Fabricating Steel Tanks to Rest On Ribs (w/ food safe inner coating / lining)

[Inner Love]

Since I plan on buying a MIG / TIG welder anyway, and learning how to use it, I have this idea of fabricating my tanks from 1/8" or thicker plate steel, so they can be slotted into the existing ribs after temporarily removing the skirting. This way I can have minimal dead space under the floor and be able to fit hundreds of gallons of tankage more easily.

As long the water never contacts steel I should never have a rust problem. This is accomplished by coating the inside multiple times with some kind of durable, food safe coating. If welded properly, they should never warp or deform. Baffles are easily added in the fabrication.

Attached is a diagram of my concept, notice how the tanks are made to rest on ribs, as the ribs are already overbuilt to hold up the buses body, they can hold up the tanks as well.

Has anybody done this or made potable water tanks from plate steel?

[ol trunt]

I'm sure you could do that but why reinvent the wheel. Plastic tanks can be ordered to fit whatever shape you want and you won't have to mess with trying to get the inside coated well enough to prevent rust. No reason not to do it other than I'm sure there are other far more challenging and worthwhile projects to spend the time on. That's how I feel about it today--I might feel different about it on another day.
Jack

[Danjo]

With the way you have drawn it, I don’t think you would realize much extra space by It doing that way. 15 gallons? That is so much extra work for so little gain.

But fitting tanks between the C channels, if you had a clear shot for the full width would fit a 75 gallon tank. Not bad.

I got a poly tank for the black tank in the C channel space below my toilet by removing the second battery compartment and shoving it to the other side.

[Samweld]

If your going to use steel, use stainless sheet metal, it's not terribly expensive and you won't have to coat the inside of it. You will however have to purge the inside when you weld it (tig!) also if your going to hang it off ribs like that then you need to make all the metal supporting the tank thicker and you definitely don't need to make the whole thing out of 1/8" plate. I have been planning to build my tanks as well but only because I already have all the tools to build them and the idea of trying to retrofit a plastic tank with a drain where I want it seems futile without it leaking at least a little bit. I would still support the tank from the bottom (the way gas tanks are always mounted). I think it's awesome that your going to learn to weld and these are the exact reasons why I learned to weld. I am almost never satisfied with a product the way I get it and I always want to modify everything. Another possibility is purchasing a pre maid tank and then modifying it, it will save you time and money.

[TJones]

That is a good idea to utilize the space between the ribs. I think it will be difficult to fit a mig gun or tig torch in the tight T and L shapes in your tank during welding. If you want the tank to fit between the ribs my suggestion would be to fabricate rectangle notches into the top of the tank with clearance for some rubber spacers. Then support the tank with straps around the bottom of the tank. The best material would be a stainless steel. I was considering building my own tanks but I will have to look at the expense and time involved with building my own vs buying plastic tanks.

Ted

[Booyah45828]

I vote plastic tanks as well.

I also vote building a support structure off the frame of the bus, and not the flooring ribs. The little bit of extra volume you'll gain by building in between the flooring ribs will be negated by the extra work to make it that way. Tanks also need venting, and to utilize the extra space between the ribs each space will have to be vented separately, or you'll have air trapped and won't be utilizing the volume.

Also, do you plan on removing the side panel to slide the tank in? I don't see any other way of removing and installing the tank without doing so, and the side of the bus gives the floor ribs rigidity by tying them all together.

Edit* I see now you plan on removing and replacing the skirt, Are you doing that by weld or how? What if you need to access the tank after install?

[Mountain Gnome]

Booyah is right. I think:


1) hang your tank from the frame, not the floor, with something (straps or solid bars) underneath supporting it. Otherwise you will need much thicker sheet metal, and the cost and weight will rise quickly.



2) add smaller separate tanks on top of the big main (semi-rectangular) one.


3) vent each tank


welding the inside corner of a 90° angle is difficult/tricky. It usually takes several passes. I'm an at-home self-taught welder, but it seems to me the problem is what Benjamin Franklin taught us: lightning (or in our case the welder's spark/fire) is attracted to pointy things; so he invented the lightning rod. The inside corner of two pieces of sheet metal is the opposite. The weld bead you lay wants to avoid that low-gap and stick to the sheet metal on either side. You have to lay a slow bead and get things to start pooling, which will very quickly destroy your material if it is too thin. A few quicker passes first will add some more material to prevent that (making a mess of the weld, which needs to be cleaned with a wire brush and pick in between laying beads - at least when using a stick-welder as I do). Lots of time and materials.



The outside corner is easy peasy, and for me with little experience, welds nice in (usually) one pass.


So I suggest you build only boxes with outside-corner welds. It will be quicker and easier to construct.


AS far as venting them, one vent system that interconnects all boxes, and a fill-system that fills the bottom and then feeds the top ones (so lowering the level in the bottom siphons the top ones, with the top ones accepting incoming fresh air). Just add some hose-connector nipples to the tanks.


I'd draw a pic, but I'm lazy

[Inner Love]

If I thought I was reinventing the wheel, I wouldn't have posted this idea. Alternate concepts needs to compete with more conventional implementations, like Iceni Johns implementation. https://www.skoolie.net/forums/f50/u...tml#post319554

If going with non-stainless steel, inner coating will be the most difficult thing to do right. You need a durable, food-safe coating that goes on liquid and is semi-rigid when cured, a specialist product. For complete application you'd need to pour it in, and roll / rotate and shake your tank for complete coverage. Then go in with an inspection camera and paint any missed spots. Since I like to build stuff to last I would coat the inside of the tank 3 times, each time scrutinizing every inch with my inspection camera.

Can anyone chime in on the food-safety of cured fiberglass? Perhaps it would be a good idea to line the inside with a minimum 3/8" fiberglass.

I didn't even mention stainless because stainless means money, unless someone has a source for bargain stainless sheets.

The whole point of building tanks this way is to make extremely durable tanks that are self-supporting, without straps on the bottom. Thick plate steel is the primary input, and I believe the whole build to cost about the same (time and materials) as competing methods. I would remove / cut the skirting for the installation, which would be connecting my PEX lines and sliding them onto the already painted and prepped ribs and z-channels, then reinstall the skirting. Minimizing time working under the bus. IMO, if you have to use straps, you are negating the main advantage of this approach.

[BeNimble]

You can get free stainless steel sheet almost everyday if you scan for free fridges.
Steel rusts, it hard to stop it no matter what you do, although hot water tanks are made of steel and seem to last using glass lining (google it) not a DIY solution.
I don't see the value of not having straps at the bottom. I do see the value of filling the gap, which calls for a custom shape and fiberglass works well for that, lots of boat tanks made this way. A bladder too can conform to odd shapes and this solves the venting problem, which is a very good point made by Booyah.

[RamRod4]

10ga Stainless 304 (preferably a #4F brushed finish) will be the minimum I would build the tanks with in the fashion you speak of. This will require no liner or interior coating. Baffles can be added as you please. The main problem is that 304SS is not cheap and getting more expensive by the day right now.

I’m planning to possibly fabricate my own tanks as well, but in a more traditional method. IMO, the design you have created is good on paper but will take a lot of unnecessary time and material for little benefit.

Lastly, stainless needs to be passivated after welding. This can be accomplished to varying degrees in a number of ways. Carbo is enough to the surface during welding and will oxidize and cause rust at the welds if not passivated afterwards. In a perfect world, you’d back purge the tank with argon while you weld the exterior. If you haven’t welded stainless before or done much welding in general, I don’t suggest making a water tank as one of your first big projects. To each their own, just my suggestions.

[TJones]

If you are determined to use mild steel for your tanks their are a number of industrial coatings for painting the inside of potable water tanks. I known rustoleum has one in there industrial coating line. The tricky part would be getting it to adhere well in an area with limited access as prepping the metal and getting the proper paint film thickness may be difficult. A sand blasted surface is preferred for most industrial coatings. You could blast prior to assembling and welding the tank. Although I would be concerned about the weld areas. Swirling paint around the inside of the tank may leave too much paint on the surface to cure properly. It would depend on the product.

Ted

[RamRod4]

Originally Posted by TJones

If you are determined to use mild steel for your tanks their are a number of industrial coatings for painting the inside of potable water tanks. I known rustoleum has one in there industrial coating line. The tricky part would be getting it to adhere well in an area with limited access as prepping the metal and getting the proper paint film thickness may be difficult. A sand blasted surface is preferred for most industrial coatings. You could blast prior to assembling and welding the tank. Although I would be concerned about the weld areas. Swirling paint around the inside of the tank may leave too much paint on the surface to cure properly. It would depend on the product.

Ted

I have used/seen used Tnemec products for applications like this. I was told they make products that can be applied underwater believe it or not. Get out your checkbook though. It's not cheap.

[Booyah45828]

Most of the coatings that I know of that are good for potable water, aren't cheap. And that's why they make potable water tanks out of 3xx stainless or fda plastic. Because by the time you fab the tank with mild steel and then apply a coating, your cost savings of going with the mild steel just evaporated.

Truthfully, I'd use aluminum over mild steel, because it's lighter and you won't need to paint the outside. But alloy isn't good for potable water, so you'd still have to coat the inside with something. And whatever coating you use, make sure it's good for potable water. Potable water means it's safe to drink, so any coating used can't deteriorate or leech into the water, and many paints/coatings just don't play well like that.

[RamRod4]

I went ahead and drew up a stainless 87gal tank today. Plan is to purge and TIG it. I’ll keep my build thread (Our Sheelhouse) updated with progress if anyone is interested.

[Inner Love]

Originally Posted by Booyah45828

Most of the coatings that I know of that are good for potable water, aren't cheap. And that's why they make potable water tanks out of 3xx stainless or fda plastic. Because by the time you fab the tank with mild steel and then apply a coating, your cost savings of going with the mild steel just evaporated.

I thought so, which is why I’m wondering about using fiberglass as the liner. I imagine with the custom shape requirements in boating that fiberglass-contacting drinking water tanks are a thing. Failing that, perhaps an inflatable plastic liner that then sticks to the steel after heat treatment (be messy with your welds so you have a lot of pinhole air leaks)?

Use your fabricated tank as a mold, with some specials fee methods you could make a extremely durable tank using fiberglass as the liner, steel for the main structure, and a truck bed liner for the outer coating. You’re not using the resin for structural support, so you can use less material here.

Being new to welding, Welding steel is challenging, welding stainless is out of my league. While fiberglassing is a craft in its own right, at least you can make adjustments before it sets.

[Booyah45828]

Maybe weld your mild steel tank in the shape/support you want, with a bottom that is bolted on, so the bottom can be removed/installed. Then you can fab up a plastic liner for the inside, insert it into your steel tank, and bolt on the bottom. Plastic can be welded with heat guns and soldering irons, and you wouldn't have to worry about coating degradation, or your steel tank welds failing and leaking. And it would be good for potable water.

Or make the liner from a single plastic sheet in a vac/thermal process the same way they do a lot of plastic tanks.

[Inner Love]

I made up version 2 in MS paint yesterday, note how the tank is simpler. Will fiberglass work?

[Deezl Smoke]

Mostly fast reading the thread so far, but 2 things come to mind from my limited welding skills and perspective.


1: Welding a water tight vessel is very difficult, even with a liner/coating on the inside.



2: The stamped/formed sheet ribs of the bus body being able to support this tank for any serious miles or years.



On the first concern, design your tank with as much bending and as least welding as possible. Plan all fitting/bulkhead piercing ahead and weld inside and out. Welding inside will help with the coating adhesion. Properly sand all internal surfaces removing any and all mill scale etc. prior to welding.


I admit not enough experience to know the exact rib design of your bus. However, IMO, if it is a stamped or formed sheet component, not a hot rolled I or H beam component, then the flange to web junction will be a weak spot and allow the flange to droop and the tank drop.


I like your idea and train of thought. Plastic has been mentioned as well as stainless. I believe you mentioned ss being expensive. Yes, it is. I run a small side business "Some Guy's Fab Shop llc." in which I do some work with ss. It is quite high price right now. Of course A36 and all other formulations are as well for the moment. But prices will likely come down a bit soon.
If you decide the bus body ribs are strong enough to support the system, then perhaps a mild steel enclosure around a plastic tank? Since this tank will not be highly pressurize, 12 gauge A36 or at most 10 gauge would suffice. I've made low pressure hydraulic reservoirs out of 14 ga A36 and have had no issues.

Another option for the steel tank, is to make a gasketed top. This would require either some hole threading for the top to be bolted down, or some nut welding to the underside of the flange. This would allow you to remove the top of the tank for any cleaning or maintenance needs down the line. After all, you spent a lot of time building this tank, it would be nice to not have to destroy it should some basic service be needed, but not accommodated.
IMO.

[musigenesis]

Originally Posted by Deezl Smoke

2: The stamped/formed sheet ribs of the bus body being able to support this tank for any serious miles or years.

I would add to this that since the support members of the floor are just bent sheet metal (usually 1/16" thick) they're prone to suffering severe loss of strength from a degree of corrosion that would just be harmless surface rust on the chassis. It's even worse because those lips/flanges are prone to catching dirt and mud that holds moisture against them (on my bus those flanges aren't even there any more in some places).

[Brad_SwiftFur]

Simply put, the floor ribs will *NOT* hold a water tank long term. You will need to support it from the frame just as the fuel tank is.