The invisible contaminant

05 April, 2023

Bubbles in a lubrication system can undermine operating efficiency, inhibit heat removal, increase wear, and, thus, lead to higher maintenance costs. Too often, equipment operators dismiss air contamination as inevitable. But there are steps that maintenance personnel can take to reduce or eliminate the harmful effects of air contamination. Gas-toLiquids (GTL) hydraulic oils offer faster air release and better foam control compared with mineral base oils of the same viscosity.

Air intrusion can take many forms, but foam and entrained air represent the biggest threats to equipment. Entrained air refers to bubbles suspended below the fluid’s surface that create air pockets in the lubricants. Foaming results when the bubbles rise to the surface of the hydraulic fluid or other lubricants. In both cases, the air pockets can impede the flow of fluids and leave equipment vulnerable to wear.

Surface foam in well-designed and correctly filled oil sumps rarely causes issues during equipment operation. However, foaming may indicate more fundamental problems such as oil contamination or degradation. Excessive foaming can cause the oil level to drop so much that the system inlet becomes exposed at the surface, which results in oil starvation. Excessive foaming also can cause the oil to overflow onto potentially hot surfaces.

Although foaming is more visible and tends to concern maintenance personnel most, entrained air causes the most damage and the greatest loss of efficiency.

Entrained air in a lubrication system can be harder to identify because it has few external or visual indicators. Air becomes entrained through normal engine vibration, flow surges from retracting cylinders, leaks, incorrect oil level control and working on slopes. Entrained air can cause cavitation, micro-dieseling and increased noise or vibration, all of which can lead to excessive component wear. It may also reduce equipment power, responsiveness, and efficiency.

Tackling foaming

The nature and extent of foaming result from the properties of a lubricant’s base oil and the additive packages in the formulation. Consequently, antifoaming additives are typically used to control excessive foaming. Unfortunately, most of these additives rely on large silicon particles that reduce the surface tension and cause the bubbles to rupture. Industrial applications such as hydraulic systems typically have filtration systems designed to screen out such large particles, which undermines the effectiveness of the additives.

Shell Lubricants has studied the causes and effects of air contamination and has developed a deep understanding of foaming and entrained air and how they interact with a wide variety of systems.

Working with equipment manufacturers and researchers, Shell Lubricants identified and tested almost 10 plausible antifoaming candidates, five of which had good filterability performance. The most promising, which are based on silicon antifoaming agents, have excellent foam control and product compatibility, and, importantly, remain effective after filtration.