Tramp oil is defined as that oil which is present in a metalworking fluid mix and is not from the product concentrate. The usual sources of tramp oil are machine tool lubrication, hydraulic and way oils, which are drained or washed back to the fluid sump. Tramp oil, if allowed to accumulate in the machine coolant, will cause the following problems:
- Reduce coolant life
- Increase bacteria levels
- Rancidity or objectionable odor
- Corrosion of the work or machine
- Unsatisfactory surface finishes
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Depending on the fluid type (Soluble oil, Semi synthetic or Synthetic) tramp oil/grease can become emulsified with the metalworking fluid, but in most cases, it will simply float on the top surface, as “free” oil. Either way, tramp oil is a leading cause of fluid failure. Fluid failure occurs when the “free” oil layer inhibits oxygen transfer to the metalworking fluid, causing anaerobic bacteria to flourish. We know the effects when pH is lowered due to acid by-products of bacteria growth and hydrogen sulfide gas is produced – this equals “rotten egg” odor. Also, tramp oil acts as a food source for the organisms, therefore, it becomes important to control tramp oil leaks into the metalworking fluid.
One common way of removing “free” floating tramp oil is by using a belt/rope skimmer. Although belt/rope skimmers have their purpose, there are problems with this method. In order for a belt/rope skimmer to work efficiently, the tramp oil droplets must have coalesced and settled to the surface in large layers, so it can be removed. Then you must move the belt/rope skimmer around the fluid sump where the tramp oil settles next. If you fail to do this, than the belt/rope skimmer will begin to pull off the coolant from the sump, which is the same as a coolant leak.
Another method is to remove all the coolant from the sump periodically, then recycle it through a recovery or centrifuge system. This method can be very time consuming and costly.
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The most efficient method is the use of a floating skimmer placed in the machine sump that can flow at least 10 gpm with a low-shearing pump, i.e. diaphragm or screw pump. The floating skimmer, with pump, will pull the surface tramp oil, air and about one (1) inch of the coolant below the surface and deliver it to a Tramp Oil Coolant Separator (TOCS). The flow rate of the pump should be enough to turn over the sump tank at least five (5) times per hour. The flow rate through the skimmer will generate a current in the sump towards the skimmer, thereby, preventing the re-locating of the skimmer in the sump. The TOCS unit must contain a high amount of surface area, a minimum of 100 sq.ft./cu.ft., and be made of polypropylene material stacked closely together, so as to enhance the coalescing process of the tramp oil droplets. Polypropylene is extremely efficient because it is oleophilic material, meaning it is oil loving. As the influent enters the TOCS unit, the heavy tramp oil will rise to the top and be skimmed off into a separate compartment of the TOCS unit. The small tramp oil droplets in the coolant will coalesce on the polypropylene media, and then rise to the top surface to be skimmed off from the coolant.
Once the separation process is complete the aerated clean coolant (effluent) will leave the TOCS unit and return to the machine sump. The many turnovers through the TOCS unit will assure clean and aerated coolant, free of tramp oil.
Note: If dirt settling is also a concern, a second suction line can be installed into the sump bottom opposite the machine pump inlet area to circulate through the TOCS unit.
For more information about tramp oil removal in machine coolant, please contact us at:
EnPro, Division of Hydrotech Inc | Leonard Ardizzone, ME | lardizzone@hydrotech.com | Phone: 513-881-7009
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