9 August, 2022

Practical contamination management for hydraulic systems

10 September, 2015

By Iain Hanson, general manager – fluid power, Brammer UK.

Integral to plant and equipment operation, hydraulic system failure can be catastrophic, resulting in unplanned downtime. By far the most frequent cause of system failure is contamination of fluid, which accounts for 70 to 80 per cent of failures. Preventative action should therefore be a top priority for maintenance engineers to prevent hydraulic equipment suffering premature component failure, extended downtime – and the costs associated with both. Additionally, as contamination can increase wear and shorten machinery and lubricant service life, any action to minimise contamination will have positive effects elsewhere in the system.


Contamination can be broken down into three categories – gaseous, liquid and solid (particulate). Gaseous contamination can impair the hydraulic medium’s lubricating properties, increasing ‘metal to metal’ contact, creating wear and a likely increase in other contamination types. Air can also cause cavitation and impact on pump performance.

Liquid contamination in the form of water can drastically reduce the lubricating properties of the hydraulic fluid, and also cause rust. Cross-contamination can also be an issue – for example, mineral oil-based hydraulic fluids are incompatible with water glycol hydraulic fluids and interfere with their anti-wear properties, which can result in the ‘varnishing’ of system components. Similarly, mineral oil also reacts with fatty acids contained in water glycol products, forming a ‘white soap’ effect which can block filters and strainers.

Particulate contamination can be further divided into three categories. Soft particles, like fibres, gasket or seal abrasion particles, rubber and paint, are likely to cause only minimal damage. However, both hard particles – iron, steel, bronze, brass and aluminium – and extremely hard particles, like corundum, scale, rust and furnace dross, are highly abrasive and can cause significant surface degradation.

Particulate contamination can also cause spontaneous outages, including valve blockages, substantial pump damage, and blown seals and gaskets.

Once any contamination occurs, a chain reaction of wear can result. Gaps grow larger, oil leakages increase in size, component operating efficiency decreases, blockages can occur, and metering edges are worn away. Many ‘sudden’ failures are actually the result of cumulative damage over time, meaning that even soft particulate contamination must be constantly countered.

The usual causes of contamination are, however, avoidable. Using incorrect fluids, inadequate oil drum and container marking, storing oil drums in contaminated environments or using unclean or contaminated containers to transfer oil, or an incorrect filter trolley to transfer fluid, can all be prevented with simple control measures and training for operatives.

In order to determine ‘how clean is clean?’, ISO 4406:99 has been developed to provide a standard for measuring and reporting particulate contamination in fluids. This is based around the number of particles of three different sizes per unit volume of fluid, and helps engineers understand how equipment performs at different cleanliness levels through evaluating the level of contamination protection needed in each application, based on operating pressure.

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