Understanding Pump Curves
The Power Characteristic
The graph below shows us the Head Capacity Characteristic (black curve) and the Power Capacity Characteristic (blue curve). The Power Characteristic has its own scale calibrated in Brake Horsepower (BHP), telling us how much power is required to maintain the constant speed (3550 RPM) at various flow rates.
Because no statement is made on the curve regarding the type of fluid, it is assumed that the curve is for water at Standard Conditions.
The Head Capacity curve is not dependant upon the type of fluid being pumped (assumed to be Newtonian (free flowing), and also Non-Compressible. But the Power Characteristic is dependent upon the type of fluid, specifically the mass of the fluid. The heavier the fluid, the more power required to maintain the pump at a constant speed. Therefore, if a fluid with a weight different than water is being pumped, the Head Capacity Characteristic remains the same, but the Power Characteristic would no longer be valid, but must be re-calculated based on the mass of the fluid.
The Power Characteristic is determined by the pump manufacturer during tests where the manufacturer measured power required to maintain a constant speed at various flow rates.
On the graph above the relationship of power to flow rate is direct and approximately proportional. This type of relationship, where higher flow rates require higher BHP, indicates that the pump has a radial vane impeller (Low Specific Speed), usually with a Specific Speed index value of Ns < 3500.
Learn more about this subject by following the links below. See pump curves illustrating how power to flow relationships are different based on pump Specific Speed, see drawings showing how fluids move through impellers and how energy is transferred from the driver into the fluid, includes detailed explanations.
Follow the links below to pages focusing on other types of information provided in the Pump Characteristic and more, including graphs and explanations.
