In centrifugal pump applications, cavitation is one of the most destructive phenomena, causing mechanical vibration, impeller damage, and severe performance drop. To avoid cavitation, the mechanical design must satisfy a strict thermodynamic relation: the available Net Positive Suction Head (\(NPSH_A\)) must exceed the required NPSH (\(NPSH_R\)) dictated by the pump manufacturer.

The Physics of Cavitation

Cavitation occurs when the local static pressure inside the pump falls below the vapor pressure (\(P_{vapor}\)) of the liquid. Vapor bubbles form instantly. As these bubbles move into zones of higher pressure, they collapse violently, generating micro-jets with local pressures up to 10,000 bar. This causes rapid surface erosion (pitting).

Calculating NPSH Available (\(NPSH_A\))

The available suction head must be calculated at the suction nozzle center line:

\[NPSH_A = P_{suction} + \frac{V_s^2}{2g} - P_{vapor}\]

For an open reservoir, this translates to:

\[NPSH_A = H_{atm} \pm H_s - H_f - H_{vp}\]

Where:

  • \(H_{atm}\) is the atmospheric pressure head.
  • \(H_s\) is the static elevation head (positive if liquid level is above pump, negative if suction lift).
  • \(H_f\) is the friction head loss in the suction piping.
  • \(H_{vp}\) is the vapor pressure head of the fluid at the operating temperature.

Best Practices to Prevent Cavitation

  1. Keep the suction line as short and straight as possible.
  2. Increase the suction pipe diameter to reduce fluid velocity and frictional head losses.
  3. Minimize operating temperature to lower the vapor pressure.
  4. Mount the pump close to or below the liquid surface to increase static head.