Fuel polishing

I seem to recall some calculations being published in a previous thread showing that condensation can't account for water in your diesel tank (well only a tiny bit). It may have been Vyv or Vic who did the calls.
The conclusion was that most was from a leaky filler. Most fillers are mounted on a flat piece of deck so it has quite a large catchment area
 
pvb said:
A non-return valve wouldn't work - air needs to go out when the tank is filling, and needs to go in as the fuel is used.
Click to expand...

I think I wrote "You'd need a vent to open to let air out when filling.
Some people have this anyway, as a small vent does not let the air out fast enough on some tanks, resulting in fuel being belched out of the filler.
Some people say petrol keeps a lot better in tanks with such valves as opposed to open vents. I would not disagree with them. Not sure if it would help with diesel, but again, most cars do not have open vents and do not have diesel bug issues.....
 
ghostlymoron said:
I seem to recall some calculations being published in a previous thread showing that condensation can't account for water in your diesel tank (well only a tiny bit). It may have been Vyv or Vic who did the calls.
The conclusion was that most was from a leaky filler. Most fillers are mounted on a flat piece of deck so it has quite a large catchment area
Click to expand...

Without being critical, such calculations are flawed. I'm a 35-year refinery chemical engineer who has inspected countless storage tanks, and I promise you that condensation formation on the underside of the roof, which then falls to the bottom of the tank where it cannot evaporate, is universal. It is only worse 2' off the water. I also have installed many fuel and lube drier systems; the need is real.

The factor that folks always forget is that once a drop falls from the roof and falls through the diesel, it cannot re-evaporate in the morning. The oil forms a trap. The other factor folks forget is that bug growth and corrosion do not depend on a big water layer, only the difference between fuel that is saturated vs. not saturated.

And yes, filler caps are the larger problem. But I can also promise you that you can create hazy fuel by leaving it in a jar with a tiny hole in the lid in a damp location. This is a common result when doing corrosion testing.
 
thinwater said:
Without being critical, such calculations are flawed. I'm a 35-year refinery chemical engineer who has inspected countless storage tanks, and I promise you that condensation formation on the underside of the roof, which then falls to the bottom of the tank where it cannot evaporate, is universal. It is only worse 2' off the water. I also have installed many fuel and lube drier systems; the need is real.

The factor that folks always forget is that once a drop falls from the roof and falls through the diesel, it cannot re-evaporate in the morning. The oil forms a trap. The other factor folks forget is that bug growth and corrosion do not depend on a big water layer, only the difference between fuel that is saturated vs. not saturated.

And yes, filler caps are the larger problem. But I can also promise you that you can create hazy fuel by leaving it in a jar with a tiny hole in the lid in a damp location. This is a common result when doing corrosion testing.
Click to expand...

There's interesting stuff in that, but surely if droplets of water fall down into the fuel, their being considerably more dense than the fuel, would suggest that the water would (and does) accumulate at the bottom of the tank. That's fine, and my tank sumps, with drains, take care of that.

Now you are introducing the prospect of "hazy" fuel, by effectively having a vent to the tank. What is it? How does it form? And what precautions do you suggest should be taken? Must say that I have never experienced "hazy" fuel in any of my boat tanks.
 
thinwater said:
Without being critical, such calculations are flawed. I'm a 35-year refinery chemical engineer who has inspected countless storage tanks, and I promise you that condensation formation on the underside of the roof.
Click to expand...

As a 35+ year Mechanical Engineer I don't doubt your observation as I know where I live I have steel roofed car ports and in the winter in particular we get lots of condensation on the underside of the steel roof in cold mornings. We don;t get so much condensation on out polycarb roofed carport due to the difference in heat transfer and thus temperature differential.

On my boat which is steel with built in steel tanks I regularly check for water in my tanks with a water indicating paste and have found non in any of my 3 tanks . All my fillers are on plinths that are way above ony possible water level, in fact they are above the bottom of my companionway. so no water can enter that way. My vents are also as high as possible under a overhang so no water can get in that way.

One of the issues is that on boats most diesel tanks are inside so not directly subjected to outside temperatures and its the difference in surface temperature causing the inside surface to drop below the dew point of the air inside the tank.

IMHO this is unlikely in most pleasure boats even those with metal tanks and how many boats these days have metal fuel tanks.
 
NormanS said:
There's interesting stuff in that, but surely if droplets of water fall down into the fuel, their being considerably more dense than the fuel, would suggest that the water would (and does) accumulate at the bottom of the tank. That's fine, and my tank sumps, with drains, take care of that.

Now you are introducing the prospect of "hazy" fuel, by effectively having a vent to the tank. What is it? How does it form? And what precautions do you suggest should be taken? Must say that I have never experienced "hazy" fuel in any of my boat tanks.
Click to expand...

I can see I have not been clear.

a. Corrosion does NOT require a free layer. It requires only enough water to increase ion availability in the fuel and to increase conductivity. Smae with bug growth; there needs to be moisture in the fuel, but there does NOT need to be a free layer.

b. Hazing of fuel due to moisture is always a very brief matter. The droplets coalesce within hours and settle to the bottom. You will not see this in the Raycor filter or with occasional checks.

These three liter bottles were stored in a temperature cycling chamber for 2 months (high humidity, 90/80F swing). All three had 12" x 1/8" ID vent hoses. The left bottle was open, the center bottle had a silica gel filter, and the right bottle had a carbon filter. The color change is primarily due to corrosion of the metals. None of the bottles had a free water layer (in other words, draining the free water doesn't help as much as you think--the diesel is still saturated). The filters were scaled to match full scale applications (they were thumb-sized).

diesel+vent+filter+jar+closeup.jpg

The point is that even tiny amounts of water can make a difference, almost as much as a real leak.

Two proven cures are vent driers and corrosion inhibitors. I have primarily tested US products, but I know that Soltron combined with Biobor does very well. However, MOST additives are not effective, so unless you see test results, you don't know anything. I believe only Practical Sailor has published corrosion data.
 
Last edited:
Thanks for the further explanation. My present fuel tanks are stainless steel, and they don't have bits of extraneous other metals in them, so I'm not worried about possible corrosion.

It seems that I don't need to worry about the hazy fuel either, if at worst it only lasts for hours, before settling out on the bottom of the tanks, which in my case means it drops into the sumps. This is helped by having tapering tanks, so that the tank bottoms are relatively small.

My previous boat had mild steel tanks, similarly equipped and designed, and gave no problems during my 30 years usage. I have never doped my fuel tanks with any form of treatment, but I am meticulous about removing even the slightest trace of water. Maybe that's better than waiting until the symptoms appear, and then treating them.
 
In my previous reincarnation water in the fuel was a big no no. There was absolutely zero chance of water getting into the fuel from the filling point, all the water that WAS in the fuel either came from the supplier or from condensation/tank breathing. Because it was a no no to supply water to gas turbines ( massive deleterious effect on nozzle entry guide vanes and first row of HP turbine blades) the fuel was centrifuged before being put into the service tanks and then pumped from the service tanks through pre filters and coalescers before being supplied to the engine. Massive capital cost of equipment and an eye watering bill to replace both prefilters and coalescer elements. There is no way that any water got into the filling connection, so while the deck cap in a yacht may or may not leak it is most definitely not the only source of water in fuel - despite the urban myth that is oft repeated and quoted here.

Leaving aside corrosion issues and patchy lubrication of fuel injection pumps, water present in fuel promotes the growth of cladisporium resinae (fuel bug) which even if treated with biocides will block filters.

Just saying
 
I still can't understand where the water comes from to condense inside the tank. I would have thought that the water in the air would only condense out once and drip into the fuel and sink to the bottom. Unless further saturated air is introduced, there is no more water to condense.
I think that was the gist of the previous argument which seems logical to me.
 
ghostlymoron said:
I still can't understand where the water comes from to condense inside the tank. I would have thought that the water in the air would only condense out once and drip into the fuel and sink to the bottom. Unless further saturated air is introduced, there is no more water to condense.
I think that was the gist of the previous argument which seems logical to me.
Click to expand...

I'm only guessing here............. if the moisture in the air has departed to the bottom of the tank, the air is of course drier. When outside temperatures change and the outside air contains more moisture than the air in the tank, the air in the tank absorbs moisture from the surrounding air. That then condenses and the cycle repeats.

Obviously talking tiny amounts, but tiny amounts that will build up. I also do not agree that the water is present in the fuel when it is purchased. A car uses a lot of fuel and i have never known one to suffer from tanks full of water (not for these reasons) or to be infected with fuel bug.
 
ghostlymoron said:
I still can't understand where the water comes from to condense inside the tank. I would have thought that the water in the air would only condense out once and drip into the fuel and sink to the bottom. Unless further saturated air is introduced, there is no more water to condense.
I think that was the gist of the previous argument which seems logical to me.
Click to expand...

The tank is not sealed (even with a tightly fitting deck connection) because if it was, the vacuum developed in the tank as the fuel is drawn off by the pump would reach a point where it would defeat the pump and the engine would stop. There has to be a breather to allow atmospheric pressure to be maintained. The more obvious example of this arrangement is your outboard motor. The action of breathing brings in air/moisture which can then condense out.
 
NormanB said:
The tank is not sealed (even with a tightly fitting deck connection) because if it was, the vacuum developed in the tank as the fuel is drawn off by the pump would reach a point where it would defeat the pump and the engine would stop. There has to be a breather to allow atmospheric pressure to be maintained. The more obvious example of this arrangement is your outboard motor. The action of breathing brings in air/moisture which can then condense out.
Click to expand...

As far as I know, my outboard doesn't breathe. It also has a closeable vent which is only open when the engine's running.
 
This is all gripping stuff - but what is the conclusion? The advice has always been to leave the tank full of diesel through the winter. Does this still make sense. Is the benefit, if any, offset by the deterioration in the stored diesel?
 
richardabeattie said:
This is all gripping stuff - but what is the conclusion? The advice has always been to leave the tank full of diesel through the winter. Does this still make sense. Is the benefit, if any, offset by the deterioration in the stored diesel?
Click to expand...

My conclusion is this, if condensation does occur (as i believe it does) keeping the tank full is a benefit.

If i'm wrong and it does not occur, there isn't going to be any noticeable deterioration in the fuel, so no downside.
 
I always fill my tanks before the winter------------------- in the fond hope that the fuel might be cheaper than it will be in the Spring.
 
ghostlymoron said:
I still can't understand where the water comes from to condense inside the tank. I would have thought that the water in the air would only condense out once and drip into the fuel and sink to the bottom. Unless further saturated air is introduced, there is no more water to condense.
I think that was the gist of the previous argument which seems logical to me.
Click to expand...
Your answer is quite correct, and although there will be a small exchange of free molecules in and out of the tank via the small breather, the actual amount of water vapour that will enter and condense out is about 1/2 teaspoonful per annum for a tank with a 1/4" vent, empty and getting cold below the dew point, each and every day of the winter.

A leaky O ring on the filler could allow a cupful or more per day by comparison.
 
You must log in or register to reply here.

Other threads that may be of interest

lustyd
Fuel Gauge Calibration
2 3
Replies
45
Views
703
paulbrown22
P
W
Budget diesel polishing
2
Replies
26
Views
686
Iliade
Iliade
S
Fuel tank cap
Replies
11
Views
356
ylop
Y

Share this page

Top