When sizing a centrifugal pump for a dewatering project, guesswork won’t cut it. You can try to use whatever pump you have on hand, but if you get it wrong, the system won’t function effectively and the pump could experience damaging cavitation, which could bring pumping operations to a halt. To size a pump properly and reduce the risk of cavitation, it’s important to understand net positive suction head (NPSH) and how to ensure a sufficient margin between the net positive suction head available (NPSHA) and the net positive suction head required (NPSHR).

What is NPSH?

Let’s start with an NPSH definition. NPSH describes the pressure at the pump’s suction port, measured in feet or meters. It’s broken down into NPSHA, or how much pressure is available—technically, the stagnation pressure available over the liquid’s vapor pressure—and NPSHR, or how much pressure is required—technically, the minimum pressure at a specified rate of flow and speed for a specified pumped liquid that cause a 3% drop in the pumped head.

A lower head drop, such as 0% or 1%, might eliminate cavitation entirely, but the Hydraulic Institute Standard guidelines use a 3% head drop value with the goal of preventing damaging cavitation as opposed to achieving zero cavitation.

For those who need a refresher, cavitation is the rapid formation and implosion of vapor bubbles in the liquid. An early sign is vibration. If cavitation gets worse, the pump will start making loud noises as shockwaves from the implosions hit the impeller, pitting the metal. In addition to potentially damaging the pump, cavitation reduces a pump’s capacity.

How to calculate the NPSH margin

The difference between NPSHA and NPSHR (NPSHA minus NPSHR) is known as the NPSH margin. Maintaining a sufficient NPSH margin is key to avoiding damaging cavitation.

How much margin is enough? A general recommendation is at least 3.3 feet or 10% of NPSHA, whichever is greater. ANSI/HI 9.6.1-2017 Rotodynamic Pumps – Guideline for NPSH Margin provides target margins for different applications and pump designs.

The NPSHR is provided by the pump manufacturer, so there’s no calculation for the pump user to perform. Performing an NPSHA calculation, on the other hand, takes some effort and is best left to a pump expert or engineer. NPSH margins are typically tight, and a small miscalculation could result in real issues with the pump and pump setup.

The NPSHA formula is this: Site atmosphere pressure or absolute suction pressure + static head (this number is positive for flooded suction applications and negative for suction lift applications) – friction + velocity head at suction port – vapor pressure at the pumping temperature.

In some cases, acceleration head is another factor. An example is positive displacement pumping that involves sudden acceleration of liquid.

NPSHA: A moving target

Changes in the operating environment or the pumping setup can change the NPSHA, so regularly monitoring a pumping operation is critical. Adjustments to the pump system design may be needed if a pump performance issue arises.

Factors that can affect the NPSHA include:

• Changes in atmospheric pressure due to a low-pressure or high-pressure weather system
• A drop in the influent rate that causes an increase in static head
• An increase in the flow rate, which can lower NPSHA and raise NPSHR
• Changes in the temperature of the liquid, which alter the vapor pressure
• A drop in the liquid level, which can increase the suction lift and decrease the NPSHA
• A change in the liquid’s viscosity or specific gravity, which can alter friction losses
• A change in the length or diameter of piping or the number of fittings, which can also alter friction losses

The value of expert guidance

Improperly calculating the NPSH margin and allowing the margin to drop too low are common, and potentially costly, mistakes. Working with a temporary pump vendor that has in-house Professional Engineers is the easiest way to avoid them from the start.

The vendor can also help you troubleshoot pumping problems. For example, if a system loses performance due to an insufficient NPSH margin, they might recommend:

• Operating the pump at lower speed or operating two pumps instead of one
• Using larger-diameter suction pipes or shorter pipes and hoses
• Surcharging the liquid level in the suction source or benching down the pumps to reduce static head
• Using a submersible pump or other intake pump if the suction lift is too high

When choosing a pump and designing a pumping system, attention to detail can mean the difference between a smooth dewatering operation and problems that reduce efficiency, damage equipment and cause unplanned downtime. The NPSH margin is one detail that’s critical to get right.

For help with all your pumping operations, contact the United Rentals Fluid Solutions group.