Which statement correctly describes Net Positive Suction Head (NPSH) and its variants?

• A. NPSH is a measure of head loss due to friction in piping
• B. NPSH is unrelated to cavitation
• C. The two values are NPSHA and NPSHR, and NPSHA should be greater than NPSHR (correct)
• D. NPSH is only applicable to air-handling units

Answer: (C)

NPSH relates to preventing cavitation in pump suction. Cavitation occurs when the pressure at the suction drops below the liquid’s vapor pressure, forming bubbles that can collapse and damage the pump. To evaluate this, two values are used: Net Positive Suction Head Available (NPSHA) and Net Positive Suction Head Required (NPSHR). NPSHA is the actual suction-head available, determined by system conditions such as suction pressure, liquid vapor pressure (temperature), elevation, and any losses in the suction piping. NPSHR is the head the pump requires to avoid cavitation at a given flow, read from the pump’s curve. To prevent cavitation, the available head must exceed the required head: NPSHA should be greater than NPSHR. The other statements mischaracterize NPSH as a measure of friction losses, claim it’s unrelated to cavitation, or limit its applicability to air-handling units, which isn’t correct.