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How to caltulate bottom's thickness of tanks using API 653?

How to caltulate bottom's thickness of tanks using API 653?

How to caltulate bottom's thickness of tanks using API 653?

(OP)
I want calculate bottom's thickness of tanks using API 653, but I have some questions about it.
I have a tank. It's over 20 years. I dont have inspection reports about pitting and bottom's thickness. Just I have design planes of the tank. In these planes thickness of the bottom = 6mm (0.2362in)

I'm assuming:
- RTbc = 3mm. (API 653, 4.4.7.4)
- RTip = 3mm.
- Or = 5 year, StPr= 0 (bottom is painting), UPr= 0.005in/year
 
Is RTip = corrosion allowance? Or Is RTip the total thickness of bottom?
 

RE: How to caltulate bottom's thickness of tanks using API 653?

From API 653, Section 4.4.7.1
RTbc = minimum remaining thickness from bottom side corrosion after repairs,
RTip = minimum remaining thickness from internal corrosion after repairs,

See Sections 4.4.7.3 & 4.4.7.4 of API 653.  In general, if you're lower than 0.1 in you're going to need to perform a stress analysis.

RE: How to caltulate bottom's thickness of tanks using API 653?

An Internal Inspx is mandatory under API-653.  No 20-year calc w/o internal measurents can be anything other than a "WAG" a wild-aaed guess.  A WAG is not engineering, nor does it qualify as Mechanical Integrety and Risk Management.

MISTRAS and several other tank inspection companies have remote-controlled bottom rovers that can video and measure thicknesses with product in the tank.

RE: How to caltulate bottom's thickness of tanks using API 653?

(OP)
Yes Duwe6, You're right. I agree with you.
I asked for last inspection report but They answered me: "We don't have it".

Now, my question is about the meaning of: RTbc and RTip.

I found la description about RTbc and RTip in API 653 4.4.7.4.

API 653 4.4.7.4. tells:

RTbc = minimum remaining thickness from bottom side corrosion after repairs,
RTip = minimum remaining thickness from internal corrosion after repairs,

But I'm a bit confused,

If I have a bottom of 5/16 inch thickness total (with 1/16 inch for allowance corrosion), then:

Is RTbc the low half (ground side) of this bottom (5/32 inch)?
Is RTip the high half (hydrocarbon side)  of this bottom (5/32 inch)?

Note: I'm attaching a draft.

 

RE: How to caltulate bottom's thickness of tanks using API 653?

Both are *minimum* remaining T.  

As written, this part of 653 is confusing to the point of being silly.  Table 4.4 gives minimum [total] bottom thickness.  Tank floor Retirement Date can be calculated by using the MEASURED top-side plus MEASURED bottom-side corrosion rates.  Next Internal Inspx has to be no more than halfway to the Retirement Date.

6.4.2 gives the maximum Inspx intervals.  With a nominal thickness, you are required to perform an Internal Inspx 12 years after the last *documented* Internal Inspx.  If your tank has both leak-detection and 100% containment [impermiable dike], the interval is 20 years from last Inspx.

Thus, your guesstimates of corrosion rates gives either 12 or 20 years per 6.4.2, or less per your calc's.  The 12 / 20 year intervals are fixed and not to be exceeded, barring a full RBI Evaluation.

RE: How to caltulate bottom's thickness of tanks using API 653?

Hi,

Can anybody help on the calculation for the wall thickness of a storage tank 98"OD x 216"straight side? The SS304L tank is vertical with cone top and flat bottom at atmospheric temperature and pressure to hold 67% HNO3 (1.41kg/l or 11.7 lb/gal). We had a proposal with 10gauge SS304L for this tank. But not sure if it is safe for tank with this height. Highly appreciate your advice.

Thank you!

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