ENGINEERING SOLUTIONS / Post date: 7 June 2019
For quite some time, the Castrol® Field Engineering team has been addressing customer questions and concerns about used oil analysis (UOA) results that show elevated silicon (Si) results and particle count flags that were triggered by the presence of additives in the oil. The intent of this article is to provide clarity on these issues for those who have been affected or who may be affected in the future.
Whenever customers implement a used oil analysis program and begin to review the reports carefully, they will find some results that require additional explanation beyond what is available on the report itself. This isn’t unusual, as there are limits to the amount of information that can be supplied on a UOA report. This is one reason why it’s important to work with a lubricant supplier that can provide basic training on how to read a report and, more importantly, value-added insights.
With this context, let’s take a look at elevated silicon results in spectrographic analysis results and elevated particle counts, which can both be caused by additives in the oil.
The components that have the most influence on results tend to be certain anti-foam (also known as “defoamant”) additives, which are used to prevent the accumulation of foam in an oil sump. Viscosity index improvers, which help keep the viscosity of the oil more consistent over a wider operating temperature range, can also influence results.
But all additives can influence results. This is a well-established effect, even if it is not widely understood or acknowledged by some in the machinery maintenance profession. An in-house study performed by Castrol has identified this impact and similar effects have been documented by others (see cited references 1, 2, 3 below).
Let’s begin with elemental spectroscopy (aka Optical Emission Spectroscopy or OES), the technology used to identify the presence of and the quantity of elements in the oil sample.
It is important to understand that this analysis only identifies specific elements and is not capable of identifying compounds or molecules.
So when the spectrometer identifies Silicon (Si) in the oil, that silicon can be from silica sand (from dirt ingression) or it can be some other compound that includes Si. For example, many engine coolants contain silicates as corrosion inhibitors. Coolant can sometimes make its way into the crankcase via failed and leaking seals and gaskets, where it mixes with the engine oil. When the oil is sampled and analyzed, the silicate additives in the coolant may be picked up by OES and reported in the UOA report as Si.
Sometimes the Si may be from silicone sealants that are used to seal up covers, plates, or other openings on a component. If too much sealant is used, some of the silicone can slough off and end up in the crankcase where it gets into the lubricating oil. Again, when the oil is sampled and analyzed, the silicone is reported as Si.
Another source of Si found in used oil samples is Silicon-based anti-foam additives that are used to enhance the release of air bubbles in the oil so they don’t accumulate into foam. A spectrometer cannot identify between Silica, Silicates, or Silicone; it can only identify the element Si, which is the main element in each of these sources.
When SI is elevated, most often it’s from Silica, which comes from the ingression of dirt. Silica is extremely abrasive, so when it gets into a component it can cause very aggressive wear, which is why UOA reports alert customers of its presence. When Si is present along with wear metals like Iron, Aluminum, Chrome and others, it is often Silica. Sometimes, Si is present, but wear metals remain at normal levels. In these cases, the Si is probably not silica and is more likely one of the other sources named above.
Si from anti-foam additives are typically reported at very low numbers… usually less than 10 parts per million (ppm). However, they can sometimes be much higher… up to double that count. So, if you are seeing higher levels of Si with low or normal wear-metal levels… and if you haven’t performed any service work on the component lately involving the use of a silicone sealant… and if you don’t see other typical coolant additives such as Sodium or Potassium, then the Si may just be from the anti-foam additive in the oil. You can compare Si results to other units/components that use the same lubricant to confirm this and you can also check with your oil supplier for the typical Si level in a sample of fresh oil.
Note that this represents no danger to the component the sample oil was drawn from, as it is a benign condition. In fact, it is a beneficial condition, as the anti-foam additive prevents the buildup of foam, which can be dangerous. So, if Si is only from the anti-foam additive and there is no elevated wear occurring in the component, take this item off of your worry list.
Now, let’s look at how additives can affect particle counts, which can skew UOA results.
Particles in a used oil sample can be measured in several different ways, but typically a commercial UOA lab uses a process known as Optical Particle Counting. The following is a very simplified explanation of how this technology works: A sample of oil is flowed through the path of a light source and particles in the oil cause a shadow to be cast onto a receiver that counts the number of shadows and reports them by size. Some other models have receivers that identify reflections of light that bounce off of particles in the oil. The intensity of the reflected light indicates the size of the particle.
These are very simplified explanations, but the important thing to understand is that both types of counters are susceptible to inaccuracies due to possible contaminants or additives in the oil, as well as the opacity of the fluid… i.e. darker fluids and dyed fluids can affect the accuracy of the count. Water droplets or air bubbles in the oil can also be counted as particles, falsely increasing the reported particle count.
Certain additives in oil, in particular silicon-based anti-foam additives and, to a lesser extent, viscosity index improvers, can cause issues as well. Some testing has even identified standard detergent and inhibitor (D&I) packages that can increase particle count results (Martini & Deskin).
So what good is particle counting if it can be influenced by additives in the oil? That’s a good question. Unfortunately, the machinery maintenance industry is at an early stage in the development and use of this technology. A significant amount of incorrect advice has already been promoted by many who lack a clear understanding of the technology and how it should be used.
Very often we see hard limits on fluid cleanliness promoted as a means of extending equipment life. However, the reported cleanliness levels of perfectly clean lubricants are often falsely flagged as exceeding these limits, causing much confusion among maintenance managers and technicians who aren’t UOA experts.
Particle counting is best used as a trending tool for the cleanliness levels of individual components… not as a tool for measuring absolute cleanliness levels. As explained above, the technology has not yet developed to the point where it can be used for this purpose.
All of this is not intended to dissuade you from using particle counting as a part of your used oil analysis program, but rather to inform you about some of the limitations of the technology.
Castrol® has a team of experienced, field-based lubrication engineers who can help you identify opportunities to improve your maintenance practices and to reduce your operational costs. Please contact your Castrol sales representative for assistance.