All pumps (both the centrifugal pumps and the positive-displacement pumps) have operational limits. Specifically, centrifugal pumps have certain limitations which, if not properly evaluated can drastically reduce their working life. The BEP (Best Efficiency Point) is not only the maximum operating point but it is also the point where the speed and pressure at the impeller and the spiral stator are equal. As the operating point diverges from the Best Efficiency Point, the speed changes which in turn modifies the pressure acting on one of the sides of the impeller. This irregular pressure on the impeller manifests itself as a radial thrust which deflects the pump shaft causing, among other things:
an excessive load on the bearings
an excessive deflection of the mechanical seal
irregular wear on the shaft bearing
The damages that might occur then consist of a shorter bearing life or a broken shaft. Radial loading is maximized when everything closed. If operating outside of the recommended operating range, damage to the pump could be caused by excessive speed and turbulence. Vortexes can create cavitation damage which very quickly can destroy the pump and impeller casing.
When choosing a pump, it's very important that unrealistic safety margins not be calculated or that inappropriate information not be included in the evaluation. The true curve could exceed the recommended operating range resulting in series consequences.
It is best to verify the actual operating conditions for the pump the during while it is in operation (using a flowmeter and/or a pressure gauge) in order to make the adjustments needed to ensure the proper working conditions and a long service life.