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Life span of storage tanks

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davincigee

davincigee

Mechanical
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Oct 28, 2012
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Hello comrades,
I have 2 questions.
1. I'd wanted to know. How is life span of a welded storage tank factored into the calculation of the shell plate thickness according to API 650
2. I discovered quiet recently that shell plates that were stored in the open and had been sandblasted for construction had their thicknesses reduced significantly. A cursory check on the thicknesses on each sample revealed that the thicknesses of the shell plates were just a mm or two below their minimum design thicknesses. My question is can these be still considered useful or they still have some usefulness but reduced life span?

 
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API-650 does not address life span directly. The owner is allowed to specify a corrosion allowance, also internal and external coatings as applicable, cathodic protection systems, various stainless steel alloys, etc. API-653 has requirements relating to corrosion rates and inspection intervals, and if that information is available, it can be factored into the new tank design as well. Note that there are steel tanks in service that are over 100 years old.

On the loss of thickness observed- that sounds like something abnormal. If the corrosion was extensive prior to the blasting, I suppose it's possible. I would also consider if the plates were mis-identified, or thickness testing performed incorrectly, etc. But in general, they should be the specified thickness when the tank is built.
 
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I agree with JStephen ..... Neither API650 nor any other Storage Tank Code or Standard specifically addresses tank life.

Be aware that storage tank life is controlled by the sole mechanism of internal and external corrosion. In nearly all situations, the tank life is not controlled by swings of pressure or thermal loadings, like would occur for pressure vessels and piping.

(An exception to this would be the flat-bottomed, high-temperature salt storage tanks in a Solar Energy Generating Plant)

Be also aware that, as the decades pass, the useful life of the tank may change due to new coatings, changes in maintenance or service, etc.

The only reasonable way to determine the useful life of the tanks that you manage, is to inspect surfaces and take thickness readings ...

I suppose, in some indirect way, when the tank purchaser decides upon a Corrosion Allowance, he is making a choice about tank life

MJCronin
Sr. Process Engineer
 
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1) I agree with JStephen and MJCronin
2) Plate thickness below design thickness is not allowed to fabricate new tank. Something is wrong with the thickness of the measured plates.
The Acoustic Emissions Examination (AEE) is the best practice for the expected service life.

Regards
 
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Interestingly API 650 does refer to "service life" in a few locations without really defining it or using it anywhere.

The best description comes here: 3.7 corrosion allowance
Any additional thickness specified by the Purchaser for corrosion during the tank service life. Refer to 5.3.2

5.3.2 doesn't add much.

If your plates are below the minimum design thickness than you can't use them and comply to API 650.

Either build a smaller tank ( less height) or buy some new plate.

I think API 650 basically assumes that the tank won't corrode more than the corrosion allowance (if any).
If it does then you need to assess it in a fitness for purpose assessment, normally to API 653 for API 650 tanks. However you can't do that for a new tank. That would not be permitted.

So yes the plates are still useful, but not for your original tank but a smaller one where the min design thickness is less than your "sandblasted" plates





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

Corrosion allowance is a de-facto tank lifetime gauge... Your sandblasted plates can serve as materials for a redesigned smaller tank.

As a point of reference, even the sellers of baked-on coatings for "Glass-fuse-to Steel" tanks only advertise a 30 year service lifetime.


MJCronin
Sr. Process Engineer
 
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Note that per API 650 4.2.1.2.2 an underrun of 0.01" [0.3mm] is allowed for shell plate. This referrs to the as-deliverred plate thickness from the mill or supplier, it was not intended for your situation and is probably too small to help. My best suggestion is to recalclate the allowable liquid heights for each plage, use all the material you have for upper tings and bring in new steel for the bottom ring.
 
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Try with joint eff. E=1

Regards
 
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