Lesson 205 – Tramp Oils
One major cause of coolant degradation is contaminant oil, such as tramp oils. In the Introductory Course, we discussed how these oils feed bacteria, and how it is very important to skim them from the fluid surface as quickly as possible to prevent bacterial growth and coolant rancidity.
Another reason to skim them promptly is because if left in the fluid pool, tramp oils will start circulating through the coolant pump and get mixed, or emulsified, into the coolant itself. Coolants contain de-emulsifying agents that help reject contaminating oil; however, they don’t prevent emulsification entirely. As discussed earlier, those coolants with high percentages of base oil are most at risk of tramp oil emulsification, partly because they also contain emulsifying agents that help mix the coolant concentrate into water! But even synthetic blends with de-emulsifiers can succumb to the effects of emulsification.
In addition, high-pressure coolant delivery systems tend to exacerbate the oil emulsification process since putting the fluid under pressure causes coolant and tramp oil particles to bombard each other. These systems tend to form an inverse layer, which looks like a milkshake or a sponge. It doesn’t fully separate but it must be treated as contaminant oil, as it contains up to 99% tramp oil. The rest is a small amount of coolant with lots of tiny air bubbles that inhibit tramp oil removal.
Effects of Emulsified Oil on Mechanical Skimmers
Emulsified tramp oil will not separate fully from coolant by gravity and the coolant will, over time, become loaded with oil. Mechanical skimmers are designed to pick up oil, so they will be rendered useless, as they steadily skim coolant from the tank. You may see this same effect on coolants with heavy base oil content, whereas synthetic coolants contain chemical lubricants that are less attracted to skimmer surfaces.
Zebra offers a solution for heavy oil-based coolants, the Diverter. It is integrated as in the Smart Disk Skimmer or sold as an accessory for the Belt Skimmer or Sidewinder. However, even it cannot separate oils that are emulsified into the coolant itself. They will only travel back to the sump since at that point, they have become part of the coolant.
As introduced earlier, an inverse layer has lots of tiny air bubbles. This can inhibit its removal when using a mechanical skimmer. The skimmer no longer recognizes it as oil given its new physical nature, which is no longer slick like pure oil. In this case, a coalescer is a better option.
Effects of Emulsified Oil on Coalescers
All coalescers have their own hold time, the amount of time the fluid stays in the coalescing tank. They will only be able to separate those oils that separate by gravity within this hold time. Emulsified oils do not separate by gravity, thus they re-circulate through a coalescing system.
However, a coalescer uses a pump to draw off the surface layer, so the foaminess of an inverse layer is less of a problem here. A coalescer is recommended for high-pressure applications, because the inverse layer can be skimmed from the machine sump, manually drained from the coalescer, and sent to oil waste. Note they are also recommended for applications with floating sludge or where biofilm is a constant problem.
Effects of Emulsified Oil on Coolant’s Function
You may ask, “If base oil in coolant is meant for lubricity, isn’t added lubricity a good thing?” Added lubricity may be good, but emulsified oils reduce cooling ability–the other important function of coolant. Tooling may overheat and wear more readily and parts finishes may be affected.