Hydraulic systems are integral to plant and equipment operation, meaning the impact of any hydraulic fluid contamination can be highly significant - indeed around 75% of all hydraulic system failures are in some way related to contamination. Moreover, as contamination can increase wear and shorten machinery and lubricant service life, any action to minimise contamination will be time well spent. Checking the system is leak-free is not enough. The simple maxim is that 'whatever is not allowed to enter the system does not have to be removed'.

Contamination can be broken down into three categories - gaseous, liquid and solid (particulate). Gaseous contamination (air) 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 impact negatively on the hydraulic medium's lubricating properties, and also cause rust. There are also issues caused by cross-contamination - the presence of mineral oil-based hydraulic fluids, which are incompatible with water glycol hydraulic fluids and interfere with their anti-wear properties, causing 'varnishing' of system components. Mineral oil also reacts with fatty acids contained in water glycol products, forming 'white soap' which can block filters and strainers. 

Particulate contamination can be sub-divided into three categories. Soft particles, like fibres, gasket or seal abrasion particles, rubber and paint, may only have the potential to create limited 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 contam-ination can also cause spontaneous outages, including valve blockages, substantial pump damage, and destroy seals and gaskets.

Once any contamination is introduced, a chain reaction of wear can result. Gaps grow larger, leakage oil-flows increase in size, component operating efficiency decreases, blockages can occur, and metering edges are worn away, creating control inaccuracies. Using incorrect fluids, inadequate oil drum and container marking, and storing oil drums in contaminated environments are three easily avoidable ways in which contamination can be introduced. Using unclean or contaminated containers to transfer oil, or an incorrect filter trolley to transfer fluid, can also cause contamination. A lack of operator understanding or training in the issues surrounding contamination also increases the likelihood of contaminant introduction.

The most common standards for measuring and reporting particulate contamination levels in fluids are the NAS class or ISO4406:99, which is based around the number of particles of three different sizes per unit volume of fluid. Standards like this are also used to help understand how equipment performs at different cleanliness levels, meaning they allow the evaluation of the level of contamination protection needed in each application, based on operating pressure. In the first instance a particle count analysis should be undertaken, with corrective action required if actual fluid cleanliness is below the desired target.

New hydraulic oil is not always clean and can contain around 50% more contaminants than would be considered acceptable under ISO4406:99. Furthermore, many people are unaware how clean their hydraulic oil should be. The correct cleanliness level can be established through a seven-step process - duty, component sensitivity, life expectancy, cost of component replacement, cost of downtime, safety, and environmental considerations. Once each of these is weighted and the required cleanliness level established, an appropriate filtration system can be implemented. 

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. Filters have a nominal or absolute pore size rating. The former 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. The filler breather life indicator must be clearly visible, enabling easy checking for when the unit should be changed. Failure to change the unit when indicated can result in contamination, 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.

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 against 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.

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, because they are inadequate for the task, not well enough maintained, 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 like 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.