Ask any temporary equipment vendor what’s the most common cause of pump failure in the field and they’ll likely say pump cavitation. It’s a frustrating but avoidable problem. When you’re having an issue with cavitation in pumps on a job, it’s often a signal that you’re using the wrong pumps or the wrong pump system design.

What is pump cavitation?

Cavitation is the rapid formation and subsequent implosion of air bubbles in the liquid being moved through the pump. The collapse of the bubbles under high pressure creates shock waves that ripple through the pump. These shock waves can remove chunks of metal from the impeller and cause seals to fail. Over time, cavitation usually leads to pump failure.

Cavitation in centrifugal pumps happens most frequently, but the problem can occur in any type of pump.

Types of pump cavitation

There are two types of pump cavitation: suction cavitation and discharge cavitation.

Suction cavitation

Suction cavitation is the result of the intake flow being too low due to low pressure at the suction end of the pump, or to high vacuum conditions.

When the net positive suction head (NPSH) drops below the liquid’s vapor pressure (the pressure at which a liquid turns into gas), it causes air bubbles to form on the intake side of the impeller. As the liquid moves toward the discharge point, the NPSH rises above the vapor pressure and the bubbles implode.

Discharge cavitation

Discharge cavitation is the result of the pump’s discharge pressure being too high. This prevents some of the liquid from leaving the pump. As that liquid recirculates in the discharge area, air bubbles can form.

What causes pump cavitation?

Pump cavitation is usually the result of choosing the wrong type of pump, sizing it incorrectly (using a pump with the right NPSH is critical) or designing an inadequate pump setup. Failing to properly maintain a pump can also contribute to cavitation.

Some specific pump cavitation causes include:

• Failure to calculate the NPSH properly or choose a pump that’s adequate for the NPSH requirement of the application
• Improper pump placement (too high above the reservoir where the liquid originates or too far away)
• Intake or discharge hoses that are too small, too flimsy or too long
• A pump that’s not powerful enough to move high-viscosity liquid
• Changes in air temperature or liquid temperature that impact the liquid’s vapor pressure
• Unaccounted-for changes in elevation that impact the liquid’s vapor pressure
• Dirty filters or clogged strainers, which could impact the pressure of the fluid flowing through the pump

Signs of cavitation

Unusual vibration is often the first indicator that you have a problem with pump cavitation. After the vibration stage, the pump may start making loud growling noises or sounding like it’s pumping small rocks instead of liquid. Another sign is an increase in the pump’s power consumption.

Upon inspection, the pump may show signs of destruction, such as damaged seals or an impeller that’s pitted or missing pieces of metal.

How to prevent pump cavitation

The most critical steps to preventing pump cavitation are choosing a pump with sufficient capacity and getting the design of the pumping system right.

Consider the site conditions, including any elevation changes along the discharge route and the distance to the fluid reservoir and the discharge point, and determine the pressure and flow rates you’ll need.

Look at your intended setup with pump cavitation causes in mind. For example, make sure the vertical distance between your liquid reservoir and the pump inlet won’t result in an NPSH that’s too low. Avoid long hoses, which can contribute to cavitation in pumps, by placing the pump as close as possible to the intake and discharge points.

Perform regular maintenance on your pump to ensure that filters, strainers and hoses are clean and free of obstruction.

The cost of pump cavitation can be much higher than the price of a replacement pump. If cavitation leads to pump failure, your schedule and budget could suffer. A knowledgeable and experienced pump vendor can help you arrive at the right pump, and the right pump system design, for your application.

For help with pump selection and system design, contact the United Rentals Fluid Solutions team today.