Condensation in fuel tank

PaulRainbow said:
That's wrong. Diesel can absorb water, the saturation point depends on temperature. Once it reaches saturation point it settles in the bottom of the tank. If your diesel contains less water than the ambient air it will absorb water, diesel is hygroscopic. So you can have times when the diesel will absorb water from the air and times when the temperature drops and that water is released to fall to the bottom of the tank.
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Yeah, yeah, but saturation in diesel is about 50ppm (parts per million), so in a 100 litre tank you can have at most 5ml of water, distributed throughout the fuel. It's hygroscopic, just not very hygroscopic.
 
JumbleDuck said:
Yeah, yeah, but saturation in diesel is about 50ppm (parts per million), so in a 100 litre tank you can have at most 5ml of water, distributed throughout the fuel. It's hygroscopic, just not very hygroscopic.
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The saturation point varies with temperature and how much biofuel is present. On a hot day it's more likely to be 200ppm plus. If the night time temp drops a lot the saturation point could fall to 50ppm. So your 5ml max will be true, but where's the other 100ml gone ? Bottom of the tank, that's where. Add some condensation the next morning when things warm up and you have even more water.

Granted, some of the free water will most likely be re-absorbed as the fuel warms up, but the net result is more water at the bottom of the tank, IMO. It's a simple matter to keep the tank full and prevent this from happening, however small the amounts may be.
 
JumbleDuck said:
Only if the temperature inside the tank falls below the saturation temperature for the air inside it.
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Isn't that the due point temperature so if the temperature of the inside surface of the tank condensation will occur on the inside surface of the tank.

The temperature of the inside surface of the tank will depend on the temperature outside and inside the tank and the heat transfer coefficient of the tank material.

396px-Dewpoint-RH.svg.png


Look here.
 
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PaulRainbow said:
The saturation point varies with temperature and how much biofuel is present. On a hot day it's more likely to be 200ppm plus. If the night time temp drops a lot the saturation point could fall to 50ppm. So your 5ml max will be true, but where's the other 100ml gone ? Bottom of the tank, that's where. Add some condensation the next morning when things warm up and you have even more water.
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Maybe, but where is that 100ml supposed to have come from in the first place? certainly not from condensation from air, in any likely situation. If this really was an issue, we'd see dead diesel cars all over the place. We don't.
 
Rogershaw said:
Isn't that the due point temperature so if the temperature of the inside surface of the tank condensation will occur on the inside surface of the tank.
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Yup. Saturation temperature = dew point. If you're interested, it's the temperature of the saturation (boiling) point of water at the partial pressure of water vapour in the air.
 
JumbleDuck said:
Maybe, but where is that 100ml supposed to have come from in the first place? certainly not from condensation from air, in any likely situation. If this really was an issue, we'd see dead diesel cars all over the place. We don't.
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Correct. Another point overlooked is that there's virtually no circulation of air in a diesel tank; the air that's in it is essentially static, so there's a limit to how much water it can possibly contain.
 
As a diesel fitter in the plant hire game l have seen, cleaned and serviced many engines operating in many situations. As long as you empty the filter bowl then that's it in a healthy system.
Most outside tanks have a four inch riser to keep the cap out of the rain. Certain manufactures moved the fitting to a flush cap fitting to be vandal proof.
After some time these began to leak as the seal began to fail but the operators claimed water in the fuel until a new seal cured the problem. Just think that most of our fill caps are almost flush with deck and when was the last time we changed the seal?
 
PaulRainbow said:
The saturation point varies with temperature and how much biofuel is present. On a hot day it's more likely to be 200ppm plus. If the night time temp drops a lot the saturation point could fall to 50ppm. So your 5ml max will be true, but where's the other 100ml gone ? Bottom of the tank, that's where. Add some condensation the next morning when things warm up and you have even more water.

Granted, some of the free water will most likely be re-absorbed as the fuel warms up, but the net result is more water at the bottom of the tank, IMO. It's a simple matter to keep the tank full and prevent this from happening, however small the amounts may be.
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Surely the fuel will reabsorb the 100ml in the bottom of the tank before sucking any more up through the air?

Best leave the tank empty then, so the volumes of water are far far smaller and it's easier to filter out. Marine16 will prevent bug... :-)
 
No Regrets said:
Surely the fuel will reabsorb the 100ml in the bottom of the tank before sucking any more up through the air?

Best leave the tank empty then, so the volumes of water are far far smaller and it's easier to filter out. Marine16 will prevent bug... :-)
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I'm not sure if it would re-absorb it all.

If there's no water in the tank, there's no need for marine 16 :)
 
PaulRainbow said:
The saturation point varies with temperature and how much biofuel is present. On a hot day it's more likely to be 200ppm plus.
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Indeed it does but when posting the 50ppm figure (for 20 degrees) I made clear it was for petro diesel with no bio. If the fuel gets as hot as 35 deg C it might get near 100ppm.
Yes biodiesel holds a lot more - about 1300ppm at 20 deg and 1700 at 35 deg.
However when you blend them, the water saturation capacity for the mix is less than a simple ratio calculation would suggest. I don't know why, I'm not a petroleum chemist.
So even if you have 10% bio content, I suspect 200ppm plus is too high.
I agree your conclusion keep the tank full - if you can be sure the fuel you're filing it with is pretty dry!
 
Freebee said:
kerbside pumps tanks that run low may suck water bottoms if they are present.
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I don't understand that part. The pick up pipe in a fuel tank is fixed - let's just say 30mm from the bottom. Once the water in the bottom of the tank rises to 30mm it gets sucked up the pick up pipe regardless of whether the tank is full or near empty. Same goes for crud that might be sloshing around the bottom region of the tank - that crud too gets sucked up the pick up pipe regardless of whether the tank is full or near empty. I think.
 
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