Very small air bubbles in Racor sight bowl. Normal?

[Scott Carter]

Does anyone have any first-hand knowledge of the concept of "dissolved air" in diesel fuel, which then forms tiny bubbles (champagne-like) in the sight bowl of centrifugal separators (Racor clear bowl style). I've been told by Racor tech support that these bubbles, which concern my client, the owner of the boat, are not necessarily a problem. Obviously this would only be true if they weren't in such quantity as to cause combustion problems.
My first troubleshooting efforts reasonably assumed air entering the fuel system from somewhere along the fuel pick-up line between tank and Racor. So I removed the supply line into the Racor unit, replaced it with a short 4' section of fuel hose led directly into a 1 gallon jug of diesel. Exactly the same thing. Very small bubbles entering the sight bowl from under the centrifuge (the plastic inverted cone in the bowl), average about 1 every 5 seconds or so.
I would have suspected an internal leak (manufacturing defect in the ball valve or something) except that this is the second brand-new unit doing this. Racor provided us with a warranty replacement for the original one showing this symptom. So unless I replaced a faulty unit with another exactly-the-same-faulty unit, this points to the fuel supply line. I eliminated that with the temporary 4' pick-up (properly clamped at the filter assembly). The engine has run scores of hours under sea-going conditions with no fuel-delivery problems.
So, this leaves me with a big ?, And leaves me the idea of dissolved/entrained air becoming un-dissolved when placed under the slight vacuum of fuel suction (registered at about 2 or 3" hg on the Racor's gage, well within normal operating vacuum). I'm not buying it 100% (neither is the boat's owner) but I'm looking for a better explanation.
Thanks for any insights or ideas.
Scott

[Carteret]

The only time that I have encountered this was from the return line of the injection pump. Was your return line feeding into your one gallon supply jug? If it was, install a clear plastic line at the injection pump where the return fuel leaves the pump. Look for bubbles there. I have also seen this problem when fuel coolers have not been installed on return lines. Hot fuel tends to foam. does your system have a fuel cooler installed?

[Scott Carter]

Good questions. No fuel cooler in the scenario, but the fuel temp. is in the mid 50s for these tests. Re. the return line into the jug: at first I did not have the return line going in, and I saw bubbles in the glass. Just fresh, undisturbed fuel being pulled. But I ran out of fuel very quickly so I routed the return line to the jug and ran another test. This let me run for much longer (more than 20 minutes). The jug wasn't clear, so I wasn't able to view the return fuel's state (in terms of foamy, bubbly, etc.) Maybe another test using a clear jug. The answer there, for the best test, would be to keep the return line going to tank and just use a much bigger temporary fuel reservoir to test from. That may be in the cards.

[kenJ]

What style racor? There have been lots of documented problems with the smaller spin on filters with the clear bowl. Air leaks from the vac gauge mount or manual fill pump have caused loss of power. Possibly your leak is too slight to cause a power loss, but enough to see the bubbles.

[Scott Carter]

This is the 75-500 Centrifugal separator/filter assembly, dual unit (Racor's term is "manifolded"). It's got the clear reservoir "sight bowls" below with the filter elements on top, switching valve between the two bowls. I'll inquire re. the vac gages. But again, I have to wonder about the odds of receiving two new units with exactly the same fault. Possible, sure. And things like that happen. Our bad luck if so (and a few more gray hairs). Thanks for the tip.
Scott

[Scott Carter]

Mystery solved. I used clear PVC tubing for the filter line in and fuel pump return to tank lines. It became very evident that the bubbles were not being picked up at any single point in the system. They were, though, being formed in the lines themselves. As vacuum increased the bubbles became more apparent. I could constrict the line with pliers and immediately start to see countless bubbles appear in the line. This formerly absorbed air was literally being boiled out of the fuel (as pressure decreases boiling point decreases). These bubbles will not be a problem for diesel fuel systems, as they will inevitably collapse as pressure begins to increase at the pump.
My final test was to raise the 1 gallon feed jug above the level of the fuel filters (essentially gravity feeding the fuel, so under almost zero vacuum) and the bubbles vanished. As I started lowering the jug of fuel, I observed a gradual but certain increase in the quantity of bubbles forming as the internal line vacuum increased to draw fuel vertically up.

You can look up the specs, I don't know but that 500 is tiny. How big are the fuel lines preceding it? It may work but may also be marginal.

[Scott Carter]

The 500 is more than adequate according to Racor and Yanmar (4LH engine), but you're on the right track in posing the question about the line size - it's questionable at 5/16. Short runs with no lift it would do, but the filters are almost 24" higher than the tank, so there's that added resistance to pull. The distance is fairly short, maybe 5' or so. To be realistic about this though, the fact that the line size is allowing full flow (lots of pump discharge on the return line) tells me that the bubbles are just something we live with all the time but most people don't know they're there, and neither do their engines.