In the world of fluid dynamics and pump engineering, few concepts are as critical as Net Positive Suction Head, or NPSH. While it may seem complex, understanding NPSH is fundamental to ensuring the efficiency, reliability, and longevity of any pumping system. Neglecting it can lead to a destructive phenomenon known as cavitation, which can severely damage your equipment and halt your operations.

What is meant by NPSH?

Net Positive Suction Head (NPSH) is a measure of the absolute pressure present in a liquid at the pump’s suction port. More specifically, it is the difference between the liquid’s actual pressure and the liquid’s vapor pressure at a given temperature. It is always expressed as a head, typically in meters or feet.

To truly understand NPSH, one must first understand vapor pressure (pv). This is the pressure at which a liquid will begin to boil and turn into a vapor at a specific temperature. If the pressure in the suction line drops below the liquid’s vapor pressure, bubbles will form. This is the genesis of cavitation.

NPSH Available (NPSHa) vs. NPSH Required (NPSHr)

The concept of NPSH is divided into two key parameters:

  • NPSH Available (NPSHa): This is a characteristic of the system. It represents the absolute pressure that is actually available at the pump’s suction inlet. It is determined by factors related to the specific installation, such as the height of the liquid source, friction head losses in the suction piping, and the pressure acting on the surface of the liquid.
  • NPSH Required (NPSHr): This is a characteristic of the pump. It is the minimum pressure required at the pump’s suction port to prevent cavitation. This value is determined by the pump’s design (impeller geometry, speed, etc.) and is provided by the pump manufacturer for different flow rates.

The rule is simple:

NPSHa > NPSHr

The NPSH available in your system must always be greater than the NPSH required by the pump. It is recommended to have a safety margin, with the NPSHa exceeding the NPSHr by at least 1 meter.

npsh curve 1

Calculating NPSH: a practical guide

While pump manufacturers provide the NPSHr, system designers are responsible for calculating the system’s NPSHa. The formula for NPSHa is:

NPSHa = Ha ± Hz – Hf – Hvp

Where:

  • Ha (Absolute Pressure Head): The pressure acting on the surface of the liquid in the supply tank. If the tank is open to the atmosphere, this is the atmospheric pressure.
  • Hz (Static Head): The vertical distance between the surface of the liquid and the centerline of the pump. This value is positive if the liquid level is above the pump (flooded suction) and negative if it is below the pump (suction lift).
  • Hf (Friction Head Loss): The energy lost due to friction as the liquid flows through the suction piping, valves, and fittings.
  • Hvp (Vapor Pressure Head): The vapor pressure of the liquid at the pumping temperature, converted to a head of liquid.

The dangers of insufficient NPSH: cavitation

When NPSHa falls below NPSHr, the pressure at the pump’s suction drops below the liquid’s vapor pressure. This causes the formation of vapor bubbles. As these bubbles are carried through the impeller to a region of higher pressure, they violently collapse or “implode.” This collapse creates powerful shockwaves that can cause significant damage, including:

  • Erosion and pitting of the impeller and pump casing.
  • Excessive noise and vibration, often described as pumping gravel or marbles.
  • A noticeable drop in pump performance (head and flow rate).
  • Premature failure of mechanical seals and bearings.

Factors affecting NPSH

To effectively manage NPSH, you must be aware of the variables that can influence it:

Factors affecting NPSHa (System-Dependent):

  • Liquid Temperature: Higher temperatures increase vapor pressure, thus reducing NPSHa.
  • Altitude: Higher altitudes have lower atmospheric pressure, reducing NPSHa.
  • Static Head: Raising the liquid level in the supply tank increases NPSHa.
  • Piping Design: Longer pipes, smaller pipe diameters, and numerous fittings or valves all increase friction losses (Hf), which in turn reduce NPSHa.

Factors affecting NPSHr (Pump-Dependent):

  • Pump speed: Higher pump speeds generally increase NPSHr.
  • Flow rate: As flow rate increases, so does the NPSHr.
  • Impeller design: The geometry of the impeller has a significant impact on the NPSHr.

How to improve NPSH and prevent cavitation

To increase NPSHa:

  • Raise the liquid level in the suction tank.
  • Lower the pump’s position relative to the liquid level.
  • Increase the diameter of the suction piping to reduce friction losses.
  • Minimize the length of the suction line and the number of elbows and valves.
  • Pressurize the suction vessel if it is a closed system.
  • Cool the liquid to lower its vapor pressure.
  • Use a booster pump to increase the pressure at the main pump’s suction inlet.

To decrease NPSHr:

  • Choose the right pump: It is crucial to select a correct pump model and have it operate at a point on its curve that requires a low NPSH. The choice of the correct model depends on all the pump selection parameters, which must be carefully evaluated.
  • Use a larger, slower pump, if possible.
  • Operate the pump at a lower speed if using an inverter (VFD).

A thorough understanding and careful calculation of Net Positive Suction Head are not just academic exercises; they are essential practices for ensuring the health and performance of your pumping systems. By ensuring an adequate margin between NPSHa and NPSHr, you can prevent the devastating effects of cavitation, reduce maintenance costs, and maximize the operational life of your equipment.

If you have any questions or require expert assistance for your pumping applications, you can contact us by writing an email to info@gemmecotti.com. Our team of technicians is ready to help you select the right pump and ensure a reliable, efficient, and long-lasting solution.