IS NPSH IN CENTRIFUGAL PUMPS AND WHY IT IS CRUCIAL TO AVOID CAVITATION

Micelli Camargo
Sep 12
2 min read

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When we talk about pumping systems, one of the most common terms is NPSH, which stands for Net Positive Suction Head.

This concept is essential to understand and prevent the phenomenon of cavitation in pumps.

Cavitation occurs when the pressure of the liquid drops until it reaches its vapor pressure, which causes the formation of vapor bubbles.

These bubbles implode when the pressure rises (as happens at the outlet of the impeller), generating shock waves so intense that they can damage the pump and its peripherals.

NPSH is the parameter we use to assess the possibility of this phenomenon occurring. It is divided into two types:

👉 Available NPSH (NPSHa): Related to the actual conditions of the system and can be calculated as:

• NPSHa = (P1 - Pv) / γ - hsuc – hp

👉 Required NPSH (NPSHr): Related to the pump itself and is provided by the manufacturer.

Where:

--> P1 is the absolute pressure at the suction reservoir

--> Pv is the vapor pressure of the fluid

--> γ is the specific weight of the fluid

--> hsuc is the suction lift (geometric suction height)

--> hp is the head loss in the suction line

To ensure the proper operation of the system, it is essential that:

• NPSHa > NPSHr

Otherwise, cavitation will certainly occur, potentially causing premature wear of the pump components, increased energy consumption, and pumping failures, possibly to the point where the pump cannot transfer the fluid.

Choosing the right pump depends not only on flow rate and total dynamic head but also on the compatibility between the pump’s NPSHr and the system’s NPSHa.

Several factors influence NPSHa:

--> Fluid temperature (hot fluids have higher vapor pressure)

--> Site altitude (higher altitudes have lower atmospheric pressure)

--> Type of fluid (volatile substances are more prone to cavitation)

--> Length and complexity of the suction line (more head loss)

This is why flooded pumps tend to have fewer cavitation problems. Considering the suction reservoir level as a reference, the lower the suction static head, the higher the NPSHa will be.

It’s worth noting that cavitation can still occur even when calculations are correct, due to factors such as turbulence or unexpected pressure variations in the system.

In more complex systems or with special fluids (like liquid ethylene), the risk of cavitation is even greater. In these cases, detailed knowledge and proper sizing are critical.

We recorded a video on this topic. To watch it, go to: https://www.youtube.com/watch?v=NBRqK3lIle0

Learn how to specify pumps correctly with our course SPECIFICATION OF PUMPS. Learn more and sign up at https://www.engenhariaecia.eng.br/en/bomba

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