It is not an uncommon question amongst many technical managers who usually introduce “small volume oil change” and expect oil condition to get better. In this write-up, I have shared my two cents’ worth on this topic with some common life analogies.
Let me get straight to the point without wasting your time.
I have replenished 25 to 50% of total reservoir volume, but the oil condition is not good yet – why?
There are many methods to replenish oil and one of the most common methods used in the maritime industry is “Small Volume oil change”. All we do is drain some oil from the reservoir and add some fresh oil, expecting an improvement in overall oil quality. This seldom happens and this is mainly due to the below-explained phenomenon.
Adding some fresh oil to the used lube oil in the system is very similar to introducing 4 healthy people into a closed room which has one person ailing from a contagious disease.
Can 4 healthy people turn a sick person healthy?
While there could be some theories or placebo to support, it is very unlikely. Rather, it is very much possible that a sick will eventually turn the other four to fall sick as well. It is the same with lubricants. Adding some fresh oil to used lube oil in the system will eventually deteriorate in its quality. It also depends on how sick or how bad the used oil was, the initial volume, and % of fresh oil added.
Another analogy to explain the above scenario is that – this is very similar to the usage of food coloring agent when you cook. A very small quantity will change the color of the food completely.
Whenever the lubricant experiences oxidation, which is characterized by an increase in viscosity, dark color, a significant increase in acid number or a drop in base number, and a pungent smell – then it is time to change the oil. A complete system flush is required to purge the infection. Oil analysis can help advising the exact time to change oil based on proper information on lubricant running hours and oil test results. All we do by adding some fresh oil in the system is extending the used oil remaining life.
Next comes the Ghost Riders!
Image Source: Kleanoil
Let’s assume we have a 20-micron filter in the lubrication system. This filter will easily separate out particles more than 20-micron and all particles less than 20-microns will flow through the filter unabated. These particles which flow throw the filters are called Ghost Riders and they stay forever in the system until the system is completely emptied and re-filled with fresh lubricant. The filter selectively removes only the particles larger than 20 microns. As new particles ingress during normal operation, the small ghost riders continue to grow in population until the oil is changed completely.
Below graph clearly explains the drop in filtering efficiency as the particle size is less than 20-microns (for a 20-micron rating) – so this is how the ghost riders escape and grow in population.
Image Source: Discussion Forum (Bob the Oil Guy)
The problem with these ghost riders is that they can do many harmful things to the machinery onboard. For instance, anywhere there is elastohydrodynamic lubrication (e.g., rolling-element bearings), these small particles can readily bridge the working clearance, damaging bearing surfaces. Where boundary lubrication exists (starts, stops, – journal bearings, slow-moving surfaces (X-head bearings/shoes), etc.), the ghost riders will cause the vast majority of the damage and continue to grow in population.
Thus small-volume oil replenishment does not necessarily mean that the used oil condition shall turn better, rather, in most cases, it is otherwise. Full-volume oil change has a significant advantage that the complete system is flooded with fresh lubricant and all ghost riders are drained out, thereby actually extending machinery life. Of course, for main propulsion diesel engines and steam turbines, it is not economical to replace the complete system oil as the volume is huge and the contaminants entering the system are relatively low.
Reference: Machinery Lubrication / STLE – CLS / Jim Fitch
Disclaimer: The opinion expressed here are authors’ own and need not be accepted as a common technical advisory. For specific machinery-related questions, feel free to write to me: [email protected]
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Source: Sriram Balasubramanian on LinkedIn(https://www.linkedin.com/pulse/oil-change-ghost-riders-beware-sriram-balasubramanian/)