Contamination control is a very broad term. Engineers use the term frequently, but with little understanding of its full impact.
It has been said that engineers frequently under-estimate the ability of contamination to enter the system and conversely over-estimate the ability of the system separation technology to remove it.
In terms of contamination, is it something we can live with? In theory, most engineers know this is a dangerous scenario leading to premature failure. However, in practice, engineers are often tolerant of contamination on the basis that machines don’t fail immediately. Thus work scheduling and cost management does not favour a focussed contamination control programme.
Contamination requires a definition. Contamination is any foreign matter within the lubricant that causes chemical and physical damage to both the lubricant and the machine. The typical contamination may be solids, moisture, acids, heat, aeration, fuel, coolants, and soot. The type and nature of the contaminant will vary based on the location of the equipment, the nature of the operation, and the machine type.
Contamination exists within the machine from three basic causes; that built in during manufacturer or maintenance activity, that ingested from outside the machine during operation, and that generated internally within the machine as a result of the degradation of the lubricant or the machine components.
Putting it mathematically:
Total Contamination = Contamination Built-in + Contamination Ingested + Contamination Generated
It is essential to understand these entry points so as to properly apply a cost effective control programme.
However,
Therefore, it makes sense to address the root causes of generated particles or wear debris (examples include balance, alignment of rotating systems) and the contamination built in and ingested. This reduces the overall impact and thus extends both the lubricant and machine life in a profitable manner.
Solid particles are the most destructive, yet most silent, form of machine wear. They are responsible for the abrasive damage and surface fatigue that results in machines. Work by Doctor EC Fitch in his book ‘Proactive Maintenance for Mechanical Systems’ 1 concluded that for one unit of improved cleanliness in the lubricant, a potential five units of life extension was feasible on the machine. Solids are the hidden enemy. In many instances, the solids are largely ingested silica from the atmosphere which is harder than many machine components. To the detriment of the machine. The size of particle that is most destructive is less than 10µm, and the limit of our vision where colour contrast allows is >40µm. Increased levels of solids will increase the viscosity of the fluid, and the fluid friction thus demanding more power to operate. As solids become crushed into smaller particles the relative toughness increases, thus increasing the wear potential. Adherent debris on machine surfaces can also increase abrasive wear.
At a chemical level, the wear debris generated from the components may catalytically react with the lubricant causing accelerated degradation. In addition, the wear material will attract the additives and thus reduce the potential service life. Degradation and oxidation of the oil from these causes can then result in varnish deposits on machine surfaces.
Moisture is deemed the second most destructive contaminant, yet it is the one most engineers respond to with urgency. Partly, this is due to the physical nature, it is more obvious as a contaminant. The usually likely causes are the mis-guided use of washdown sprays, or outdoor equipment and plant equipment exposed to extreme wet conditions. Physically, the water will cause an increase in the viscosity of the oil, which will impact the efficient operation and result in additional operational problems such as poor oil flow and increased temperature issues. As it also affects the film strength, scuffing may result in sliding contacts. In rolling contact surfaces, water may penetrate fatigue cracks and result in hydrogen embrittlement of the component. Water also encourages aeration problems and this will result in possible foaming and cavitation issues. Water will also cause premature filter blockage, and the filter medium may be weakened allowing further damage to result as it splits.
Chemical damage from water will include the increase in the oil’s oxidation, by as much as ten times. Because of the polar nature of the water, the additives may be attreacted into the water away from the normal target components, and hydrolysis may also take place thus creating further acid issues. Water may also be responsible for the cause of bacterial growth within the system, which, on hydraulics, could have detrimental effects on the operation and filter life. Most importantly, though, water is the main cause of rust, and in combination with acids, gives the greatest potential for corrosion. The rust, or ferrous oxide, formed is generally weakly magnetic, so the chance of detecting it is reduced, but rust is also much harder than normal iron, so will increase the wear rate.
Aeration will increase the presence of oxygen, thus promoting oxidation. Oxidation is a naturally occurring process in the oil that is slowed through the use of additives. However, oxidation results in the increase of the acid levels and the viscosity of the oil. The extended impact is a resultant sludge and varnish problem within the machine. The air bubbles will deplete the anti-foam additive. Moreover, the air bubble will compress in high pressure zones resulting in heat generation, the possible formation of soot from micro-dieseling as the vapours within the bubble combust by compression ignition, and the lay-down of varnish, particularly on screens, filters and servo-valves. Apart from the cavitation damage, the partial blockage of the suction strainers will impact the pump performance and increase the risk of further cavitation. Some case studies suggest that half of most hydraulic pump repairs are the result of cavitation damage. Aeration can also increase the risk of rust and corrosion problems.
Whilst high temperature may be beneficial in ridding the system of water and aeration problems, it is detrimental. High temperature encourages oxidation, and on mineral oils, the Arhenius Rule states that the oil life is halved for every 10°C rise above 40°C. The high temperature will result in lower viscosity at the working surfaces, with reduced film strength, for a given viscosity grade of oil. In time the damage will result in a thickening of the oil which will become apparent in cold starts. The increased oxidation is again a cause of increased risk of varnish laydown, and increasing acid levels causing corrosion.
Lastly, since many cranes utilise engines as well as hydraulics and gears, it is important to also consider the nature of the contamination risks peculiar to engines. Soot is an inevitable consequence of diesel engine operation but is controlled to manageable levels (<0.1µm in size) by a dispersant additive. This additive may be damaged or the soot levels may increase as a result of other contaminants, but under normal conditions the dispersant additive has a defined life. Extended oil operation will result in soot particles in excess of 1µm in size which becomes a danger as discussed in solid particles. Fuel dilution is another risk in engines, and the sulphur in the fuel may react with water to form sulphuric acids. The fuel will decrease the viscosity, and becomes a potential safety risk in operation as the flash point is lowered. Coolants, usually a glycol product, will also damage the oil. Whilst the water generally evaporates owing to the high temperature operation, the glycol reacts to form ‘oil balls’, increasing the viscosity, and these ‘oil balls’ can block filters and oilways. It will also reduce the film strength, and result in sludge formation.
I have been deliberately generic in the use of the word lubricant rather than oil, as the principles apply equally. Whilst grease may be an effective seal against contamination in some applications, the oil within the grease soap is equally susecptable to damage.
