Tank vent water vapor filter
- Thread starter Roger Long
- Start date
- Status
Yup, you've got that right.
You can also make your own by using clear (polycarbonate, etc.) container and fill with prepared Silica Gel and specially purchased 'dyed' silica gel - that turns from blue to pink when 'saturated with adsorbed water vapor'. To regenerate, simply put the SiGel (but not the container) into an oven at 350 deg. F. for ~6-8 hours, then quickly put it into a sealed metal container (with valve) to let it cool; but, not in contact with the atmosphere. Simple Silca Gel is available from 'craft' supply shops, the 'water indicating' dyed SilicaGel is available from 'technical or industrial hardware suppliers.
You can also make your own by using clear (polycarbonate, etc.) container and fill with prepared Silica Gel and specially purchased 'dyed' silica gel - that turns from blue to pink when 'saturated with adsorbed water vapor'. To regenerate, simply put the SiGel (but not the container) into an oven at 350 deg. F. for ~6-8 hours, then quickly put it into a sealed metal container (with valve) to let it cool; but, not in contact with the atmosphere. Simple Silca Gel is available from 'craft' supply shops, the 'water indicating' dyed SilicaGel is available from 'technical or industrial hardware suppliers.
For marine use, it should be installed with a bypass and two small "joker" valves so it only inhales through the desiccant but exhales through a full diameter vent, bypassing the dryer. That way if you get a burp when filling, the diesel foam won't ruin the desiccant. There may be a ABYC/Coast Guard(?) guideline that says you can't have something like that in the fuel tank vent line.. MaineSail may have that reference.?? Seems like I remember reading something that would not allow a restriction like that in the vent?? Dunno..
Last edited:
Done that for years. Just add a water strainer and fill the inside with desiccant. No need for expensive stuff. BTW, I never fill the tank like most people believe they need to. I only bring enough and always have fresh fuel.
There is a lot of myth on the diesel. Just keep moisture out and use clean fuel.
It is more important to check the "O" ring on the deck fill. It let in more water than anything else.
There is a lot of myth on the diesel. Just keep moisture out and use clean fuel.
It is more important to check the "O" ring on the deck fill. It let in more water than anything else.
Hit the nail on the head, here. Always the first thing to check. I had one boat with a really poor design that just couldn't be properly fixed (it was also located in a run-off channel). Finally solved the problem by replacing the fitting and moving it.
Not a trap...an arch
We're only talking about a fuel tank vent (same applies to water tank vent, btw)...NOT a waste tank vent. You want a waste tank vent to be a straight "shot" to the tank.
A trap will be a place where the water coming into the line via the thru-hull will run down into the trap and pool, blocking the vent. What's needed is an ARCH, so that the vent line runs uphill to the top of it. Any water you take on through the vent will just run back down and out the vent.
We're only talking about a fuel tank vent (same applies to water tank vent, btw)...NOT a waste tank vent. You want a waste tank vent to be a straight "shot" to the tank.
"What's needed is an ARCH, so that the vent line runs uphill to the top of it. Any water you take on through the vent will just run back down and out the vent."
Unless I re-locate the vent lower on the hull this solution is not practical.
The trap will need to drain into a clear bottle that can be monitored.
Unless I re-locate the vent lower on the hull this solution is not practical.
The trap will need to drain into a clear bottle that can be monitored.
I have been sailing in Maine for years where we have large temp swings from day to night and I have never had a problem with moisture in any of my boat's tanks. I usually leave my tank mostly empty in the winter and then clean it in the spring. I will find a very little bit of crud and virtually no moisture. Why would I need a product that removes moisture from my vent line?
Witz / Peggy -
Such a desiccant trap is not for 'free' or 'visible' water but for 'vapor phase' (gaseous) water. Many desiccants, especially SilicaGel, will shatter to a fine powder and be rendered totally useless if in contact with 'free/visible' water.
Petroleum distillates are from high temp. process that essentially delivers a 'bone-dry' product. Water (in vapor phase) from the atmosphere (called relative humidity) enters the fuel by 'chemical equilibrium'. Chemical equilibrium dictates that there WILL be water (in molecular form) dissolved into the oil depending on the temperature and pressure (called the 'law of partial pressures'). The vent is the 'pathway' for water (vapor) to equilibrate into the oil until the conditions of temp. and pressure are met ... a 'saturated' condition. A desiccant trap simply preferentially 'strips' (again by equilibrium) the water vapor (and some oil vapor) from the air AND oil onto the surface of the desiccant ... and in 'both directions' of the 'air-flow' and no matter how full or how empty the tank.
If you have 'free' water getting into your vent .... thats a whole entirely different problem. ;-)
Such a desiccant trap is not for 'free' or 'visible' water but for 'vapor phase' (gaseous) water. Many desiccants, especially SilicaGel, will shatter to a fine powder and be rendered totally useless if in contact with 'free/visible' water.
Petroleum distillates are from high temp. process that essentially delivers a 'bone-dry' product. Water (in vapor phase) from the atmosphere (called relative humidity) enters the fuel by 'chemical equilibrium'. Chemical equilibrium dictates that there WILL be water (in molecular form) dissolved into the oil depending on the temperature and pressure (called the 'law of partial pressures'). The vent is the 'pathway' for water (vapor) to equilibrate into the oil until the conditions of temp. and pressure are met ... a 'saturated' condition. A desiccant trap simply preferentially 'strips' (again by equilibrium) the water vapor (and some oil vapor) from the air AND oil onto the surface of the desiccant ... and in 'both directions' of the 'air-flow' and no matter how full or how empty the tank.
If you have 'free' water getting into your vent .... thats a whole entirely different problem. ;-)
TimR. --- because you keep an empty tank (winter) not enabling the 'time' for the fuel to totally saturate with water; and probably only take onboard fuel enough for your immediate needs, are probably buying 'fresh' (non water saturated) fuel ... you dont need a desiccant trap.
If you were to keep your tank full or 'always topped-off' whenever possible, keep your boat in an area that predominantly has constant high relative humidity and higher temperature .... you'd soon realize that your fuel is water saturated/contaminated and you would then need to think about ways to remedy it.
some of you guys have your boats in pretty cool areas. In florida where humidity can be in the 90s water in the fuel is very common. Full tanks do help. these are some pretty interesting gadgets but on a lot of boats the vents lines are very hard to access.
I believe the design of the vent deck fitting is poor and is allowing liquid water to enter the system as well which is why I want to install a trap. I hear you Rich on the need to use a desiccant. I have a couple thousand Mil-D-3464 Unit Paks that I can utilize to dry out the incoming vapor. Just need to design, package and install the system.
One thought
Looks like a nice idea, but I like Witz's added water trap, and $129.99 for the smaller one is a bit steep, or am I just missing something. I would also think that eventually you would have to replace the dessicant and from the look of the straps, replacement of the whole thing looks more likely.
I think in Florida, you have two opposing and aggravating situations causing your problem:
1. high humidity of the ambient atmosphere
2. You guys are ALWAYS running your air conditioners !!!! and that can keep your tank and its contents at a much lower temp than the ambient atmosphere.
...... If I lived in FL, etc. Id be damn sure that my fuel tank was ISOLATED and INSULATED from the Air Conditioned cooler air !!!!!! A COLD tank in a WARM/HUMID environment ... is a freaking - condensate WATER PUMP.
- Status