Net Positive Suction Head (NPSH)

Definition - What does Net Positive Suction Head (NPSH) mean?

Net positive suction head (NPSH) for a pump is the difference between the liquid pressure at the pump’s suction and the vapor pressure of the fluid being pumped. It is the amount of energy available to exert pressure on the fluid and is expressed in terms of height of a liquid column i.e. in feet (not in pressure gauge reading).

In hydraulic systems, the NPSH is usually kept positive to avoid the vaporization of fluid, which could lead to cavitation corrosion and damage to the pump.

Corrosionpedia explains Net Positive Suction Head (NPSH)

The net positive suction head is a measure of the available pressure at any given datum point. This point can be at the suction nozzle centerline, at the impeller centerline or the top of foundation, depending on the specific circumstances.

The net positive suction head varies according to the specific operating conditions and application. It is influenced by:

  • Friction losses in the suction line
  • Atmospheric pressure
  • Fluid temperature
  • Pressure in the suction tanks
  • Distance of the suction tank fluid surface from the pump suction

NPSH can be divided into two components:

  • NPSH Available (NPSHA) - Absolute pressure at the suction port of the pump
  • NPSH Required (NPSHR) - Minimum suction pressure required by a pump at the impeller centerline

The NPSHA is a function of the pump system and is usually calculated, while NPSHR is provided by the manufacturer. For a pump to function properly, NPSHA must be more than NPSHR to provide sufficient pressure to push a specific amount of flow rate into the impeller and avoid cavitation. Cavitation occurs when pressure at a pump inlet drops below the liquid's vapor pressure. The liquid forms bubbles at the inlet, which then move toward the pump's discharge, where they collapse and take small pieces of the pump.

Insufficient NPSH may lead to cavitation erosion/corrosion and damage to the system as well as reduced efficiency.

Understanding NPSH helps in avoiding unnecessary noise and system vibration, minimizing impeller damage and maximizing seal and bearing life. It is important to set the correct value to extend the system’s life and increase productivity and reliability.

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