Diesel and Condensation--It Is NOT a Myth

3-23-2013, rev. 8-1-2024

 

Not enough condensation to generate a water layer, but enough to generate haze, encourage bio-growth, and promote fuel system corrosion.

 

The bottle with the silica gel filter is noticeably clearer

Last year I performed dozens of tests for several Practical Sailor articles relating to vent filters and their ability to keep gasoline and diesel dry. Most of the focus was on w-10 gasoline;  I have gasoline engines and self-interest rules, and a regulatory change is requiring vent filters on new boats. We learned that the vent filters conserve fuel, reduce oxidation and reduce corrosion in e-10 gasoline. Also, I did do one quick test on diesel. Sure enough, when the temperatures and dampness were just so in the spring condensation formed in the jar with no filter and the jar with the silica gel filter stayed clear. There were small water drops on the bottom that mostly re-dissolved when temperatures rose.

 

 

The left bottle is sealed the right bottle is not

 

This year I repeated the test, including corrosion coupons in galvanicly coupled pairings, to measure any moisture effect on corrosion. And guess what; the sealed jar stayed clear and the vented jar clouded right up when the weather got damp in March (the test was started in December and the jar stayed clear until this week--the conditions must be just so). The corrosion coupons in the cloudy jar seem a bit worse, but I have not yet disassembled the bundle for examination. There are no added corrosion inhibitors or added corrosive elements (seawater or acid).

It also seems that the silica gel filters do not last as long on diesel (multi-season on gasoline but perhaps only a few months on diesel). I need to confirm this and to think about why it might be so. One reason is that the ethanol in e-10 gasoline tends to scrub the water out of the filter during exhalation cycles, a benefit that a diesel vent filter will not experience.

So much for those that proclaim water condensation in diesel to be a myth. Not too surprising, when we consider that water solubility in diesel is only about 500 ppm at 32F and that the diesel we buy is not completely dry (has some dissolved water). The effect seems quite reproducible, though the small sample size clearly exaggerates breathing. The 1/8-inch x 18-inch vent lines is based upon EPA work on fuel tank breathing and is intended to mimic a 1-year exposure in 1-month, based upon a 40 gallon tank.

What are the practical ramifications?

1. Keep the tank full to limit breathing? Maybe, but there is also an upside to adding fresh fuel in the spring. Do 2 and 3, and then top up in the spring.
   
2. Silica gel vent filter. I would.
   
3. Corrosion inhibitors. Absolutely.
   
4. Fuel polishing? 2 and 3 should prevent the need. If it recurred, I would build a system. based on a lower pick-up and a separate pump. High volume, because you want to catch the crap before it settles. A slow polish system only catches the stuff that didn't settle, and not the stuff that will get stirred up in rough weather, which is what worries me.