Contaminants of hydraulic fluid 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 hydraulic fluid, corrective action is required to prevent damage to both the fluid and system components.
Air can be present in four forms:
- Free air - such as a pocket of air trapped in part of a system.
- Dissolved air - hydraulic fluid contains between 6 and 12 percent by volume of dissolved air.
- Entrained air - air bubbles typically less than 1 mm in diameter dispersed in the fluid.
- Foam - air bubbles typically greater than 1 mm in diameter that congregate on the surface of the fluid.
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 fluid degradation problem.
Why is entrained air bad?
Negative effects of entrained air include:
- Reduced bulk modulus, resulting in spongy operation and poor control system response.
- Increased heat-load.
- Reduced thermal conductivity.
- Fluid deterioration through increased oxidation and thermal degradation (dieseling).
- Reduced fluid viscosity, which leaves critical surfaces vulnerable to wear.
- Cavitation erosion.
- Increased noise levels.
- Decreased efficiency.
Gaseous Cavitation
As pointed out above, hydraulic fluid 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 fluid 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:
- 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.
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.
External Ingestion
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 fluid 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 gland of double-acting cylinders, and return fluid plunging into the reservoir (drop-pipes extending below minimum fluid level should be fitted to all return penetrations).
Prevention is better than cure
Like 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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"Thanks for the great work on the two publications, Insider Secrets to Hydraulics and Preventing Hydraulic Failures. I have been in the hydraulics business for the past 20 years and it is very difficult to find any decent material on hydraulic maintenance, troubleshooting and failure analysis. These two books cover it all in easy to understand language... I conduct hydraulic training courses and plan to purchase copies to distribute to my students to share your practical approach to understanding a not so understandable subject."
Paul W. Craven, Certified Fluid Power Specialist
Motion Industries, Inc.
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