Fuel Tank Stainless or Alum 3

[Christo_AU]

Hi All. We have to fabricate a 200 litre (50 gallon) fuel tank that will be underslung a trailer. This fuel is not for the truck, it is for a generator mounted on the trailer. Someone told me that it is better to fabricate it from alum sheet, because they said stainless steel sheet will crack. Can someone please validate this and explain why stainless steel sheet will crack but not aluminium sheet when used for a tank on a trailer?

 

[MintJulep]

Why don't you ask "someone" the reason for their recommendation?

Metals (and other materials) crack when the load exceeds the strength, or when cumulative fatigue damage exceeds the fatigue tolerance of the design. You need to understand this before you start fabricating things. Especially things filled with inflammable liquids used on public roads.

Many fuel tanks are made of aluminum.

Many fuel tanks are made of stainless steel.

Many fuel tanks are made of steel with various coatings inside and out.

Many fuel tanks are made of aluminized steel (which I guess counts as a "various coating").

Many fuel tanks are made of plastic.

Fuel tanks are cheap and easy to buy in many different shapes. It seems unlikely that you "have to fabricate" one.

There are probably laws and regulations about fuel tanks that you need to comply with.

 

[IRstuff]

Please refrain from duplicate posting

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[Christo_AU]

Apologies, I have deleted the duplicate post.

 

[TugboatEng]

Aluminum is generally superior. Thicker plate can be used which requires fewer bulkheads for reinforcement.

Stainless steel also has issues with road salts and dirt. It requires frequent washing to maintain it's "stainless" properties. High concentrations of salt on the surface can initiate stress corrosion cracking. Aluminum has similar sensitivities to dirt but is not as severely affected by salt.

 

[BrianPetersen]

If this is for a mobile application - and I don't care that the fuel is going to be used for a generator mounted on the trailer as opposed to for propulsion, it can still leak and catch fire and it can still be involved in motor vehicle collisions - your first step is to consult the relevant regulatory bodies in the place where you are going to be using this trailer, and establish what standards apply and what those standards have to say about it.

Buy, if at all possible. Don't make.

 

[CWB1]

If you have to ask the question then you should be outsourcing this to a supplier with experience designing onroad fuel tanks.

 

[EdStainless]

And ask TBE what happens when you get salt water inside of tanks.
I would think that you can find a pre-manufactured Al tank that will work fine.
Providing very good support is critical.
The dynamics of liquid sloshing inside of a tank while being moved can impart some very high forces.
I am a SS guy but the risks of external corrosion on SS concern me.
I have used SS tanks in past, and the outsides were always coated with very good products.

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[TugboatEng]

The good news is that with the modern ultra-low sulphur diesel fuels the microbial induced corrosion isn't so much of a problem anymore. I wouldn't worry too much about the inside of the tank.

 

[ctopher]

Being mounting under a trailer, there must be some codes somewhere that dictate material and size/location.

Chris, CSWP
SolidWorks
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[LittleInch]

Well it will depend on what you're doing with the sheet?

Are you forming the sheet into a rectangular tank with some sharp edges?

Aluminium is "softer" than Stl steel, I.e. has a lower yield, but doesn't work harden as much which is where you might get "cracking"

The radius of any bent shape will be crucial as well. Make it 10 to 15mm and you shouldn't have an issue with either.

Make it sharp edged or <5mm then you might.

So much depends on details you haven't told us....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Christo_AU]

Thank you very much for all the replies, much appreciated. The reason for making this post is for me to understand WHY st steel tanks will crack but alum tanks won't. I think there might be a gap in my materials science understanding here.

As requested, I have added a typical cross section of the tank, see link below. Bottom section is bent from 5052-h32 bendable marine sheet. Internal radius is 9 mm. End plates are tig welded from outside and mig welded from inside. Will have baffles every +- 400 mm. Top sheet is only welded form outside.

LittleInch, thank you very much for your feedback. I do have some questions regarding the following you said:
"Aluminium is "softer" than Stl steel, I.e. has a lower yield, but doesn't work harden as much which is where you might get "cracking""

I don't understand how this can be true if the ultimate strain of alum sheet is less than stainless sheet? This was actually the original reason why I made this post, because in my mind it doesn't add up why st steel would crack before alum. I think I might be missing something?
5052-h32 ultimate strain: roughly 10%
5052-0 ultimate strain: roughly 25%
316 st steel ultimate strain: roughly 40%
So doesn't this imply that 316 can be bent (strained) much more than al before cracking.

As TugboatEng and EdStainless have suggested, maybe this relates to corrosion cracking.

I also don't think fatigue could be the culprit? Does anyone have any insights?

I really do appreciate the feedback.

 

[3DDave]

The reason for making this post is for me to understand WHY st steel tanks will crack but alum tanks won't.

You are trying to understand why someone told you that. You need to ask them.

Every suitable metal will crack if used improperly. Fatigue cracks are independent of plastic strain capability as they occur in the elastic region.

 

[SWComposites]

Yes, who is the “someone”? And why do you believe them? If you do, ask them to explain their reasons in detail.

 

[Christo_AU]

It is the fabrication manager. Unfortunately, he doesn't know why. Experience and observation has led him to this conclusion.

I believe that he is correct. However, I would like to understand WHY this is true. Because if I understand the underlying scientific principles I can apply it elsewhere.

 

[3DDave]

Same thickness, same weight, same cost, same details, same fuels?

Plenty of companies make stainless steel fuel tanks. If your fabrication manager doesn't have confidence in stainless steel, don't use it.

In a brief search I see the US Coast Guard recommends 316 and care taken in making welds; a number of of comments like monel for boat tanks over stainless and aluminum.

 

[TugboatEng]

OP asked a very valid question. I don't see the reason to dog pile. Stainless steels are challenging materials to work with that don't succeed in many environments. I don't think a lot of engineers know this and I see over-application of stainless steel everywhere. The issues associated are unusual and not always the result of design or quality control.

 

[LittleInch]

I think this is more about welding than forming cracks. Welding SS is not easy and needs careful work, especially on thin sections.

But you really to talk to a metallurgist or welding engineer.

There is also mention of SCC which may be more prevalent in bent sections.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LittleInch]

Ask the fab manager where exactly he sees cracking?

In the weld?
At the bent sections?
Somewhere else?

That will give you something to go on at least.

3mm is not very thick so will heat up and cool down rapidly and my be harder to weld without cracking as a thin sheet than maybe pipe or thicker plate

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MintJulep]

he doesn't know why. Experience and observation has led him to this conclusion

So without understanding the why, he just declares stainless bad - never to be used.

It's commendable that you are trying to understand.