Net Positive Suction Head (NPSH) is critical for lubricating and feed pumps because what?

Net Positive Suction Head (NPSH) is critical for lubricating and feed pumps because what?

• A. NPSH is the margin above vapour pressure available at the pump suction.
• B. NPSH is the margin above vapour pressure available at the pump discharge.
• C. NPSH is the vacuum in the pump casing.
• D. NPSH is the atmospheric pressure.

Answer: (A)

The important idea is preventing cavitation on the suction side. NPSH is the margin of pressure available at the pump suction above the liquid’s vapour pressure. This margin keeps the liquid from boiling as it enters the impeller. If the suction pressure falls too close to or below the vapour pressure, vapor bubbles form and then collapse inside the pump, causing loss of flow, noise, vibration, and damage to the impeller and bearings, as well as poor lubrication and cooling. So NPSH represents the head that ensures the liquid stays in the liquid phase at suction. The other concepts aren’t what NPSH measures: it’s not about pressure on the discharge side, a vacuum in the casing, or atmospheric pressure, but about maintaining enough suction pressure above the liquid’s vapour pressure to avoid cavitation.

The important idea is preventing cavitation on the suction side. NPSH is the margin of pressure available at the pump suction above the liquid’s vapour pressure. This margin keeps the liquid from boiling as it enters the impeller. If the suction pressure falls too close to or below the vapour pressure, vapor bubbles form and then collapse inside the pump, causing loss of flow, noise, vibration, and damage to the impeller and bearings, as well as poor lubrication and cooling. So NPSH represents the head that ensures the liquid stays in the liquid phase at suction.

The other concepts aren’t what NPSH measures: it’s not about pressure on the discharge side, a vacuum in the casing, or atmospheric pressure, but about maintaining enough suction pressure above the liquid’s vapour pressure to avoid cavitation.