Water in hydraulic oil? Effective filtration helps!

Hydraulic oil is not just any old operating medium. As an important part of the machine, hydraulic oil is crucially significant to the design, operation and maintenance of hydraulic systems.

Modern hydraulic systems are noted for shortened cycle times, higher temperatures and pressures, reduced clearance gaps between components and more compact designs, with smaller tanks and higher speeds of revolution. The requirements for the quality and cleanliness of the hydraulic oils used have therefore continually risen over recent years.

Awareness of the importance of fluids and their expert selection, care and monitoring is essential for users and designers of stationary and mobile hydraulic systems to ensure the efficient operation and value retention of plants and their components.

More than 3/4 of all failures on hydraulic systems, however, are caused by contaminated hydraulic oils.

In practice, many kinds of contaminants can have damaging effects on hydraulic oils. Contaminants are classified as solid - liquid - gaseous.

Liquid contaminants include oil mixtures and water.  After solid contaminants, water is the second most frequent cause of failures and malfunctions in hydraulic oil systems. For most hydraulic systems, a diagnosis of water contamination of hydraulic oil is serious, possibly leading to higher wear and leakages.

If there is too much water in the hydraulic oil, the hydraulic system suffers a number of negative effects: condensation water in the oil tank, for example, leads to hydrolysis and premature ageing of the hydraulic oil.                                                                               

Increased acid formation has the following consequences:

• Acceleration of oxidation and oil ageing
• Increased viscosity from oil oxidation
• Loss of lubricating quality
• Corrosion of oil-wetted surfaces made of materials such as bright metal alloys or iron
• Metal surfaces are attacked, releasing particles that can damage hydraulic hose lines and other system components

Hydraulic hose lines and any elastomeric seals in contact with water can swell and cause leaks. The direct consequences are more frequent maintenance, higher need for repairs and longer standstill times. Water can cause hydraulic oil to break down, leading to the complete failure of the hydraulic system.

Effects of increased water content in hydraulic oil:

• Corrosion of ferrous and non-ferrous metals
• Hydrolysis – ageing – oxidation
• Cavitation – partial vaporisation
• Reaction with additives – e.g. sticking, gumming
• Filtration – blocked by saponification products
• Reaction with components – swelling
• Increased friction with associated greater wear

There are various gateways through which water can enter a hydraulic system. Water always seeks a way. For example, fresh oil has an inherently elevated water content. Moisture can also enter the system past the rod sealing element of the cylinder.  Further gateways for water entry include: cleaning with steam jets or allowing rainwater to stand for too long on the lid of an oil drum. Likewise not to be underestimated are the effects of incorrectly stored fresh oil, a damp environment and oil-cooler leaks. Many people fail to consider the most frequent mode of entry: ventilation. Air has a high water vapour content and is sucked directly into the tank through the breather filter. When the air cools, the moisture it contains condenses on the surfaces inside the tank. With the dew point temperature being very close to the initial temperature for air with a high relative humidity, it takes only a very small fall in temperature inside the tank to cause condensation water to form on the walls and cover. These temperature fluctuations may occur on a daily basis in some systems. The dangerous results are soon visible in the form of droplets and rust.

The gateway for moisture entering the oil through the tank breather can be closed by using adsorber filters. They are mounted on the breather opening of the tank as a replacement for the conventional breather filter. The air sucked into the tank flows through the cylindrical adsorber filter. The moisture is taken up by a silica gel desiccant and bound into the granules. The integrated 3 μm filter element also separates all relevant particles and is the last element before the air flows into the tank. After passing through the filter, the relative humidity of the air is just around 10 %. The dew point temperature drops to such an extent that no condensation can occur in the tank even with larger temperature fluctuations.

The silica gel has another practical feature: as it becomes saturated, its colour changes from orange to green, starting from the bottom of the adsorber filter. If the colour changes from the top, the moisture has already been removed from the tank interior, which is now sustainably dehumidified. If the whole of the adsorber filter is green, then the filter has reached its maximum moisture adsorption capacity and must be replaced. A visual inspection of the filter at regular intervals is enough to be sure it is still working properly. Adsorber filters have the advantage that they can be retrofitted without the need to modify the hydraulic system. The work involved is limited to removing the conventional filter from the breather and installing the adsorber filter.

Other ways in which water can be removed from the system:

• Partial flow filter systems with water-separating filter elements
• Vacuum drying
• Oil change

Solutions for water-free hydraulic oil during operation:

• Always fill with fresh oil using fine-mesh filtration
• Continuous partial flow filtration with water-repelling filter elements
• Condition monitoring
• Regular independent laboratory analyses
• Use of an adsorber - breather dryer

Adsorber filters protect hydraulic systems against damage from condensation water, extend maintenance intervals and reduce the risk of unplanned stoppages. However, not all systems need breather dryers. It greatly depends upon the environmental conditions: the warmer the ambient air, the more water it can absorb and then release when it cools in the hydraulic system. In general, where high ambient temperatures prevail, such as in tyre-making factories, rolling mills or foundries, the risk of water damage increases. The same applies for environments in which high relative humidities prevail, for example in canal lock drives, sewage systems, hydraulic tanks on ships or hydraulic systems in hydro-electric power plants.

In these situations, it takes only small temperature fluctuations to cause condensation.

Irrespective of relative humidity and high ambient temperatures, large temperature fluctuations have the greatest potential for damage. If a plant is regularly exposed to a temperature difference of 10 °C or more, condensation water is bound to form in the hydraulic tank. Particular care is recommended when using bio oils. These rapidly biodegradable fluids are highly hygroscopic and therefore absorb atmospheric moisture through their surface without condensation necessarily occurring. The use of an adsorber filter on a hydraulic tank with bio oil is therefore always recommended as essential.

A wide range of solutions in the field of adsorber filters are available. Whether single-use or refillable filters with valves for long maintenance intervals: the optimum solution depends on many factors. Our cooperation partner HANSA‑FLEX has therefore developed a special questionnaire for determining the correct adsorber filter. There are also special solutions for use in accordance with ATEX 2014 or in the offshore industry available that guarantee compliance with EU conformity rules. Numerous components are available for the installation of adsorber filters. These include flange adapters, wall brackets, adapters for concurrent operation of partial flow filter systems and oil droplet separators.

All types of adsorber filters must protect the hydraulic oil used and the hydraulic system against condensation water damage, extend maintenance intervals and reduce the risk of unplanned, expensive stoppages.