Why must NPSH_available exceed NPSH_required in a pump suction to avoid cavitation?

• A. To increase the flow rate.
• B. To prevent cavitation by keeping local pressure above the vapor pressure.
• C. To minimize friction losses.
• D. To maintain a constant temperature.

Answer: (B)

Cavitation in a pump suction happens when the pressure at the impeller eye drops to the liquid’s vapor pressure, causing tiny vapor bubbles to form and then collapse, which damages the pump and reduces performance. To prevent this, the suction system must provide enough energy head so that the actual pressure remains above the vapor pressure. NPSH_available represents the total head available at suction (from static suction head, velocity head, and atmospheric pressure, minus suction-line losses). NPSH_required is the minimum head the pump needs to avoid cavitation for the given liquid and operating conditions. When NPSH_available exceeds NPSH_required, there is a safety margin above the vapor pressure, so cavitation is avoided. If the available head is not large enough, vapor bubbles form and cavitation occurs.

Increasing flow rate isn’t a guarantee against cavitation and can actually raise the required NPSH, while friction losses and temperature influence the situation but do not replace the fundamental requirement that available head exceed the cavitation threshold.