Practical contamination management for hydraulic systems

10 September, 2015

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.

Categorisation

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.

Seven steps

The correct cleanliness level can be established by looking at each of the following seven steps in turn:

• Duty/intended usage.

• Component sensitivity.

• Life expectancy.

• Cost of component replacement.

• Cost of downtime.

• Safety.

• Environmental considerations.

Once each of these is weighted and the required cleanliness level established, an appropriate filtration system can be implemented.

Two main types of filtration system can be specified depending on the needs of the application. A dedicated off-line filtration system operates at a constant flow, maximising filter life and performance, while trolleys can offer a secondary filtration system if connected to the power unit. The likelihood of contaminant introduction can also be reduced by using flat face couplings in conjunction with offline filter trolleys.

Any filtration system is only as good as its filters, which have either a nominal or absolute pore size rating. A nominal rating describes the ability to retain the majority of particulate at the rated pore size, while the absolute rating refers to the filtering media’s capability to retain all particulate of that size. Beta ratios can also be used to determine if a filter is designed for highly efficient removal.

Once installed, the filler breather life indicator must be clearly visible, enabling easy checking for when the unit should be changed. If the unit isn’t changed when indicated, contamination can result – particularly water ingress. Off-line filtration systems should be kept clean, with QRC couplings wiped with a clean, lint-free cloth before connection to minimise the risk of contamination between QRC faces.

To assist in contamination prevention, oil drums should be stored in clean conditions and clearly labelled stating whether they contain clean or dirty oil and what type of oil is contained. Drum tops should always be kept clean. Taps should be fitted correctly, with the tap at the bottom of the drum pointing downwards. The bung should be slightly unscrewed when filling from the drum and immediately tightened to prevent contaminants entering.

When topping up hydraulic power units, the suction pipe should be wiped with a clean, lint-free cloth before lowering into the drum. Spare parts should always be stored in a clean, dry, dust-free environment, with packaging checked to ensure it is intact to prevent contaminant ingress.

Each filter trolley should be clearly marked stating which fluid or oil type it is suitable for, while dust caps should be fitted in all ports of hydraulic cylinders and valves. Any filters which appear damaged should be replaced.

Regular sampling

Once the management system is established, regular sampling should be undertaken to establish any changes in the fluid’s physical or chemical properties and excessive water or particulate contamination. The latter will indicate that the filters are not keeping the system clean, either because they are not suitable for the task, not maintained properly, or the system is subject to excessive ongoing corrosion and wear. Samples should be taken at least monthly.

Contamination management is just one aspect of best practice in hydraulic system maintenance and should be combined with other functions including hydraulic hose inspection and power unit temperature testing as part of a complete maintenance regime.

A specialist maintenance, repair and overhaul (MRO) service provider can advise on implementing complete hydraulic systems maintenance regimes and provide all necessary consumables.

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