Case Study: Hydraulic fluid degradation

A particleboard plant used water-soluble oil in its high-pressure hydraulic system. The oil consisted of 98% water, 1.5% soluble oil and 0.5% ISO100 AW100, an oil that improves the fluid’s lubricating and rust-preventive characteristics. The 7,000-gallon hydraulic fluid system started to produce large chunks of debris and slimy material, which clogged filters and strainers. This chronic problem ultimately led to a plugged poppet valve, which caused the particleboard in the press to cook onto the platens. This 48-hour catastrophic shutdown of the particleboard line cost one million dollars in lost production and cleanup costs.

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When the problem was first observed, several parties were brought in to identify the source. Analysis using standard techniques led to the conclusion that oxidation was generating insoluble by-products. The fluid was drained and replaced. Shortly thereafter, however, the catastrophic plant shutdown reccurred.

Obviously, oxidation wasn’t the root cause of the problem.

The plant then performed a detailed analysis of all of the possible contamination sources including:

  • Water used in the system
  • Soluble oil
  • AW100 hydraulic oil
  • Resin (used in gluing the particle board)
  • Hydraulic filter debris
  • Air filter debris (for potential sources of airborne contaminants)
  • Debris from strainers
  • Debris separated from oil

Analytical tests included physical and chemical separation methods, FTIR characterization, SEM/EDS elemental analysis, ICP and TGA to identify and characterize the contaminants and fluids. The organic material consisted primarily of oxidation products and phosphates. The inorganic material was identified as magnetite, a form of iron oxide.

The debris consisted of approximately 20% iron oxide, 20% water and 60% oxidized hydrocarbon products. This information suggested that there were two causes of the debris formation.

The first source was oil oxidation as stated by the first round of analysis. However, the second source was iron oxide from the pond water used in blending the hydraulic fluid mixture. The water contained unacceptably high levels of soluble iron. Once in the system, the iron precipitated out as iron oxide. The source of other contaminants, such as silicon and calcium, were traced to the pond water.

The insoluble iron oxide collected on the filters and acted as a catalyst, causing severe oxidation of the hydraulic fluid. The oxidation produced an oil-insoluble tar-like substance.

The lubricant contained the anti-wear additive zinc dialkyl-dithiophosphate (ZDDP), which also was supposed to provide antioxidant protection. Because ZDDP additives are unstable in an aqueous environment, the lubricant had no oxidation control system. The ZDDP decomposed to phosphate, which was observed in the debris, and wasn’t able to control the oxidation. The accelerated oxidation by the iron was a second cause.

After identifying these causes, the plant drained and cleaned the hydraulic system twice and recharged it with purified water. In addition, the plant stopped using AW100 containing ZDDP. The plant built a purification system for the incoming water to remove the iron contamination. These actions stopped the deposit formation and the hydraulic system has since been running without incident.

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