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How to Prevent Pump Cavitation Using NPSHa Calculations

by PSG

Keeping your centrifugal pumps running smoothly is a constant balancing act of monitoring various gauges, levels and components. One of the most common phenomena that can lead to damaged pumps is cavitation. If a pump is cavitating, excessive vibration and rapid degradation of hydraulic components can occur, leading to premature seal, bearing or other part failures.

In this article, we’ll break down how to identify when your pump is cavitating and how you can prevent it through consistent pump monitoring.

What is Pump Cavitation and Why Does it Happen?

Pump cavitation is a phenomenon in which pumping system’s net positive suction head available (NPSHa) is not in balance with the net positive suction head required (NPSHr). When an NPSH deficit is created, it leads to the formation of vapor bubbles in the liquid. When these vapor bubbles are subjected to higher pressure during a pump’s pressure stage, they implode. This implosion causes the liquid walls to collapse and produce shock waves of incredible force that cause destruction upon impact.

The most obvious signs of pump cavitation are vibration and noise during system operation, often sounding like the pump is being filled with bits of gravel. Other signs of cavitation include a reduction in discharge pressure or flow, increased power consumption and debris in the discharge liquid.

How to Calculate NPSHa

The NPSH margin separates the NPSHa from the NPSHr for the pump to operate without cavitation. In order to stay on top of your pump’s health, consistent monitoring of the NPSHa is critical. Always ensure that your pump is operating within acceptable limits of the NPSHr.

To calculate a pump’s NPSHa, use the following equation:

NPSHa = hs - hvp + hvel + hg - hf

hs = suction head (ft)
hvp = vapor pressure (ft)
hvel = velocity height (ft)
hg = gauge height (ft)
hf  = friction loss (ft)

To take the required readings, start by taking the temperature of the fluid running through the system. Either make use of your system’s built-in temperature sensors, if applicable, or use an infrared temperature gun to take the temperature at the suction of the pump. This will guide us to our corresponding vapor pressure at pumping temperatures.

Next, take the pressure measurement at the suction of the pump by recording the number displayed on your system’s absolute pressure gauge.

Finally, plug in your system’s specifications and the numbers you recorded to arrive at your system’s NPSHa.

Here is an example calculation for a system with the following specifications:

  • hs = 37.6 ft (converted from 16.3 psia)
  • hvp = .76 ft (vapor pressure of water at 67°)
  • hvel = 5.2 ft (based on 3” pipe and 400 gpm flow)
  • hg = 1 ft (height of suction gauge)
  • hf  = .35 ft (based on 3” pipe and 400 gpm flow) 

NPSHa = 37.6 - .76 + 5.2 + 1 - .35 = 42.7 ft

Compare the NPSHa with your system’s NPSHr to ensure that the pump is operating within acceptable limits. If it isn’t, take steps to correct the pump’s pressure and velocity.

Taking a baseline vibration reading when the pump is not cavitating and comparing it to acceptable limits can also help to predict and prevent cavitation.

Be sure to consult with trusted pump manufacturers when purchasing pump equipment to ensure you’re getting the right system for your needs. Pump designers use NPSH to ensure that pumps will operate without internal damage caused by cavitation under all specified operating conditions.

Read More about Determining available system NPSHA