API 650 : Tank - Out of Plumbness tolerence. Kindly help me in this!

[0984]

When we go for inspection of tank, plumbness check. We had the readings like 0° +3, 45° +6, 90° +20, 135° +15, 180°+7, 225° +2, 270° -3, 315° -22. Below is the API 650 code states that height/200. eg:7000mm/200 = 35mm. Now the confusion is, is it +/- 35mm or altogether 35mm. Kindly advice.

API 650 code:
Plumbness
The maximum out-of-plumbness of the top of the shell relative
to the bottom of the shell shall not exceed 1/200 of the
total tank height. The out-of-plumbness in one shell course
shall not exceed the permissible variations for flatness and
waviness as specified in ASTM A 6M/A 6, ASTM A 2QMl
A 20, or ASTM A 480MlA 480, whichever is applicable.
The 1/200 criteria shall also apply to fixed roof columns. For
tanks with internal floating roofs, apply the criteria of this
section or Appendix H, whichever is more stringent.

 

[Bonswa]

Those readings should be for a local area only, not altogether. You're worst spot (90°) was still within the tolerable range of 35mm, so you're fine.

What i dont get is this...

for a new tank, you're allowed 1/200, however once the tank is in service, you're only allowed 1/100. HMM.... Can anyone clarify this?

From API-653:
10.5.2 Plumbness
10.5.2.1 The maximum out-of-plumbness of the top of the
shell relative to the bottom of the shell shall not exceed 1/100 of the total tank height, with a maximum of 5 in. The 1/100 criteria, with a maximum of 5 in., shall also apply to fixed roof columns. For tanks with internal floating roofs, apply the criteria of this section or API Std 650, Appendix H, whichever is more stringent.

 

[JStephen]

I interpret it that the shell can be 35mm in or 35 mm out relative to the base.

 

[0984]

My question is, in my case it comes +20 and -22 in the same tank so the total is 42. Is it acceptable or not?

 

[Bonswa]

Dont look at it that way. The +20 on one side is within and the -22 on the other side is within. It's not an overall. you dont take the worst tilt of one side and add it to the tilt of the other side. If you did that then you maximum would be +/- 70mm. still, you wouldnt do that. Its an individual calculation for each location. you can have up to +/- 35mm on any or all of the locations and still be within the range.

Think about it like this. If you're +20 on one side, and your circumference is consistant throughout, then you should expect to be -20 on the other side. it's as if someone took the top of your tank and pushed it 20mm over.

 

[JStephen]

for a new tank, you're allowed 1/200, however once the tank is in service, you're only allowed 1/100. HMM.... Can anyone clarify this?

L/100 is twice as big as L/200.

 

[Bonswa]

I'm giving myself the facepalm for not thinking.

Thanks.

 

[0984]

As you said, i also thinked that if one side is +20 and the other side is -20. U r correct. but my tank is +20 at 90D and -22 at 315D. its supposed to be 270D right. that y i confuse.

 

[IFRs]

API says "out of plumb". Since they do not otherwise state, the tank could be out of plumb in or out. So the tolerance is +/- 1/200 from plumb. Note that tank can be an exact cylinder but still be out of plumb - just leaning. Sometimes it is necessary to deduct the planar tilt from the plumbness survey to determine what the floating roof seals will see in deviations from round. Peaking and banding are additional criteria. The only time I worry about dents opposite each other is if they combine to hang up a floating roof. Yours are not anywhere near that magnitude. Note also that if the tank has experienced differential settlement, it will usually have associated shell buckles.

 

[786392]

API's intent

with limiting the 'Out of Plumpness' and 'Out of Roundness’ values is nothing but Presumably to ensure the uniformity of Tanks constructed/erected

to provide satisfactory service life and least maintenance and ensuring MI(Mechanical Integrity)during operation.
Experts Please correct if I'm wrong.

Now when this comes to localized 'out of plumpness' and 'out of roundness’; I consider that this should be the AHJ or End User

to check and permit acceptance or otherwise.

Since even localized inward bulging may severely impact any EFR's or IFR's if these are to employed subsequently

Thereby hampering the satisfactory sustainable operation in service is a big question mark.

Hope this is somewhat helpful.



Best Regards
Qalander(Chem)

 

[786392]

Sorry for typo error
Thereby the satisfactory sustainable operation in service is a big question mark.

Best Regards
Qalander(Chem)

 

[0984]

Thanks for all of your advice. I'm the client inspector. Thats y i'm in the position to take decision. Anyways thanks. I asked them to adjust the shell plates to bring into good tolerance level.