Air is both a normal ingredient, and a contaminant, of hydraulic oil. It’s a normal ingredient because hydraulic fluid typically contains between 6 and 12 percent by volume of dissolved air. There’s no avoiding this. And provided this dissolved air stays dissolved, it poses no problem to the hydraulic system or the fluid.
But if this dissolved air comes out of solution, or air is ingressed via the pump intake for example, the result is entrained air: air bubbles typically less than 1-millimeter in diameter dispersed throughout the fluid.
Entrained air increases noise levels and decreases the efficiency of the system by reducing bulk modulus (fluid stiffness). And when entrained air bubbles are compressed at the pump outlet, extremely high peak temperatures result.
This compression of entrained air is said to be adiabatic, that is without transfer of heat to the surrounding fluid. However, localized heating of the oil film in contact with the air bubbles results in high-temperature ‘cracking’, oxidation and nitration of the oil (nitrogen in the air oxidizes to form highly reactive compounds), all of which accelerate degradation of the fluid. If that’s not bad enough, these rapidly imploding air (gas) bubbles hammer the pump’s valve plate and other critical surfaces resulting in erosive wear (cavitation erosion)–see inset photo above.
Prevention is really the only cure. Certain conditions cause dissolved air to come out of solution. When hydraulic oil temperature increases or static pressure decreases, air solubility is reduced and bubbles can form within the fluid. This release of dissolved air is known as gaseous cavitation. Decrease in static pressure and subsequent gaseous cavitation often occurs at the pump inlet, as a result of:
–Clogged inlet filters or suction strainers.
–Turbulence caused by intake-line isolation valves.
–Poorly designed inlet (diameter too small, length excessive, multiple bends).
–Collapsed or otherwise restricted intake line.
–Excessive lift (vertical distance between pump intake and minimum fluid level).
–Clogged or undersized reservoir breather.
Air entrainment can also occur through external ingestion. Like gaseous cavitation, this commonly occurs at the pump as a result of:
–Loose intake-line clamps or fittings.
–Porous intake lines.
–Low reservoir oil level.
–Faulty pump shaft seal.
Other causes of air ingestion include faulty or incorrectly adjusted load control valves, which can result in air being drawn past the rod-seals of double-acting cylinders, and return oil plunging into the reservoir. Drop-pipes extending below minimum oil level should be fitted to all return penetrations to prevent this.
Bottom line: allowing a hydraulic system to become contaminated with air is a costly mistake. And to discover 6 other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.