fittings for integral aluminum tanks

[BrianR]

Here's a diagram showing my perception of how to fabricate the pipe/ doubler / coupler arrangement. If I make them this way, the doubler, coupling and pipe get welded together at the bench, then slid into a hole in the tank top and the doubler is then welded to the tank top. Then I believe I can reach into the tank through he access lid to weld the pipe to the gusset. So if this will work, then it means I don't have to position any of the tank fittings yet, which is conceptually kind of hard to do, what with the boat upside down, and in such an early stage.

[KevinMorin]

Norm, I agree that taking out the tank bottom junk out of a tank into the filters is good practice but the screens aren't for water, they don't prevent it flowing up to the filters. The screens/strainers on the draw are just for the "big pieces" that can clog the entire fuel line- or that class of particulate. Typically they're several inches long and are laying along the tank bottom so the effective cross sectional filtering area is about the same as a 1-1/2" to 2" pipe. Not too easy to clog but still leaves chips and weld bee-bee's and teflon tape in the tank, but picks up all the free water rolling around at the level of the pickup.

And Brian; they are located at the bottom of the fuel draw on a plastic liner/riser tube which is inside the draw tube, and they come out by opening a through-bore type fitting -thread ferrule capped type- at the top of the tank's draw tube welded-on thread fitting.

There is no good way, I've found, to get the real water bottoms out without a sump or drain which I use even on may gasoline tanks event though it may not be DOT approved practice. The corrosion cells form of long term 'resident' water ten to fifty drops that lay in the corners and flat on the bottom in small puddles that become acidic. Those are really rough on aluminum tanks and are hard to get out with anything but a level/gravity separation geometry.

My main experience on tank bottom corrosion is from two sources. One is [repair of] recreational boats with very shallow tanks that are built to go in under shallow decks in small craft and the other is cannery and larger boats that had both integral and removable tanks.

Consistently the most common failure factor was water bottoms and by the size of many of these corrosion sites the volume of water was very small- less than an ounce; I'm discussing aluminum tanks and don't think steel is as sensitive to water bottoms. So, if Brian's tank bottom is integral to the keel; my view is that he can't get the water out, thoroughly enough, with a draw [alone] unless it is in a sump.

A good compromise might be put the fuel draw down in a sump and take out the tank bottoms during regular running operations? The bottom fuel screen fittings don't plug up unless there is lots of material left in an aluminum tank- but I have seen them plug due to MIG weld soot/smut/black dust. The draws on our current boats haven't fouled in three-four decades- but they're mostly gasoline and that might make a difference?

Brian, the socket weld I was referring to is like a boiler tube in a tube sheet where the rise/down comer is inserted into the tank top doubler about 1/32" and the thickness of the doubler and wall of the pipe form a 90 deg fillet at the end of the pipe's wall and inside the doubler. TIG welding this in one or two passes will drop/sag a back weld into the tank volume surrounding the outer pipe wall under the doubler- which is why the tank top is 1/16" to 1/18" larger diameter than fuel down comer. Another method is to bevel the OUTSIDE of the pipe and make the end flush to the doubler plate TOP surface and TIG weld this grooved fillet to make the seal.

Brian your sketch is one of the methods used, if you can weld in the tank after the top is on- then your ideas all seem valid from here.
Since the down comer is attached inside the tank by welding the seal weld at the top has no need to be back welded to achieve full integrity -there is no torque or deflection loading.

All the risers/down comers in the tanks can be sched. 40 6061 T6 as I don't see much corrosive action on those pipes regardless of size. The bottom sump is a different affair- potentially.

A grain of salt about this topic, in rural Alaska the fuel quality hasn't been know as very clean, maybe in the warmer latitudes water bottoms in aluminum tanks aren't as critical- cleaner fuel; more rapid turnover of supply; higher quality filtration and delivery? I'm used to more or less 'defensive' boat building here, otherwise some of our skippers wouldn't be fishing. Maybe I'm just being over cautious?

[BrianR]

Kevin-
I think a sump with a clean out/ pump out tube is a good idea.
One other question:
Can the fuel supply feed tube inside the tank be larger than the tubing that then runs from the tank top fitting to the engine? Is it okay to make this 1/2" S40, with the same type of doubler-coupler design as the down comer?
Sorry for my ignorance, but I'm treading virgin ground here...learning so much and having a blast doing t!
Thanks!
B

[KevinMorin]

Brian, yes the 1/2" pipe will work for all the small bore pipes in the tank; draw and return. We typically use 1/2" in the tank- sometimes we'll go with 3/8" pipe if the installation is an outboard and we have that pipe on hand-, then neck to 3/8" SS tubing for the runs and then down to 1/4" or smaller at the injector pump.

[BrianR]

Thanks Kevin. But now it gets even trickier. There is not much room on the tank tops to locate the fill pipe coupler! There's enough room for a small doubler and the 2" coupler, but after the tank top is welded in, it is not going to be ideal welding the doubler/ pipe/ coupling assembly in as previously described. That method will work well for all of the other connections. For the down comer, I have come up with an alternate method that will allow me to weld the doubler and coupling in place before welding the tank top to the boat. The coupler extends into the tank, where the down comer would screw into it from below. If I cut the pipe slightly shorter than the distance between the lower edge of the coupler and the tank bottom, then I will be able to insert the threaded end into the coupler and tighten it, thus raising the pipe's lower end the required distance above the tank bottom, about an inch. Then I can weld the pipe to a gusset for support. The only question I can imagine with this scenario is thread sealing: IF I don't seal the thread inside the tank, but the nipple and other pipe fittings that attach to the top side of the couple are sealed with teflon tape ( is that diesel proof?) will it leak at the threads? I don't see how. But then again, my thinking can get myopic at times like these! I'm loathe to put any kind of thread sealant into the tank itself. Whew! Thanks for all of your input!

Brian

[KevinMorin]

Brian, before you do this sacrifice on coupler and one pipe nipple and thread the aluminum pipe into the aluminum coupler.

just to see what happens.

[BrianR]

Good idea...you're thinking that the threads of the coupler will be distorted by the welding? It is certainly possible, and with my ( slowly getting tighter) giant welds a definite possibility. I think I will thread a steel nipple in there on both sides to hold the shape and heat sink it. The couplers that I got are 6061, not cast 345, so they should weld well. Definitely will experiment! Stay tuned...
Bye the way, Kevin, how cold is it there in Kenai? It was 20 here this morning....probably considered a balmy day in N. AK, eh?

Tally Ho!
B

[Conall]

On a pickup tube, the smallest of leaks will cause you to loose your prime and cause much grief on the getting the engine started. Gasoila seems to be on of the most reliable thread sealants I've ever used.

Conall

[nofacey]

Norm,a re you installing any kind of fuel level gauge in your tanks?

b

Yep - thinking a simple mechanical float/dial for day tank, electronic for rest
Moller makes the mechanical unit, WEMA the electronic.