What Is cavitation in a hydraulic system and how does fluid affect it?

Hydraulic systems are built upon one promise that is fundamental: an incompressible fluid transfers force consistently, without loss. Cavitation is a breach of this promise. It is among the most destructive effects of fluid power, and most importantly, it is among many that are misunderstood. Engineers tend to view cavitation as a problem with pumps, but in actual fact, it's a systemwide failure mechanism that has the hydraulic fluid at the heart of it.

This post will explain the causes of cavitation, how it occurs, what harm it causes, and the most important thing: how your choice and handling of hydraulic fluid will determine whether cavitation will remain a unique occasion or become a frequent nightmare.

What is cavitation?

Cavitation is the process of the forming and rapid collapse of bubbles of vapor within the hydraulic fluid. It occurs when the pressure within a fluid decreases to below the liquid's vapor pressure at operating temperatures. When that happens, the fluid doesn't just flow, but it partially vaporizes, creating tiny gas-filled cavities or voids in the flow stream.

The voids in these are only temporary. As the fluid flows from the low-pressure region into the higher-pressure zone downstream, bubbles begin to collapse and then explode with tremendous force and speed. The energy released by this implosion causes damage. The metal surfaces around the collapsed bubbles are exposed to tiny jets of fluid that move at speed that pits, erodes, and even fatigues the steel that has been hardened.

There are two kinds of cavitation that occur in hydraulic systems. The first is cavitation that is truly occurring, also known as vaporization cavitation, in which the fluid itself evaporates because of low pressure. The other is pseudo-cavitation or gaseous cavitation. Gas or air that is dissolved escapes from solution when pressure decreases, creating bubbles that fall as pressure increases. Both of them are harmful, but their underlying causes and conditions of the fluid that trigger them differ.

Where does cavitation take place?

Cavitation typically occurs close to or around the pump's inlet. Pumps that use hydraulics make suction at their ports for the inlet to pull fluid out of the reservoir. When the inlet pressure is below the vapor pressure of the fluid (due to a suction strainer that is blocked or an insufficient suction line or a lengthy or high suction run or the fluid is too viscous to flow at a sufficient speed -- the process of vaporization begins at the point that pumps are trying to draw the fluid into.

When they are formed, these bubbles are transported through the compression zone of the pump, which is where they break down against internal surfaces like pistons, gears, vanes, and valve plates. The damage is gradual and cumulative.

The cavitation can also happen at other locations in the system, including orifices and control valves where pressure drops dramatically and in return lines that are not properly sized and filter elements with high restriction, or anyplace the velocity of fluid increases and the pressure decreases in a similar manner in accordance with Bernoulli's theory.

Recognizing the signals

Cavitation can be detected. The first sign is sound—the characteristic crackling, rattling, or grinding sound coming from the pump. It is sometimes called gravel or marbles falling into the housing. The sound is the result of bubbles bursting.

Other indicators include unstable actuator movement, decreased system flow and pressure, and elevated temperature of the fluid (since cavitation boosts the rate of dissipation of energy) as well as visible foaming or aeration in the reservoir. In time, a closer inspection of the components of the pump will reveal pitted or cratered surfaces on parts made of metal—the hallmark damage of cavitation over time.

It is costly to ignore these warning signs. The replacement of a pump is one among the most expensive maintenance activities within a hydraulic system and cavitation is the primary reason for premature failure of a pump.

How can hydraulic fluid directly influence cavitation?

This is when the selection of fluids and their management becomes essential. The hydraulic fluid isn't a passive medium—its physical properties determine its vulnerability to cavitation in various interconnected ways.

Viscosity, flowability and viscosity

Viscosity is perhaps the most crucial property of a fluid when it comes to cavitation. A fluid that's too viscous is unable to flow quickly enough to fill up the pump's inlet at the speed the pump requires. This causes inlet starvation and a decrease in pressure on the suction side, which causes cavitation. This is the reason cold starts are especially dangerous because most mineral hydraulic fluids are significantly viscous at lower temperatures as well as running the system at full speed prior to the fluid's warming to the operating temperature produces ideal cavitation conditions.

Making sure you use the right ISO viscosity grade that is appropriate for the temperature range for your application isn't optional. It's a fundamental protection against cavitation during startup.

Vapor pressure

Each fluid has a specific "vapor pressure," which is the point at which it starts to evaporate at a certain temperature. Fluids that have higher vapor pressures will vaporize much more easily in low-pressure conditions, which makes them more vulnerable to true cavitation. The best hydraulic oils that are formulated with high-quality base stocks have lower vapor pressures, which makes them less prone to vaporization, even under the stress of suction.

Hydraulic fluids that are water-based, like water-glycol and high-water-content fluids, tend to have much higher vapor pressures than synthetic or mineral oils and are therefore more susceptible to cavitation. Systems that operate on water-based fluids need to be designed with larger suction lines, lower pump speeds, and tighter filtering to counteract this.

The release of the air as well as properties that defoam

Pseudo-cavitation, the formation of bubbles due to dissolved air or entrapped air, relies heavily on the fluid's ability to release air rapidly and stop foam. Hydraulic oils are infused with anti-foam as well as air-release ingredients for precisely this reason. Fluids that hold suspended air carry the air to the pump, in which the change in pressure causes bubbles to form and then collapse.

Fluid degradation causes the destruction of the additives. If contaminated, oxidized, or thermally stressed, the fluid loses its release of air ability and is more susceptible to foam formation and prone to forming foam, which dramatically increases the risk of gaseous cavitation, even in well-designed systems. This is the reason why the monitoring of fluid condition—and not only levels of monitoring—is important.

Fluid condition and contamination

The contamination of water, particles, and oxidation-related byproducts all reduce a fluid's capacity to stop cavitation. The presence of water in oil is particularly difficult to deal with because it raises its effective vapor pressure in a fluid mixture, increases the formation of bubbles, and also promotes foaming. Even water contamination as low as 0.1 percent by volume could profoundly alter the cavitation process in a delicate system.

Cleaning and maintaining a dry, clean fluid isn't a secondary matter, but it is the prevention of cavitation.

Temperature of the fluid

As temperatures increase, vapor pressure increases and viscosity decreases. At higher temperatures, the fluid is more susceptible to vaporization cavitation and moves more easily. However, the lower viscosity may decrease the strength of the film and reduce lubrication, causing wear on pumps in areas where cavitation damage is already taking place. A high-temperature operation that does not have adequate cooling results in a failure cycle. Damage to the cavitation results in heat, which decreases viscosity and increases the pressure of vapor, and the conditions for further cavitation are made worse.

Strategies for prevention

To prevent cavitation, it's dependent on maintaining the proper inlet pressure and maintaining that fluid in good working order. In terms of practicality, this involves keeping suction lines straight and properly measured by using suction strainers with adequate flow ratings and changing the strainers on a regular basis, providing adequate warm-up times before high-load operation. Also, checking the fluid's viscosity, contamination levels, and water content on a regular basis.

On the fluid end, that means selecting an oil that has the right viscosity for your specific temperature, as well as ensuring that the oil has air-release and anti-foam characteristics and setting a fluid change interval based on the condition monitoring instead of the calendar's time alone.

System designers must also ensure that the conditions of the pump's inlet meet manufacturer specifications, which typically include the minimum pressure inlet being between 0.7 and 1.0 bar absolute and that the tank baffles and return line positioning reduce air entrapment into the suction area.

Cavitation is not just an issue with mechanical origins It is an issue of fluids. The temperature, vapor pressure, and air handling characteristics as well as the general condition of hydraulic fluid will determine whether the system functions properly or is tearing itself up one shard at one time. Repair of the component and pump are essential responses to damage caused by cavitation; however, they address the cause. The cause usually can be traced back to an unsatisfactory design or a flaw in the selection of fluids and maintenance.