Dirty Oil – An Introduction
This condition may exhibit no signs of an impending problem within the system, but if this condition (dirt in the oil) is leading to severe cutting wear or fatigue wear then abnormal noise, heat and vibration may also be evident within the system. If so, it suggests the oil cleanliness is having an impact on the condition of the system. In this case, oil analysis is the best method for early detection of fluid cleanliness, helping to reduce the chances of this problem leading to abnormal wear in the system.
Many studies have been performed by equipment manufacturers and filtration companies to demonstrate that cleaner oil can extend the life and reliability of equipment. This is because cleaner oil leads to less abrasive wear, hence longer component life.
To reduce contamination, such as environmental dirt in the system the best solution is to try preventing it getting there in the first place.
A visual inspection of your sample can be a starting point – any visual dirt or debris in your oil sample suggests there may be a cleanliness issue with your equipment that needs addressing.
Dirty Oil – Prevention
All systems breathe, which means that air, which contains dirt and water can and will enter the system during normal machine use, unless there is something to stop it. So if your system involves a bulk storage tank, then consider fitting air filters on breathers to combat dirt ingress and desiccants to remove water.
Systems such as engines or compressors are normally fitted with air filtration to prevent dirt ingress. Hence a visual inspection of air filters and trunking joints at the next planned maintenance or offline shutdown period is crucial in reducing dirt ingress into the system.
When new parts or components are fitted or new machines installed on your site the system should be flushed with clean oil to remove any build-debris or casting sand likely to cause wear on start-up. This procedure is an important fact to discuss with your OEM and lube supplier prior to the very first start-up. Additionally, when new oil is delivered, even with the cleanliest and strictest quality control procedures in place, there is always room for improvement. So especially for expensive, critical machines or large oil volume systems it is often worthwhile pre-filtering the oil before it enters the system. Your lube oil or filtration supplier will be able to give advice, on the most cost effective solution for your equipment.
If the system is pressurised then it is recommended to monitoring the pressure of the system and noting any pressure changes. A drop in pressure may indicate a leak (sight glass drops can confirm this) or significant increase – indicating filter blocking. A laboratory particle count test may confirm this possibility and may in turn suggest seals or pipe connections as a potential source of contamination in the system.
Acoustics monitoring is often a useful way to identify leaks within pressurised systems (particularly air leaks). Likewise inexpensive options, such as the manufacturers approved soap-based fluids, which if placed on the outside of suspected areas of leaks, foaming will indicate a leaking seal etc. Again, dirt ingress may cause a pressure increase, but if the pressure gets too high within the system or across the filter, the by-pass valve may trip – meaning contamination is no longer being removed, but at least unfiltered lubricant is reaching vital components; a much better option than no lubrication.
If the system is fitted with a drain tap, then it is possible to first allow the oil to stand and settle for a period, then running off a small volume of oil until the oil clarity improves; thus removing some of the sludge or sediment collecting at the bottom of lube oil systems.
The most common cause of a flagged contamination result on a report is poor sampling and not an indication of true contamination of the system. If it is suspected this is the cause of the contamination – then please see our advice on “how to take a good sample” shown in the previous sections.
Dirty Oil – Removal
When discussing oil cleanliness it is not possible to have zero particles present, even if using the best filters available, so there is always room for improvement. This is why contamination control should be considered an ever improving target and not a fixed cleanliness level, claimed will cause no wear. The cleaner the oil – the better the expected component life, but also with each improvement the more difficult it gets to improve further. Hence it is often a balance of working towards best component life against what can be achieved for a given at a sensible cost. Target cleanliness levels should only be set if it is realistic and achievable for your given equipment and working environment i.e. don’t set yourself unreasonable NASA cleanliness levels if your equipment is working in quarries for example, excavating rocks and earth all day. Your filtration supplier and equipment manufacturer will be able to assist you in finding the ideal balance.
The choice when it comes to handling dirty oil often comes down to three options:
- Change it – This is often the best approach where oil changes are relatively inexpensive or the practicalities of filtration make this a too costly solution. Changing the oil may also be the best approach if there are other factors, other than dirt affecting the oil, which cannot be completely resolved by filtration, such as viscosity changes or spent additives, leading to loss of key lubricant properties.
- Filter it – usually the best approach if the contamination is particulate or the system is large. Oil changes may prove too costly, because the price of oil or waste handling is too expensive and the oil is otherwise in good condition. Or, of course, if shutdown is not a commercial option at the time dirt ingress is detected, then the filtration option is best.
- Leave it – Stricter and stricter cleanliness targets will eventually reach a point where it is impossible to achieve further significant benefits from additional cleaning. This point may be the ideal cleanliness level achievable at an acceptable cost, but the system should be closely monitored thereafter for signs of an increase in abnormal wear. This may also be the approach if the equipment is at the end of its expected life and plans to replace the equipment have been agreed. In any event “leave it” should be the practice, if waiting for a resample result to confirm the original result..
When changing oil on large systems, it is always wise to have the oil tested before and after changing and indeed when changing supplier or product types to ensure that the fluids are compatible – the lubricant supplier should be able to provide all this information and give advice if flushing the system is necessary.
When considering filtration there are usually two distinct methods:
- Conventional full flow Inline filtration – Inline filtration is the typical filtration systems adopted, usually using filters with pleated designs to give a high surface area to maximise filter efficiency and reduce pressure drop across the filter. These may be single filters, or part of a series of filters throughout a system. They are often under high pressure, operate at fast flow rates and be installed after the main system pumps. These high pressures, stop/starts, cyclic conditions mean they generally struggle to be efficient at removing particles. They are usually between 5 and 30 micron ratings and designed to catch large solid particles. In modern systems these are often combined with off-line filters that do the majority of the cleaning and then the in-line filtration is just a “last chance” filter to catch anything before it reaches the critical components. The groups of in-line filters can be grouped further into:
- Pressure inline filter – as described above, are generally fitted after the pump and before components.
- Suction Filters – Designed to reduce wear to the pump, so are generally coarse filters 25+ micron (to reduce risk of cavitation) fitted before the pump/booster pump to remove large particles.
- Return line filter – these filters remove any contamination or wear particles from components upstream of the filter and returns clean oil to the reservoir tank. When assessing wear levels it is important the sampling points is before this filter otherwise wear metals cannot be accurately determined.
- Combination Suction Boost Return line filters – These are often used as return line filters where space requirements for a main oil tank is at a premium and smaller tanks are used and when the pump requires finely pre-filtered oil, which otherwise would risk of cavitation with other filtration options.
- Offline Filtration – These are often called filter caddies, or kidney filters in so much they work offline away from the system and able to filter at less than full system pressure with slower flow rates. The ratings tend to be relatively small, ranging from 0.1 to 5 micron ratings. Unlike traditional filters these tend to have higher capacity for holding dirt and although more expensive than standard inline filters, they benefit by requiring less frequent changes. There is a simple calculation available, which is the cost of removing ‘x’ amount of dirt. Typically this may take the form of cost of removing 1kg of dirt = cost of filter insert / dirt holding capacity. This can enable comparisons to be made between two or more products. However, additional features – such as efficiency and beta ratio ratings and material type should also be considered when selecting which filter to buy.
When fitting any filtration, lubricant delivery or storage system to improve oil cleanliness – it will prove a major expensive investment. Hence it is important when planning any of these systems, to take regular oil samples before and then after implementation to truly establish the benefit of the new equipment. Often on-line particle counters, which may be part of the new filtration system, will probably show an improvement after installation. Nevertheless, unless the filter supplier or the machine owners use an independent ISO 17025 accredited laboratory to verify these findings, the results may not stand up to scrutiny. Hence it is recommended to use independent laboratories, who participate in regular round robin analyses, to ensure the values being obtained are true cleanliness levels and have no bias towards any one set of data. This is because differences between instruments and methods of measuring particles can result in significant particle count variations.