Contaminants of hydraulic oil are broadly defined as any substance that impairs the proper functioning of the fluid. Air fits this definition and therefore when air becomes entrained in the oil, corrective action is required to prevent damage to both the oil and the other components in the hydraulic system.
Air can be present in four forms:
Of these four forms, entrained air is the most problematic. Pre-filling components and proper bleeding of the hydraulic system during start-up will usually eliminate free air. Small amounts of foam are cosmetic and generally do not pose a problem. However, if large volumes of foam are present, sufficient to cause the reservoir to overflow for example, this can be a symptom of a more serious air contamination and/or oil degradation problem.
Why is entrained air bad?
Negative effects of entrained air include:
As pointed out above, hydraulic oil can contain up to 12 percent dissolved air by volume. Certain conditions can cause this dissolved air to come out of solution, resulting in entrained air.
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 release of dissolved air can occur at the pump inlet, as a result of:
Other causes of decreased static pressure include changes in fluid velocity through conductors and orifices, flow transients and faulty or incorrectly adjusted anti-cavitation or load control valves.
Air entrainment can also occur through external ingestion. Like gaseous cavitation, this commonly occurs at the pump as a result of:
Other causes of air ingestion include faulty or incorrectly adjusted load control valves, which can result in air being drawn past the gland 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).
Prevention is better than cure
Like all other hydraulic problems, proper equipment maintenance will prevent the occurrence of most air contamination problems. As in all troubleshooting situations, when air contamination does occur, an understanding of the problem and a logical process of elimination are required to identify the root cause.
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