then the pump selection leads to an oversized pump. Hence more attention shall give to system curve calculation.