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Old 27-08-2015, 21:29   #1
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TEE the diesel fill line

I want to add an additional diesel tank under the floor so about 8 ft forward and 3 ft lower than the existing port tank. And I am thinking to TEE into the existing fill hose. So while filling the port tank, it would likely fill the lower tank first and then continue to fill the higher tank. Connect the same vent??
So after I empty the existing tank, I would use a small Facet pump to refill it from the new lower tank. This would eliminate a third fill connect thru the deck. Current tanks are aluminum, new tank is a Moeller rotomolded tank. Is there any good reason NOT to do this? Use an aluminum TEE?
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Old 28-08-2015, 02:21   #2
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Re: TEE the diesel fill line

At the very least I would put a valve in the fill line after the tee so you could fill the existing tank independently of the new tank should the need arise. I would use a seperate vent.

Personally I would perfer a completely seperate deckfill but YMMV.
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Old 28-08-2015, 03:07   #3
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Re: TEE the diesel fill line

I don't think it is going to work. One vent means the first tank being filled has to vent through the second tank, and with the randomness of just a tee in the fill line, the second tank may have too much fuel in it to allow the vent system to work properly. Tanks are usually connected at a lower point with valves in the cross over plumbing to control fuel use. You may vent the new tank by a tee into the existing vent so that there is only one vent exposed, but I don't think the original idea will function properly. A fill and a vent for each tank would be the correct way.
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Old 28-08-2015, 03:55   #4
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Re: TEE the diesel fill line

Tanks at different levels can get a bit risky.


I agree with Wotname. Without a valve on the lower fill line and using a shared vent, the bottom tank would fill 100% before beginning to fill the upper tank. Any temperature related expansion on a warm day would then cause fuel to run out your vent line.
Also be very careful with your fuel supply lines. If you ever accidently leave valves open allowing the upper and lower tanks to equalize, you could have overflow problems. Your bottom tank vent needs to be at least as high as your top tank vent to minimize this risk.
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Old 28-08-2015, 09:12   #5
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Re: TEE the diesel fill line

thanks for the input. There is not good location for an additional deck fill - hence my idea to tee the existing fill. I did a quick search and did not find any 3 way valves this large suitable for diesel. Current tanks are under the berths with large, difficult to move mattresses.
  • Quote:
    bottom tank would fill 100% before beginning to fill the upper
    do I care? I can add a small Facet pump to pump to the upper tank
  • Quote:
    allowing the upper and lower tanks to equalize
    they cannot, upper tank filled only by the pump
  • concerning the vent - I would tee these well above any fuel level.
  • a
    Quote:
    you could have overflow problems
    - how? when I fill now, I know it is full when fuel comes up near the top of the fill hose. I think the vent line must also be nearly full.
So I see the biggest problem to be making sure the hose connection to the lower tank is well sealed as there will be slightly more head than the current tanks but I would have this same problem with a separate deck fill.
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Old 28-08-2015, 09:40   #6
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Re: TEE the diesel fill line

I've never heard of it being done but also can't see why it wouldn't work.

Some suggestions:

Tee the vent lines as high as possible - just before the hull fitting. This will keep both vent lines working for their respective tanks. Make sure the vent lines are of a fuel approved hose.

Also tee the fill lines well above the top of the high tank. You really don't want fuel running back and forth between tanks in a sea.

Immediately after filling, pump enough fuel from the lower tank into the upper tank to empty the fill and vent lines for the lower tank and allow for expansion. This will also relieve any static pressure against the top of the tank from the fuel in the fill hose.
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Old 28-08-2015, 09:46   #7
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Re: TEE the diesel fill line

Just because it's T'd doesn't mean the fuel will go one way or the other when filling. Depending on the T orientation, the fuel will flow the easiest direction first.
Vents are often problematic on separate tanks with separate vents as it is. So vent each tank... even if those vent come together at the top near the exit.

If you must do it, put in valves, or a 3 way valve instead of a T on the fill. I had a boat with one fill to two identical tanks, once one tank was full fuel burped out the fill fitting all the time trying to fill the other tank... it was a mess.
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Old 28-08-2015, 10:47   #8
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Re: TEE the diesel fill line

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Originally Posted by CarlF View Post
I've never heard of it being done but also can't see why it wouldn't work.

Some suggestions:

Tee the vent lines as high as possible - just before the hull fitting. This will keep both vent lines working for their respective tanks. Make sure the vent lines are of a fuel approved hose.

Also tee the fill lines well above the top of the high tank. You really don't want fuel running back and forth between tanks in a sea.

Immediately after filling, pump enough fuel from the lower tank into the upper tank to empty the fill and vent lines for the lower tank and allow for expansion. This will also relieve any static pressure against the top of the tank from the fuel in the fill hose.
I would agree with Carl, it can work. Obviously, there is an increasing risk of a mess if a hose pops. As long as the vent line is adequate there is no reason whatsoever that would stop it. Depending on access, you could also fill the second and lower tank by tapping into the first tank at high level so that it overflows to fill the other tank. That would ensure the primary tank fills first. The only difficulty might be ascertaining when the lower tank is full? Might be worth adding a level gauge of some description?
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Old 28-08-2015, 11:21   #9
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Re: TEE the diesel fill line

Bulawayo - the new lower tank already has a sending unit and I think I can orient the tee with a slight incline to the lower tank so the upper tank will fill first. I am primarily interested in doing this while exploring the Outer Bahamian islands and southern coast of a large NW Caribbean island both of which do not have an abundance of diesel readily available.
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Old 28-08-2015, 11:38   #10
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Re: TEE the diesel fill line

Hmm. IF I understood correctly, your method of making sure the tanks are full is to see the level of fuel rise up in the deck fill. IF so, then the vent lines would need to be higher then the deck fill.

Also, vent lines should not have fuel in them. The vent lines is to provide air to allow fuel to leave the tank without creating a vacuum. Not sure your approach allows for this.
And it seems like your approach to filling the lower tank is overflow from the higher tank. Seems like you will have a venting problem there.

One value of having separate tanks is to protect yourself against contamination. It seems like you would contaminate both the upper and lower tank if you have a problem.
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Old 28-08-2015, 12:45   #11
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Re: TEE the diesel fill line

I've seen this before. Aside from the observations made, your engine fuel pump may be inadequate for that kind of lift over that distance.

You would need the engine to run off the main tank and when it got low, use a transfer pump to refill the main tank from the secondary.

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Old 28-08-2015, 13:01   #12
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Re: TEE the diesel fill line

Hi Sailorman, it makes sense. Our own arrangement is to use a Vetus fuel bladder inside a heavy duty, reinforced canvas 'pillow case'. It is a rather different as we are on a cat.
The fuel shall go up the vent pipe until it equals that of the upper tank but it the boat is heeled it shall go higher so ensure your vent pipe outlet is sufficiently high to prevent spillage.
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Old 28-08-2015, 13:52   #13
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Re: TEE the diesel fill line

Cabo - this 3rd tank (31 gal) is longer term storage, I said earlier than I would use a small Facet pump to move to the upper tank.

Bulawayo - I am also a cat, my current tanks are only 12 gal each, heeling is not an issue and I do not want to line extra jerry cans along the rail.

Going to rough this out this WE and see what I will need. Looks like the TEE will have to be fabricated

thanks to all for your thoughts - Always good to get ideas other than my own. Ed
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Old 28-08-2015, 14:04   #14
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Re: TEE the diesel fill line

Can you tee the vents too?
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Old 28-08-2015, 16:03   #15
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Re: TEE the diesel fill line

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Originally Posted by Sailorman Ed View Post
Cabo - this 3rd tank (31 gal) is longer term storage, I said earlier than I would use a small Facet pump to move to the upper tank.

Bulawayo - I am also a cat, my current tanks are only 12 gal each, heeling is not an issue and I do not want to line extra jerry cans along the rail.

Going to rough this out this WE and see what I will need. Looks like the TEE will have to be fabricated

thanks to all for your thoughts - Always good to get ideas other than my own. Ed
Good luck with whatever you decide - I agree that deck jerry cans are ugly, likely to get ripped off in bad weather and subject to theft.
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