ALS Tribology eSource
February 28, 2012

Cavitation in Lubrication Systems
by David Doyle, CLS, OMA I & II
Vice President & Operations Manager

After contamination, cavitation is one of the most damaging processes in pressurized lubrication circulating systems. Gear pumps can be particularly susceptible to damage by cavitation.

The cavitation process starts when a sudden pressure drop below the existing vapor pressure of the fluid creates tiny air or vapor bubbles. Cavitation occurs when the fluid returns to high pressure and the air or vapor bubbles collapse.  When the fluid rushes in to fill the void created by the collapsing bubble the process occurs so quickly and violently that collision of the fluid surface inward from 360 degrees creates a miniature implosion, which in turn creates damaging shock waves that impact metal surfaces. The instantaneous collapse of the air or vapor bubbles create microjets as the liquid surrounding the bubble moves towards the center, creating intense shock waves.

The damaging impact of these shock waves can create pitting, metal fatigue, and surface erosion of metal surfaces adjacent to the area of cavitation or regions not in direct contact by the effect of a wave-hammer. High heat is also created during the cavitation process which can lead to oil oxidation, associated oxidation contaminants and varnish precursors.

Inherent design of lubrication circulating systems cannot always eliminate regions of transition from high pressure to low pressure and back to high pressure, but measures can be taken to minimize air entrainment that feeds the cavitation process. By eliminating sources of air entrainment cavitation can be greatly reduced. Poor tank design and maintenance can add to air entrainment. Reservoir fluid level can also to be a factor.  A low fluid level allows air to be sucked into the circulating pump, or an overfilled fluid level can create turbulence and churning of the fluid in the reservoir.  Inadequate reservoir settling time (residence time) will reduce the opportunity for air release.  Poor flow from the reservoir into the pump can also create air entrapment. This can be a result of insufficient headspace to the pump, plugged strainers, or a poor suction line design for the fluid viscosity.

Lack of system maintenance is also a factor in causing air entrainment. Poor seals and/or, leaking seals will allow air to enter. It’s logical to assume that if fluid can leak out of a system while it is running, air can leak in once the system shuts down and the internal pressure drops. Water contamination can lead to vapor bubbles resulting in cavitation in the same manner as entrained air. Some changes in system design can also help reduce some of the cavitation mechanics. This can include; increasing pressures of pump inlet suction and valve outlets, minimizing sudden pressure drops due to opening of directional control valves, and using a fluid with lower vapor pressure or lower viscosity.



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