Fabricating Fuel Tanks with Internal Baffles

About two lives ago, I built a tank like that for use in a hydraulic dynamometer, the only significant difference being the length, which was about 10 feet. However the baffles used in it were secured only bottom and sides. Do they really need to be secured along the top, and if so, could you stitch weld them through maybe 6 - 8 slots cut through the top? I suppose you could put in access ports, and weld through them, using mirrors... but how masochistic are you? Of course, the top of the tank could be welded on in sections to match the baffle spacing -- but that seems like a hassle

Hi Ken, I considered using a shoe box style design. It all depends on whether the inspectors would pass it.

It also occurred to me that if I add an 'L' flange to the top it will stiffen the vertical plates. The shoe box design wil allow the welder access for three sides. Adding inspection cleanout ports in every second compartment should also facilitate internal access to the top seam. I eliminated a 12"step along one side so the total ullage is now 1000 gallons per tank; and tank width is now 61". There are two such belly tanks per vessel. In rough seas we can't afford to have something come adrift and break a seam open. The vessels are ocean going so I have to expect them to encounter 30 foot waves from time to time. When you cut the slots and fill them with a welding bead is that called a plug weld?

regards,

Elnav

Ok here is another one if the rack does not work, In aviation i have to make or work on all customk tanks, most aviation tanks are a series of tank installed in he wing or the wing itself is sealed up and used as tank using prc as a sealant the a large wing panel has screws to access the tank the sealant s applied the closed up, anotherway would be to use bladders that are custome made to fit in the wing, the bladders can be made with baffles, the bladders are rolled up and fit inside a 8 inch access hole, the bladders also are made with filler and outlets built in. the bladders can be made to any size. but the bigger the bladder the bigger the access hole is needed.

Make the baffles in a rack slide the rack into the tank, weld the tank lid on,

or get the foam that nascar uses in the race cars.

How much ullage is lost to the foam? Does it stand up to diesel as well as it stanjds up to gasoline?

Elnjav

it is a open cell polyureathane and not affected by diesel that i know of.probly take about 20 percent of the volume.

That's what I was afraid of.

Normally the safety margin is 15% so a loss of 20% just for the foam is unacceptable. In this case 20% amounts to 200 gallons X 2 or 400 gallons total. That is simply no to going to be accepted by potential buyers. These vessel must have extremely long range of 2000+ miles or more on full tanks.

Something else is how to clean out crud from the foam. Some parts of the world delivers unbelievably dirty fuel. Or in the case of fuel barges the last load could have been bunker C and the diesel simply acts as a wash for the tank sides and this crud now enters the fuel system. We can handle that, but not the tank foam contamination.

Elnav

Somebody else has already said it, but the slide in rack is the answer, welded to one end of the tank which is removeable for cleaning in the future. Do not forget some inspection holes on the top.

Try use only one internal baffle made with metal bended as zig zag. The breaking force of light weight fluid (diesel) inertial over the baffle is so low (at 24" centers) that you can weld only one of each three baffles. If baffles are not significant in structure reforcement and only control fluid waves, not really need ride to bottom to top. You can leave one inch at top and other at bottom.

If the baffles are to be placed on 24" centers why not build the tank as a series of rectangular sections each welded with the baffle in place?

Thanks! This is likely what will have to be done. It means many more feet of welded seam to inspect Since the tanks are not removable and much of the outer surfaces inaccessible once the vessel is built around them, the slide out idea will not work. Nor will the zig zag patterned single baffle meet relevant standards requirements spelled out for this class of vessel. If an inspector won't approve something, the vessel is uninsurable.

I just hope the yard has fixturing suitable to deal with such a large assembly to keep it within dimensional spec to fit the molded in supports.

Elnav,

I like the slide-in rack approach already suggested.

The rack structure need not impose much of a weight or volume penalty if you design it and fixture it in the tank in such a way that the forces on the baffles will always have the supports (rack structure) in one direction in tension.

Alternately, the tank could be welded around and to the baffle frame (rack). Or some combination of the two, all factors of how you will fabricate the tank being considered.

Greg

Greg I too am intrigued by the rack idea. However these tanks will be imbedded into the vessel structure with no way to access any of the exterior sides so as to make them removable. I'm not even sure I will have access to the various cleanout ports I designed into it.

In most vessels the removal of tanks involves cutting apart the hull, extracting the tanks, sometimes in pieces then after replacement tanks are installed; rebuilding the hull.

As an example; the typical Taiwanese built trawlers dating back to the 1980's are all now facing tank replacement. At that time diesel tanks wer all black iron. Current market value of these boats run $80K - $125K but tank replacement typically cost $20K. Its usually the biggest deal breaker after dry rot in the wooden superstructure.

Aluminum typically offers longer service life, provided you mount them correctly and avoid moisture collecting near bottom. The demand for longer cruising range has forced all builders to provide bigger and bigger tankage which in turn exacerbates tank replacement and even access for cleaning. This particular boat will carry 3400 gallons in four tanks.

This is a very interesting project you have on your hands. I have been tossing a few notions around. I'm not an authority on this type of project, nor do I pretend to be, but maybe a little food for thought will help in some way.

I don't know what material, or size of sheet stock you intend to use, or what fabrication tools you have available. I would presume stainless would be a preference for corrosion resistance. I would also presume TIG would be the recommended welding method.

One thought, was to fab a trough with baffles that would not need to contact the lid at 48", and then to have alternating baffles on the lid that would not go to the floor. The lid baffles could be TIG welded to the side walls through your inspection covers, or the sides could be slit for the baffle panels and welded shut on assembly.

If the baffles had to contact all four surfaces, with this plan the baffles could be mapped out to meet up with slotted, or drilled positions on the floor, and top panel. With a good deal of accuracy, and maybe with tabbed baffles, these positions could be welded shut on assembly.

Another idea was to build L shaped tank halves that are a mirror image of each other with the baffle plates offset at 48". Once mated, the baffle welding would likely have to follow one of the methods above.

Another thought was that you might consider body panel adhesives as an option for securing your baffles. Of course, you would need to consult the manufacturers of these products before engineering this into your plan. These are wonderful products though, and may well be worth your consideration.

I Know, someone will give me flak for that notion, but it is a thought and it might help.

Somehow, I feel that the idea of a long baffled tank is stronger, better for the vessels structure, and maybe even more dependable for the integrity of the engine fuel supply than would be several smaller tanks. I am also sure it will be easier, and more reliable to plumb two tanks, as opposed to ten or more individual fuel cells.

Thanks for the input. I'm not an expert either but since I kept discovering shortcomings and mistakes made by the naval architects in the design office the department head asked me to take over. I'm a generalist plus I actually have experience in the field crawling around in the bilges of yachts and ships. Although my area of expertise is electrical I have had to deal so often with piping and mechanical systems associated with electrical system that I have a good working background.

The design and engineering is done here in North America but unfortunately the actual fabrication wil be done in mainland China. I was told not to assume any skill or expertise on the part of the chinese labor. Everything I would normally assume a North American trades person would know, now has to be spelled out in extreme detail. I can't even assume the welder is familiar with various metal techniques. He may be skilled only in one metal and one type of weding machine.

Fuel tanks will be aluminum. Potable water will be stainless steel.

We have seen problems with rotomolded poly tanks. GRP tanks are becoming cost prohibitive due to cost of raw materials ( petroleum based) and cost of tooling molds for low volume production

Another option is CNC cut plate in 20 foot X 8 foot sheets. But that requires laser cutting in N America and then shipping to the Chinese yard. Its mainland China not Taiwan, and not in the traditional boat building areas. I'm told they do have facility with regular heliarc aluminum welding of plain sheet stock.

For strenght it would be best to have all abutting plate seams welded. Until you have seen the force and acceleration imparted to a vessel being tossed around in 30 foot seas, its hard to imagine. There have been instances of under built tanks having the end walls punched out by sloshing fuel in half or quarter full condition.

I have seen a partially loaded tanker truck literally hop ahead as the fuel sloshed in the tank after it had to do a panic stop on the highway. When a 12 ton tractor and trailer gets pushed ahead six inches like that, it is easier to visualize the forces involved.

I like the idea of the L shaped tank halves. Definitely worth a more detailed look. One fixture could be used to make the half and be repeated as required on low volume production runs.

One of our naval architects is a specialist in GRP and adhesives for composite construction. I will ask him about that approach.

Tank dimension is defined by the hull configuration and need for longitudinal and transverse ribs for hull integrity. We fabricate the tanks to fit into existing hull cavities. Four tanks is considered the best compromise since not only volume but mass displacement and location in hull must be considered. Fuel weight has a tremendous affect on hull trim and stability. The ability to shift fuel to alter trim is essential for safe operation in various sea conditions. Two tanks are located above the fuel pump on the propulsion engines. Then when everything else fails you still have gravity feed to the diesel engines. For safety sake these must be able to continue running even with total electrical power failure. The ability to segregate contaminated fuel and cleaned fuel is also a consideration.

We cannot afford a totral reliance on electric fuel transfer pumps. Unfortunately placing all the tankage high up would destabilize the vessel and compromise its righting ability in storm conditions. Which brings me back to having to nest the belly tanks into existing cavities down low in the hull.

Elnav

Complete 3 sides from plain plate-bottom+two verticals+2 ends. 56"X240"X18" INSIDE.(If U need OUTSIDEs,--MAKE IT SO!). Mark+Drill 2x9=18 holes dia 1/4"thro' the corners at points where the 9 Baffles will come.

PAUSE!

Make plain top plate 239.8"x55.9" with whatever Inlet/vent/siphoning exhaust nozzles you need.On its lower face- downhand both side fillet 20% weld the 9 Baffle plates with height 17.9 inchX55.9". Corner radii 1/4"(or to clear fillet welds /bend radius of the 3-sided tank-see above--. Top plate now ready.Cut /drill 10x2=20 holes dia 1/2"thro' this Top Plate 6"away from edges at half distance from baffles/ends

PAUSE!

Fit ReadyToplate into the 3-sided tank shell.Raise the whole with bottm at 3'above the floor where the welder walks.Align all walls and make a few holding Tacks downhand .

1 or 2 assistants "fish/push the 9x2=18 baffle corners to align with the 18 nos 1/4" holes--and the welder tacks outer skin with baffle corner .He goes on to thus tack all 18.Now he tilts to 60 degrees and seal welds all 9 in a row. Repeats for other 9.

Completes the job downhand all around. If yoy have a TIG MMA welding machine -use that.

PAUSE AND INSPECT

Your Boss should now give you a raise. He has no other choice.

Enjoy!

IF you are still needing send me your email and I can send you a sketch of the sheetmetal parts and were to put your welds.