For the hydraulic calculation (selection) of a centrifugal pump, in general, the flow rate (Q), the head (H) and the speed (n) are needed. When used with a HQ curve, these ratings make up the design point of the selected centrifugal pump. This point often does not match the true operating point. Reasons for this include: uncertainties related to the system characteristic curve, properties of the fluid handled differing from those used for initial calculation, manufacturing tolerance, system-dependent control of the centrifugal pump, uncertainties related to the calculations and characteristic curves selection chart of series-produced pumps.
The aim often is to match the design point with the operating point at the best efficiency point or suction characteristics. The actual transient flow conditions significantly differing from those assumed for the design point may result in loss of efficiency in low flow operation and high flow operation, and in faults due to vibrations and cavitation.