Repair of Sulfuric Acid Tank Question

[bldickin]

Hello,

I was placed in charge of repairing a 10,000 gal Sulfuric Acid tank that had ruptured. Originally it was thought that the tank material around the drain pipe had thinned and leaked. After some thickness checking, I found the tank skin to be of good thickness all the way up to about 1" around the exit pipe weld. The actual leak happened through the drain pipe, directly below the weld.

The tank is made of A-285-C with the drain pipe made of A-53-B. I had read in other articles that the weld heat area around any weld could affect the materials abilities to resist corrosion. I originally thought it may have to do with the flow, but this particular side is rarely if ever opened, so it basically sees no flow.

I was wondering how I should go about repairing this area of the tank. My initial thoughts would be to cut a circle big enough to encompass any thinning, replace with new a new A-285-C circle welded to a new drain. Will this work? Should I go about it a different way? Any standards I should check?

Any help would be much appreciated.

Thanks.

 

[MJCronin]

Lots of good info on the internet regarding concentrated sulfuric acid handling and storage.

What concentration are you storing?

What kind of venting system do you have on the tank? Is there a dessicant

Basic concepts:

- Non moving 98% is OK with carbon steel (A285-C)
- Where the acid begins to move faster than 3fps, (ie nozzles) you can either thicken the nozzles/piping or change to stainless steel.
- A phemomena known as hydrogen grooving can occur at the top of piping/nozzles that can cause failuer. Your NDT of the system should consider this.
- You may find a zone near the top of the tank where a dilute layer of acid has thinned the tank wall. The acid becomes dilute as it absorbs water from ambient air and a layer of more corrosive dillute acid slowly eats the upper wall of the tank.

Like love, sulfuric acid tanks were not meant to be forever.....how old is the tank ?

More information needed

 

[bldickin]

MJCronin:

The concentration is 93%. Atmospheric venting. No dessicant, I believe.

The tank is 15 years old. I am not surprised it failed, 15 yrs seems to be a reasonable life. The only reason I am interested in any suggestions is the fact that it was a very localized thinning (right around the drain pipe in stagnant conditions). This caused me to think that the welding may have had something to do with it, or the joint of two metals. I read some on pitting in welds. I also read some on the possibility of the weld being cathodic and the surrounding metal being anodic (or other way around). I was wondering if there were any welding or other materials that may be better? I know 15 years is probably very good, but I think that because the thinning was localized, it may be a problem area and I could make a change that would let it last for more than 15 years.

This situation had already happened on its twin tank and the engineer at that time welded a square with a new pipe over the old using the same materials. I could do this, but having checked every where else on the tank for thinning and finding minimal, I want to try and stop this localized problem.

Sorry for being long winded.

Thanks for the help.

 

[sshep]

Hey bldicken,

I think MJCronin is giving you the most likely cause of corrosion in that location. This hygroscopic phenomenon is a well known problem in sulfuric acid tanks- atmospheric moisture is absorbed causing corrosion near the interface.

best wishes,
sshep

 

[MJCronin]

Suggest that you review:

http://www.tankteam.com/sulfuric_acid.htm

and

http://www2.dupont.com/Sulfur_Techn...nloads/Equipment_FAQs_Sulfur_Technologies.pdf

NACE sp0294-2006 is a good guideline on H2SO4 tank design..... spend the money & buy the guideline.

http://www.nace-txlagulfsection.org/images/NACE_Standard_Application_Example.pdf

 

[bldickin]

Thank you for the help. I have the repair designed and some extra testing plans in place now.

Thanks again.