Bleed & Feed

Could someone explain this technique as used in lubrication field?

Where do you get the the terms?

I heard some oil people who say that instead of completely draining the oil, a fresh amount is fed.

On most oil sumps there is a drain valve and filler cap.
I guess they just open the drain and at the same time pour in new oil at the filler! This would tend to have a "flushing" effect but obviously one then needs to shut the drain and continue filling until the correct level is reached.
DO NOT DRAIN COMPLETELY FIRST AND THEN FILL - IT MAY BE TOO LATE!!!

I would add:

I assume what we're talking about is when you have a sump and you want to reduce the concentration of a contaminant (such as water) or to increase the concentration of additives (such as oxidation inhibitor) within the oil, and the oil cannot be completeley drained (perhaps the sump continues to serve operating equipment).

One option for contaminant removal would be to filter. Another option would be feed and bleed.

If you drain a fraction X of the total volume and then add back that fraction X of new oil, then assuming uniform mixing, the remaining concentration of the contaminant after you're done is C0*(1-X) where C0 is initial concentration.

If you repeat the process, the concentration is
C0*(1-X)^2

If you do N drains refills, each time a fraction X of the total, and we ASSUME uniform mixing, the concentration is
C0*(1-X)^N

If X is 20%, then you get down to 50% of your original concentration in 3 bleed/feed cycles.

If X is 5%, then it would take you 13-14 cycles to get down to half your initial concentration.

It might be smarter to add first and then drain if you were already near a low level limit. The fastest results would come from draining to near the low level limit and adding to near the high limit. The math is a little different.

If the contaminant is water, you get the biggest bang for the buck draining off the low points in the system.

This message has been edited. Last edited by: electricpete,

So it sounds technical.

Is it true we are not supposed to top up more than 10% of new oil at any one time?