Hydraulic Fluid Contamination: Solids, Liquids and Gases
Hydraulic fluid contamination is responsible for the majority of system failures, making contamination control essential for optimal performance and longevity.
In this article, the first in a series produced in collaboration with Ikron, we explore the three main types of hydraulic fluid contamination and their impact on system reliability.
Contaminants in hydraulic systems are categorised into three primary types:
- Solid contaminants (particles and debris)
- Liquid contaminants (water and other fluids)
- Gaseous contaminants (air and other entrapped gases)
This classification serves as a critical first step in contamination analysis, helping to identify the likely source of hydraulic oil contamination and guide appropriate corrective actions.
Solid Contamination
Manufacturing and Assembly
During production, hydraulic components may retain manufacturing residues such as metal shavings, welding slags, casting dust, rubber fragments, and rag fibres. Protective oils used in storage can also trap significant amounts of dust, introducing contaminants into the system.
System Operation
Continuous use of hydraulic systems generates wear-related particles:
- Friction between moving parts produces metallic debris and sealing material fragments, which remain in suspension before settling in the reservoir.
- External-facing components, such as cylinder rods, driveshafts, and control valve stems, accumulate dust and airborne contaminants despite scraper rings.
- Inadequate tank breathers lacking proper filtration allow environmental dust to enter the system unchecked.
Maintenance Activities
Improper maintenance practices can introduce abrasive substances into hydraulic fluid:
- Component replacements, disassembly, and reassembly may introduce contaminants if performed without proper cleanliness.
- Fluid refilling or replacement can allow foreign particles to enter if not executed with precision.
Ensuring rigorous contamination control across manufacturing, operation, and maintenance is crucial for system longevity and optimal performance.
Liquid Contamination in Hydraulic Fluid
Among the various forms of hydraulic fluid contamination, water is the most common and can significantly impact system performance.
How does water enter the system?
Water infiltration can occur through multiple pathways, including:
- Improper oil tank storage, allowing moisture accumulation.
- Leaks in water/oil heat exchangers, introducing unwanted fluids.
- Condensation, forming in storage tanks and hydraulic reservoirs due to temperature fluctuations.
- Ventilation system exposure, where water vapour enters through unprotected openings.
- Moisture absorption through cylinder rods, particularly in humid environments.
- Cross-contamination, from liquids used in machine cleaning processes.
Effects and Acceptable Limits
The risk of hydraulic fluid contamination is especially high in machines operating in humid conditions or utilizing water-based substances in their processes. While trace amounts of dissolved water may have minimal impact, excessive moisture can degrade fluid properties, accelerate component wear, and lead to system failure.
For reference, an upper limit of 300 PPM (parts per million) is generally acceptable to maintain system reliability.
Gaseous Contamination in Hydraulic Fluid
Among the various forms of hydraulic fluid contamination, air intrusion is a common issue that can affect system performance.
Dissolved Air in Hydraulic Fluids
Under normal conditions, hydraulic fluids contain dissolved air at the following approximate percentages:
- 3–4% in glycol-water mixtures
- 5–6% in water-oil emulsions
- 7–9% in synthetic fluids
- 8–10% in mineral oils
These levels can increase due to several factors, including:
- Pump suction line leaks, allowing air ingestion.
- Faulty seals, enabling air penetration into the system.
- Low oil levels, exposing fluid to excessive aeration.
- Filling and maintenance activities, inadvertently introducing air.
Impact of Air Contamination
Excessive gaseous contamination can lead to cavitation, foaming, and reduced lubrication efficiency, ultimately compromising hydraulic system reliability.
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