How to caltulate bottom's thickness of tanks using API 653? 1

[carlosllv]

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?

 

[zelgar]

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.

 

[Duwe6]

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.

 

[carlosllv]

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.

 

[Duwe6]

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.

 

[bydublin]

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!