Pump Cavitation: What It Is and How to Avoid It

What’s that sound coming from your pump? If it sounds more like gravel getting pushed through pipes than a liquid, the culprit may be pump cavitation. But what exactly is pump cavitation, and why is it so harmful for pumps and their components? We’ll explain all about this common, yet damaging, phenomenon, and what you can do to stop it before it damages your system.

What Is Pump Cavitation and What Causes It?

Pump cavitation occurs when the pressure in a pump falls below the vapor pressure of the liquid, causing it to vaporize and form bubbles. These vapor bubbles typically form on the suction side of the pump, where pressure is lowest. As the fluid reaches higher-pressure areas, such as near the impeller, the bubbles collapse.

Imagine a pot of boiling water on your stove. Vapor bubbles come up from the bottom and explode upon reaching the surface of the water. In this scenario, the boiling occurs due to heat. But the boiling point is also impacted by the pressure above the water. At sea level, water boils at 212°F (100°C), but at 10,000 feet elevation, where atmospheric pressure is lower, it boils at approximately 193.6°F (89.8°C). As the atmospheric pressure drops, the boiling point of water decreases.

In a pumping system, a similar principle applies. If the pressure inside the pump drops below the liquid’s vapor pressure, the liquid begins to boil, even at ambient temperatures. This localized boiling creates vapor bubbles within the fluid. As the fluid continues through the pump and encounters higher pressure near the impeller, the bubbles rapidly collapse. This sudden implosion creates intense shockwaves that hammer the internal surfaces of the pump, particularly the impeller blades, leading to pump cavitation.

What’s so Bad About Bubbles? The Consequences of Pump Cavitation

Cavitation bubbles may be tiny, but they present big problems for your pump. These vapor bubbles typically form on tiny imperfections in the pump’s metal surfaces, such as the impeller of a centrifugal pump or the piston/gear of a positive displacement pump, on the suction side. As these bubbles move to the high-pressure discharge side, they collapse violently, creating shockwaves and microjets that impact surfaces with extreme force. This implosion creates intense, focused shockwaves that slam into the very surface where the bubbles formed.

These repeated impacts cause pitting, a type of surface erosion that starts small but worsens over time. When a pump is new and the surfaces are smooth, it’s more resistant to cavitation. But as imperfections develop, the damage accelerates. Eventually, parts of the impeller or other components may look pitted or sandblasted. Large pieces of material may even be missing from parts.

The violent reaction also damages bearings and seals, shortening their lifespan and incurring skyrocketing maintenance costs from frequent replacement. Pump cavitation results in reduced efficiency, vibration, noise, and ultimately, pump failure.

What’s the Difference Between Suction, Discharge, and Cavitation?

“Suction” and “discharge” refer to different sides of a pump. The suction side is where the fluid enters the pump, typically at a lower pressure. The discharge side is where the pump pushes the fluid out, usually at a higher pressure.

Cavitation, on the other hand, is a problem that can happen within the pump. It’s not a part of normal pump operation, like suction or discharge. It’s a damaging condition caused by low pressure that leads to vapor bubbles forming and collapsing.

Here’s a simple way to think about it:

Suction: Fluid entering the pump at lower pressure

Discharge: Fluid exiting the pump at higher pressure

Cavitation: Fluid vaporizing and causing damage due to pressure being too low (especially at the suction side)

While suction cavitation is most common, discharge cavitation can also occur when a pump operates at low flow rates or far from its best efficiency point. This causes fluid recirculation on the discharge side, forming vapor bubbles in low-pressure zones that collapse and damage components. Both types of cavitation have similar symptoms, like noise, vibration, and erosion, but they require different prevention strategies.

So while suction and discharge describe how a pump moves fluid, cavitation describes what happens when conditions inside the pump go wrong.

Early Signs of Pump Cavitation

One of the earliest signs of pump cavitation is unusual noise coming from the pump. This noise is often described as the sound of gravel rattling around in the pump housing or pipework. However, it’s easy to miss this key sign in noisy factory settings.

Other early warning signs include:

• Unusual Vibrations: Vibration monitoring can detect changes in a pump’s vibration signature and reveal cavitation.
• Erratic Pressure Fluctuations: Erratic pressure gauge readings, especially on the suction side of the pump, may signal cavitation
• Reduced Flow Rates: Reduced pump performance can indicate that cavitation is affecting the flow of fluid or has already damaged the pump.

How to Prevent Pump Cavitation

The easiest way to prevent pump cavitation is to design the pumping system so that it won’t happen under normal operating conditions. This requires understanding what Net Positive Suction Head (NPSH) is and considering it during the design process.

There are two parts of NPSH that need to be considered.

1. NPSHR (Net Positive Suction Head Required): This is the minimum pressure the pump needs at its suction side to avoid cavitation. This number is quoted in meters or feet, and it is calculated by the pump manufacturer.
2. NPSHA (Net Positive Suction Head Available): This number must be calculated during the system design process. It takes the atmospheric pressure, fluid level, fluid temperature, and liquid boiling point into consideration.

If NPSHA is greater than NPSHR, the pump should operate without cavitation. However, if NPSHA is less than NPSHR, the pump doesn’t have enough pressure to keep the liquid from vaporizing, ultimately leading to cavitation.

To avoid suction cavitation, ensure the Net Positive Suction Head Available (NPSHA) exceeds the Net Positive Suction Head Required (NPSHR) by at least 0.5–1.0 meters, as specified by the pump manufacturer. To avoid discharge cavitation, operate the pump near its best efficiency point (BEP) to avoid low-flow conditions that cause recirculation. Using a correctly sized pump or installing variable frequency drives (VFDs) can help maintain optimal flow rates.

How to Correct Pump Cavitation

If cavitation is already occurring, address it as soon as possible to prevent damage. Correction methods vary depending on whether it’s suction cavitation (caused by low suction pressure) or discharge cavitation (caused by low flow or recirculation). Here are a few solutions to try:

• Optimize Suction Piping: Small, long, or complex suction piping can restrict flow, reducing NPSHA. Use larger-diameter piping, shorten its length, or reduce bends to improve flow and prevent suction cavitation.
• Relocate Pump or Fluid Source: Positioning the pump closer to or below the fluid source reduces suction lift, increasing NPSHA to reduce suction cavitation.
• Check Filters and Strainers: Clogged filters or strainers on the suction side can starve the pump, causing cavitation. Clean or replace them, and ensure proper filter sizing to maintain flow.
• Check Valves and Components: Partially closed valves or excessive fittings on the suction side can restrict flow. Ensure valves are fully open and minimize unnecessary components. For discharge cavitation, verify that discharge valves are not overly throttled, as this can cause recirculation.
• Install a Booster Pump: A booster pump can increase suction pressure, raising NPSHA to prevent suction cavitation, especially in systems with long suction lines or elevation changes.
• Adjust Operating Conditions: For discharge cavitation, increase flow rates to operate the pump closer to its best efficiency point (BEP). Install VFDs or adjust discharge valves to maintain adequate flow and prevent recirculation.
• Replace the Pump: If cavitation is still occurring, replace the pump with one better suited to the system. For suction cavitation, select a pump with a lower NPSHR. For discharge cavitation, choose a pump sized correctly for the system’s flow and head requirements.

All of these options can be expensive and time-consuming, but avoiding pump cavitation will ultimately prolong the life of your pump and improve overall system efficiency. Use monitoring tools, such as a vibration monitor, to confirm that cavitation is resolved.

The Bottom Line on Pump Cavitation

Pump cavitation is an expensive problem to have. It increases maintenance costs, damages components, and can lead to shortened pump lifespans. But it’s also preventable. Vibration monitoring combined with vibration analysis services can offer an early warning sign that cavitation is occurring, giving you time to intervene before costly, irreversible damage is done.