February 27, 2014

Lubricant Foaming
by David Doyle, CLS, OMA I, OMA II
Vice President, Americas

Excessive air in lubrication circulating systems is a form of contamination. Air in oil generally exists in several conditions:

  • Dissolved air – the oil looks clear. Most oil contains 8 – 10% dissolved air at room temperature.
  • Air entrainment – bubbles that are finely dispersed in the oil. The oil can look hazy or cloudy.
  • Foam - large bubbles that sit on top of the oil.

Foaming is the most severe form of air contamination in a lubricating system. This usually occurs when the surface tension of the fluid is too high to allow air bubbles to break after they form and rise to the surface of the fluid. Foam reduces the effectiveness of the lubricant, resulting in accelerated wear, overheating and housekeeping issues in severe cases.

Industrial lubricant formulation (gear oils, hydraulic oils, turbine oil), engine and transmission oils are normally treated with an antifoam additive in their formulation. Generally finished lubricants used in gear and circulating systems are tested by industry standards for foaming tendency and the degree of foam stability after foam has formed to ensure the product performs within specification. Often in service the antifoam additive may deplete or does not remain effective due to contaminants, fluid degradation or system design and maintenance issues.

Laboratory testing can confirm whether a foaming problem is lubricant related or system/mechanical related. If a lubricant used in the system that is experiencing foaming problems passes a foaming test according to industry standards then the problem can be attributed to system design or mechanical influences. Circulating systems that contribute to excessive air entrainment and foaming problems can be due to several factors:

  • Air leakage into pump suction through pipe fittings and bad seals
  • Inadequate fluid residence time in reservoirs
  • Cascading of oil into reservoir
  • Low oil level - air whipped into oil by gears or pumps
  • Overfilling of the sump with splash lubricated compartments

Systems that are well designed and maintained to prevent air entrainment and foam can still have issues when in-service lubricants develop poor foaming characteristics, which can be due to several causes:

  • Dirt and other particulate contaminants entering the lubricating system
  • Contamination with grease
  • Water contamination
  • General fluid degradation
  • Mixing or contamination with an incompatible lubricant
  • Depletion of antifoam additive

Note: Caution should be taken when recharging a system with additional antifoam additive. Too high of a treat rate can have the opposite effect.

Excessive air and foaming in a lubricating system contributes to a variety of problems:

  • Increased wear due to pump cavitation and lack of proper lubricating film on moving components
  • Oil oxidation
  • Safety issues and poor housing keeping due to foam overflowing into the work area
  • Pump shutdown due to lack of head pressure
  • Vibration and spongy, erratic hydraulic control

There are a couple of design adaptions in reservoirs which can help eliminate foam that is created. Increasing reservoir residence time of the fluid will aid dissipating air and foam. Getting air out of the system can be done by adding 100 mesh screen in the reservoir, approximately 30° from horizontal to coalesce entrained air and allow larger bubbles to rise to the surface when reservoir velocities are low. Also, a baffle installed in the return side of a reservoir will force liquid to travel under the baffle towards the suction side of the reservoir while keeping foam on top of the fluid level on the return side of the baffle.

Working with your lubricant supplier, system design expert, with the support of laboratory analysis, can determine the cause of foaming problems and determine the best course for corrective action.

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