Thursday, March 22, 2012

Fuel Tank Vent Filters--Better Boat Keeping?

What's the problem with diesel stored in tanks?   Moisture accumulates in the bottom of the tank due to the tank "breathing" with atmospheric pressure and temperature changes.  And then this moisture supports colonies of bacteria who make that water acidic, corroding your tank, and when they finally die, their corpses either clog your filters (if you are lucky), or trash your injection pump.  Over at Sail Delmarva, Drew is attacking this issue from the point of view of ethanol "enhanced" gasoline, but it applies just as well to diesel.  Read on...
For all the talk and trouble water in the gasoline causes--far worse with e10--to me it's rather conspicuous by its absence that neither owners nor builders ever took a serious look at vent filters. Cars have had sealed tanks fitted with both pressure controls and filters since 1971. I considered this a few times--I've installed very large descant traps on very large chemical tanks--but figured if it was so against the conventional wisdom in the boating community, it couldn't be right. Funny.... That's not like me.

In a prior post I began a discussion of some testing for Practical Sailor. We've got two test boats going at this time. I've started monitoring the humidity inside and outside. Yup, it's drier inside the tank, after filtration. We expect some positives that should add up to a good value proposition for the owner:
  • Drier gas/fuel.
  • Less evaporation, to the tune of $8-$12/year. The unit should last ~ 10 years without service, so that will nearly pay for it.
  • Less loss of volatiles means better starting, particularly in cold weather.
  • Less loss of volatiles means better resistance to phase separation/emulsion blobs. I have only been able to recreate true phase separation from atmospheric absorption if the ethanol evaporates after it is saturated, and I've tried many combinations. This is why we generally only see it in carbs (they're small).
  • Less loss of volatiles mean less gum formation (better solvency).
  • Less loss of volatiles is good for the environment. Yes, that counts.
  • Less oxygen (less convection, more vapor space) means less gum formation.
It won't stop these things from happening in the carburetor, but it should significantly extend gasoline stability in the tank considerably. Preliminary calculations suggest at least double.

But can we demonstrate how this actually effects the fuel over time on a small scale, in a controlled manner? Calculations only go so far, since water condensation and absorption, differential evaporation, and fuel oxidation work together in complex ways. Science project time.

One liter bottles with 500 ml e10, starting levels marked with tape. From left to right:
-  Plain 1/8-inch ID vent.
-  10 ml silica gel descant
-  10 ml activated carbon  adsorbent

Actually, both carbon and silica gel are both adsorbents that pull water and organic vapors from the air. Silica gel (the packs you find in with your new DVD player) has a high affinity for water, while carbon has a high affinity for organic vapors. However, both adsorb reversibly; that is, if exposed to high temperatures and either clean air, or an excess of something else, they release what they have previously adsorbed. Carbon can be flushed by water vapor and air, while silica gel can be flushed with hydrocarbon and air. Both should have the effect of keeping the tank drier and reducing evaporation, adsorbing and desorbing with each day/night breathing cycle. Both should reduce oxygen in the tank. But how much?

Results in the fall. The long version will be in Practical Sailor.These things can't be rushed. I expect we are going to find the improvements are small and that a sailor won't "feel" any difference. But his engine will start a little easier, he'll buy a little less gas, and have a few less problems. The math works, it just won't be obvious, not like some new bit of deck hardware $100 he could buy. More like changing the oil; "pay me now, or pay me later."  

But meanwhile, like the holding tank vent filter that I installed (and am very pleased with), I've got installed a carbon vent filter on Shoal Survivor and I'm not planning on taking it off.


Note for the spelling or technically obsessed: sometimes I say absorb, and sometimes I say adsorb; both are correct. Alcohol absorbs water. Silica gel adsorbs water. The mechanism is very different and the reversibility is very different. This is the reason adsorbents are so useful.


Although the test filters photographed above were fabricated from PVC, this is NOT safe practice for permanant installation. PVC is not highly resistant to gasoline vapors and the adhesive is quite vulnerable over time.  While it won't fail in this laboratory setting, based on refinery expereince with PVC, the joits will fail if I add heat, vibration, and wait several years.


  1. Just after I posted this I set up an identical set with diesel fuel. The vent-only jar became cloudy within 2 weeks, while the jar with the silica gel filter remains clear and bright. Such a visible change was a real surprise and quite a convincer!

    The gasoline jars are not changing apearance so quickly, because e10 can easily disolve signifigant amounts of water. With gasoline I am actually just as intersted in the effect on evaporation oxidation, so those tests will run all summer. Practical Sailor, winter 2012?

  2. I am definitely going to install some moisture traps on my diesel vent lines.

    Just another thing to do that this blog has brought me...



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