Cavitation

Cavitation refers to the formation and subsequent sudden collapse of vapor-filled cavities (cavitation bubbles) in a liquid. This phenomenon occurs when the hydraulic pressure in a flowing medium locally drops below the vapor pressure of the liquid – typically at narrow points or at high flow velocities. When the pressure rises again, the bubbles implode, releasing microscopic shock waves that act on nearby surfaces.

In fluid systems such as pumps, valves, or nozzles, cavitation is an undesirable effect because it can cause mechanical damage to component surfaces and impair system efficiency. The phenomenon is especially critical in high-pressure systems, where high flow velocities and large pressure differentials occur.

It is important to distinguish cavitation from gas cavitation, which does not involve vapor formation but the release of dissolved gases into bubbles. These gas-filled bubbles generally collapse less violently and have a lower erosive effect. In contrast, true vapor cavitation often leads to significant material erosion and audible side effects.

The formation of cavitation can be explained by fundamental fluid dynamics – especially Bernoulli’s principle. It states that in a frictionless flow, an increase in velocity is accompanied by a decrease in pressure. In areas of high velocity – such as nozzles, impeller blades, or narrow pipe sections – local pressure can drop below the vapor pressure of the fluid. At this point, the liquid begins to vaporize even though the temperature remains below its boiling point. Vapor bubbles form within the medium.

How does cavitation occur?

1. Flow through a technical system: A liquid (e.g., water) flows through a component like a pump, valve, or nozzle. Velocity and pressure change locally.
2. Local pressure drop due to acceleration: When the flow passes through a narrow section or over a fast-oscillating plunger, fluid velocity increases. According to Bernoulli’s principle, this leads to a local pressure drop.
3. Pressure falls below the liquid’s vapor pressure: If the pressure drops below the vapor pressure of the liquid, vaporization begins without a temperature change. Small vapor bubbles form in the flow.
4. Transport of vapor bubbles to higher-pressure zones: The bubbles are carried with the flow. Downstream – after the constriction or in the pump’s pressure stroke – pressure rises again.
5. Implosion of vapor bubbles (collapse): When the surrounding pressure exceeds the bubble stability, they collapse violently. Local pressures of several hundred bar and temperatures of several thousand kelvin can occur in very small volumes.
6. Formation of microjets and shock waves: The collapse generates shock waves or microjets that strike adjacent solid surfaces (e.g., the plunger) at high speed.
7. Material stress and potential damage: Repeated mechanical impacts can cause erosion, cracking, or surface damage. The structure may be “pitted” or “perforated.”

Not all cavitation is the same. Depending on its cause and development, different forms can be distinguished – each with unique physical characteristics and effects on technical systems. Especially in pump technology, understanding these variants is important to assess risks and apply countermeasures effectively.

The most important types are:

Vapor cavitation

This classic form occurs when local pressure falls below the vapor pressure of a liquid. As a result, vapor bubbles form and collapse during subsequent pressure recovery, generating intense mechanical loads. Vapor cavitation is the most common and also the most critical type due to its strong erosive effects on material surfaces.

Gas cavitation

In contrast to vapor cavitation, gas cavitation is caused by the release of dissolved gases (e.g., due to pressure drop), not by vaporization. These gas-filled bubbles have lower density and collapse more slowly and weakly, resulting in less damage. Nonetheless, gas cavitation can impair system performance and damage components.

Gap cavitation

This special form occurs in very narrow gaps or spaces, such as valve guides. Tiny vapor bubbles form under restricted flow conditions. Due to the confined geometry, the bubbles cannot be fully flushed out, leading to concentrated local erosion. Gap cavitation is often overlooked but can cause premature wear in pumps.

Cavitation is undesirable in many hydraulic applications because it affects both the mechanical integrity and the performance of systems and components. The consequences extend beyond surface damage to include acoustics and fluid delivery behavior.

Typical effects include:

• Material loss: Local erosion due to bubble collapse, often visible as crater-like surface damage.
• Noise: Rattling, knocking, or metallic clicking sounds during operation.
• Loss of performance: Reduced pump efficiency due to disrupted flow and pressure drops.
• Fluid alteration: Cloudiness, temperature increase, or unstable delivery behavior.
• Pulsations, pressure spikes.

Cavitation in Pump Units

Cavitation is common and particularly critical in centrifugal and positive displacement pumps because of the high velocities and pressure differences – ideal conditions for bubble formation and collapse. Cavitation typically occurs in the pump’s working chamber.

The consequences are significant: Material erosion on plungers, seals, or housings can greatly shorten component lifespan. At the same time, reduced cylinder filling lowers the pump’s efficiency, leading to malfunctions and inefficient energy use. Acoustic signals or performance fluctuations are early warning signs.

If left unaddressed, cavitation increases maintenance needs, shortens service intervals, and can even cause unplanned downtime – especially in continuously operating high-pressure systems with tight tolerances. This is why early detection and effective prevention of cavitation are critical in pump design and operation monitoring.

Are there low-cavitation pumps?

Yes – by using flow-optimized designs, ideal inlet conditions, and well-defined operating points, pumps can be designed to significantly reduce cavitation. KAMAT develops high-pressure pumps with a strong focus on cavitation-critical applications. Thanks to thoughtful geometries and application-specific design, these pumps are built for stable operation even under demanding conditions.

The most effective measure against cavitation is to prevent it from occurring in the first place – through proper system design and appropriate operating conditions. A key factor is the so-called NPSH value (Net Positive Suction Head). A distinction is made between available NPSH (NPSHa) and required NPSH (NPSHr). Cavitation can be safely avoided only if NPSHa is consistently greater than NPSHr. Both values can be calculated during the design phase.

Pump design, material selection, and hydraulic conditions also play a major role in determining cavitation risk. Depending on the application, the following measures can help reduce it:

• Larger pipe diameters: Reduce flow velocity and prevent local pressure drops.
• Temperature management: Lowers the liquid’s vapor pressure – especially relevant for media with low boiling points.
• Adjusting speed and delivery head: Reduces mechanical stress and minimizes extreme pressure conditions in the system.

A carefully coordinated overall concept – from pump design to system planning and operation – is essential to permanently avoid cavitation, especially in high-demand applications such as high-pressure systems.