NPSH available is what the system gives the pump at its suction; NPSH required is what the pump needs to avoid the liquid flashing to vapour inside the impeller eye. If available is less than required, vapour bubbles form and collapse, and the collapse erodes the impeller.

The damage is characteristic and unmistakable: pitting on the impeller vanes, noise often described as pumping gravel, and vibration. A pump can be cavitating quietly for months before anyone connects the noise to the damage.

What reduces available NPSH is usually the installation rather than the pump. A long or undersized suction line, a partly closed suction valve, a lift rather than a flooded suction, or a hot liquid close to its vapour pressure all eat the margin. Hot water and thermal oil are particularly exposed because the vapour pressure is already high.

Tell us the suction arrangement as well as the duty point: whether the pump is flooded or lifting, how long the suction line is, and the liquid temperature. Those three decide whether the correct hydraulic selection will actually survive.