API-650 Annular Bottom 1

[jimmyhutmacher]

Could really use some clarification.

API 650 5.5 "Annular Bottom Plates" states that you can use lap welds in lieu of butt welds in the annular bottom if the bottom shell course is of a material in Group IV, IVA, V, or VI and the maximum product stress (see 5.6.2.1) for the first shell course is less than or equal to 23,200psi or the maximum hydrostatic test stress (see 5.6.2.2) for the first shell course is less than or equal to 24,900psi.

Are the "maximum product stress" and "maximum hydrostatic test stress" referring to the actual or the allowable stresses? If they are "allowable stresses", which I believe they are based on 5.6.2.1 and 5.6.2.2, it'd be nice if they clearly specified it like they do earlier in the paragraph.


Also, S.3.1.3 "Annular Bottom Plates" states that butt welded annular bottom plates are required when either the bottom shell course maximum product stress is greater than 23,200psi or the bottom shell course
maximum test stress is greater than 24,900psi.

Again, are the "maximum product stress" and "maximum test stress" referring to actual or allowable stresses?

Thanks in advance for your help!

 

[JStephen]

First question, it sounds like the intent is to base this on the allowable stress of the material used.
Second question, there is not a 5.3.1.3 in the current standard, so I assume you are looking at an older edition.
I think there are some people on here in touch with the committee, they may have a more definite answer.
You can submit a request for interpretation to API, but those usually take long enough that they don't help THIS time.

 

[HTURKAK]

The mentioned stresses at 5.6.2.1 and 5.6.2.2 are allowable stresses ..See (table 5-2b—(USC) Permissible Plate Materials and Allowable Stresses )for the same.

The limitation for the use of lap welds in lieu of butt welds in the annular bottom ( if the bottom shell course is of a material in Group IV, IVA, V, or VI and the maximum product stress (see 5.6.2.1) for the first shell course is less than or equal to 23,200psi or the maximum hydrostatic test stress (see 5.6.2.2) for the first shell course is less than or equal to 24,900psi.).
for the actual hoop stresses developing at first course during test and service loading.

The subject stresses may be calculated using the formulas given at section 5.6.3.2 .


IIf you provide D, H, G, and first course plt thickness, i can calculate the stresses thaat you are looking for..

Good luck..

P.S. I have the API 650 11.TH edition. The reference numbers could be different at the latest edition.

 

[Geoff13]

For most tanks, the actual shell stress and allowable shell stress should be almost identical. The exception I can think of would be of a small tank where minimum thickness governs, but then you'd be unlikely to use high strength steel.

Because of this, like JStephen, I would be inclined to use the allowable shell stress for both the basic code (5.5) as well as Annex S (S.3.1.3). If you have an unusual tank where the actual is significantly lower than the allowable I would consider using engineering judgement to argue that the code doesn't use "allowable" in the definition. Please make sure to understand the mechanism at the shell-to-bottom junction. I like the Zick & McGrath paper "Design of Large Diameter Cylindrical Shells" referenced in the footnote for the variable design point method.

 

[IFRs]

You have picked (or there exists) a material for the shell. API tells you in section 5.6.2 what the allowable stresses are. These are the stresses referred to in section 5.5. Section 5.6.2.1 defines the maximum allowable product stress, 5.6.2.2. defines the maximum allowable test stress. Table 5.2 lists these stresses for all materials covered in the standard. Use these stresses to determine if you can use lap-welded joints.

Note that Section 5.5 also allows lap-welded joints "When annular bottom plates are not required by this standard, lap-welded or butt-welded bottom plates cut into annular shapes may be installed under the shell, but they would not be considered annular bottom plates (see 3.1)."

 

[HTURKAK]

Plate A 537 class 2 is in Group VI. and has Hydrostatic allowable test Stress St =34300 psi and still could be choosen with a St= 24,500 psi..

Pls notice that the code does not allow the use lap welds for higher strength materials together with higher test stress ( but not allowable test stress ) ..

IMO, the code is clear enough.

 

[jimmyhutmacher]

JStephen, My second question was referring to Annex S, S.3.1.3, not 5.3.1.3. Thanks for your response and I'm curious what your interpretation of S.3.1.3 would be.

Geoff13, If you use the allowable stress for S.3.1.3, every single tank made out of 201-1, 201LN, 304, 316, 317 and 317L would require an annular bottom due to St > 24,900psi. It has to be the actual stress then right? Also, would you be willing to share the Zick & McGrath paper "Design of Large Diameter Cylindrical Shells?

IFRs, If you used Table 5.2 allowable stresses then you could never use lap welded joints in the annular ring because all the Group IV, IVA, V and VI Allowable Product Design Stresses in Table 5.2 are > 23,200psi and all the Allowable Hydrostatic Test Stresses are > 24,900psi. Let me know what you think.

HTURKAK, You could be on to something. You're basically saying that the lap weld vs. butt weld portion of 5.5.1 is dependent on the allowable stresses. However you could use a lower allowable stress in your calculations when using a higher strength steel in the first course in order to use lap welds in lieu of butt welds in the annular ring?

I'm leaning towards an interpretation that they're referring to the actual stresses in the bottom shell course.

 

[IFRs]

jimmy - thanks for keeping me honest.
After re-reading 5.5.1, I think it says this, substituting "strength" for "Group" to make it clearer:
Higher strength shell plates require a butt-welded annular ring.
If you use higher strength steel but don't take advantage of its higher strength annular plates are not required and you can use a normal lap-welded bottom design.
If the actual design stresses are below the listed thresholds for product and test you don't need an annular ring despite the fact that you are using higher strength steel. Actual design stress = maximum product stress or maximum test stress based on highest density product, filled to the design liquid level, the shel lthickness would be without corrosion allowance.

I could easily still be wrong...

 

[HTURKAK]

I am basically sayıng that the lap weld vs. butt weld portion of 5.5.1 is dependent on the ACTUAL stresses that the 1st course will experience during test and service life...

The actual stresses may be calculated using the formulas given at section 5.6.3.2 .

That is ,

ft = test stress, t= thk. of 1st course ft =2.6D(H – 1)/t

fs = service stress fs=2.6D(H – 1)G/(t- CA)


.. and if ft ≤ 24,900 lbf/in.2 and fs ≤ 23,200 lbf/in.2 , lap-welded bottom plates may be used in lieu of butt-welded annular bottom plates.



You are right ... just for making clear, the design liquid level could be total height of the tank depending on the roof type, overflow level etc..