Contamination: Action needed

About 75% of hydraulic system failures relate to contamination, which can also increase wear and shorten machinery and lubricant service life. Taking action to minimise contamination is time well spent, as Kevin Lacey of Brammer UK explains

Contamination can be gaseous, liquid and solid (particulate). Gaseous contamination, ie air, can impair the hydraulic medium's lubricating properties, increasing metal-to-metal contact, creating wear and a likely increase in other types of contamination. Air can also cause cavitation and impact on pump performance.

Liquid contamination (water) can affect the hydraulic medium's lubricating properties, and cause rust. There are also issues caused by cross-contamination - mineral oil-based hydraulic fluids, which interfere with the anti-wear properties of water glycol hydraulic fluids, 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.

There are three categories of particulate contamination. Soft particles like fibres, gasket or seal abrasion particles, rubber and paint, may only have the potential for limited damage. However, hard particles (iron, steel, bronze, brass and aluminium) and extremely hard particles, like corundum, scale, rust and furnace dross, are highly abrasive and can significantly degrade surfaces.

Particulate contamination can cause valve blockages, pump damage, and seal and gasket destruction. A chain reaction can result. Gaps grow larger, leakage oil-flows increase, component operating efficiency decreases, blockages can occur, and metering edges wear away, creating control inaccuracies.

The most common standards for measuring particulate contamination levels are the NAS class or ISO4406:99, which considers the number of particles of three different sizes per unit volume of fluid. These standards also help understand how equipment performs at different cleanliness levels, allowing evaluation of the contamination protection needed in each application, based on operating pressure.

A particle count analysis should be undertaken, with corrective action required if fluid cleanliness is below the desired target.

Filtration 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 level can be established by considering duty, component sensitivity, life expectancy, component replacement costs, downtime costs, safety, and environmental considerations. With each of these weighted and the required cleanliness level established, appropriate filtration can be implemented.

A dedicated off-line filtration system operates at a constant flow, maximising filter life and performance, while trolleys connected to the power unit can offer a secondary filtration system. Contaminant introduction can also be reduced by using flat face couplings with offline filter trolleys.

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; the latter refers to the capability to retain all particulate of that size.

Good housekeeping The filler breather life indicator must be clearly visible, enabling easy checking for when the unit should be changed. Off-line filtration systems should be kept clean, with QRC couplings wiped with a clean, lint-free cloth before connection to minimise 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. Drum tops should 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 contaminant entry. 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 be stored in a clean, dry, dust-free environment, with packaging checked for intactness against contaminant ingress.

Each filter trolley should be marked stating for which fluid or oil type it is suitable. Dust caps should be fitted in all ports of hydraulic cylinders and valves. Filters which appear damaged should be replaced.

Regular sampling - at least monthly - will establish any changes in the fluid's physical or chemical properties and excessive water or particulate contamination. The latter will indicate that 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.

Contamination management is just one aspect of best practice and should be combined with functions like hydraulic hose inspection and power unit temperature testing.

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