New oil is often looked at as “cheap insurance”; changing the oil in small reservoirs can often be done quickly and easily. However changing out a 5,000 to 10,000 gallon, or larger, reservoir becomes increasingly more difficult and costly. In these cases, the question usually arises, Can we change out some of the oil to raise the reservoir back into the acceptable range? This will save a bunch of money right….?
While it may save a few dollars in the short term, it will cost you significantly more in the long run. This partial change-out strategy is often referred to as a bleed and feed and can be done while the equipment is running or during a shut down. So let’s talk about when to and when not to conduct a bleed and feed.
If the oil is breaking down, it likely has consumed some of its additives, much of its antioxidants, and often has oxidized base oil. Doing a partial change out will restore some of the additives giving an initial helpful bump to performance. More importantly though, it will not improve oxidation performance as expected. Oxidation is not linear; it is a self-catalyzing process, meaning that oxidized molecules speed up the rate at which other oil molecules oxidize. Therefore, the more oxidized molecules, the quicker the oxidation accelerates. In this situation, the oil remaining in the reservoir is already partially oxidized and will therefore drastically shorten the life of the new oil entering the system. To put that more numerically, a bleed and feed of one third of the used, oxidized oil will not increase the useful life of the reservoir back to a third of the new oil’s useful life. This is also in part because the fresh antioxidants are consumed quickly by attacking the oil degradation products already present in the oil. So, if you expect your oil life to be extended by one third through a one third bleed and feed process, you may end up with a much shorter oil life in reality. By effect you could end up paying significantly more than you should have for that oil in the long run and only barely delaying the inevitable oil change.
When the oil is oxidized, a bleed and feed is not very cost effective for a long term solution; but there are cases where it may be a good option. First, if you can’t shut down the equipment, a bleed and feed can be the only way to replenish needed additives or reduce the oxidation number. While this is not efficient, it can be necessary as a short term measure until the next full change out can be accomplished. A bleed and feed can also be useful if the asset is going to be removed or taken out of service soon. Instead of doing a complete change out, a bleed and feed can help replenish the lubricant qualities to meet the short term need.
It is always important to consider the long term costs before moving forward with a bleed and feed strategy. You will usually discover that while a change out might sound expensive today, it is far cheaper than the alternative.
If you have questions on if a bleed and feed strategy is right for you, “Ask the Expert” on the forums tab or call the Tech Help desk at 1-800-MOBIL-25.
Thanks Justin, good article. We carried out "sump sweetening" and monitored the results on a Waukesha gas engine as TAN and TBN crossed and the customer could not shut down. we drained and topped up several times over 2 days totaling about 80% of the sump volume but only recovered about 40% of the TBN and dropped TAN by similar amount.