API 650-Plumbness Tolerance for one shell

[Frank Zhang]

Dear expert,
We are constructing a tank with 41.4m diameter.
As per API 650 stated:
7.5.2 Plumbness
a) 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 A6M/A6, ASTM A20M/A20, or ASTM A480M/A480, whichever is applicable.
My carbon steel shell plate size is 2875 x 9303 x t8 mm. As per ASTM A6M/A6 the flatness is 38mm.
So this one shell course plumbness after assembly is 38 mm or 38/2=19mm?

Thank you
Frank Zhang

 

[JStephen]

That sounds excessive, I suspect you are looking at the wrong number in the standard. It should be the flatness tolerance measured across the small dimension of the plate, not the waviness measured along the length.

(Edit: Just had a chance to pull the standard out, and yes, Geoff13 below has it correct. I assumed the thinking in this was that it wasn't reasonable to expect a tank to be any straighter than the plate it was made from.)

 

[Geoff13]

You have quoted API 650 and ASTM A6 correctly. API does not require the A6 value to be reduced by half. Thus the tolerance would be 38 mm for the ring you described.

I have never understood why API allows such a large single ring tolerance, but the 1/200 limit for the total tank height will keep the tank reasonbly vertical.

I would be concerned about the constructability and final appearance of the tank if you have a ring that needs the full 38 mm tolerance to meet Code, though with 8 mm plate you're obviously at the top ring or two.

 

[Frank Zhang]

Dear JStephen, Geoff13
Thank you for all your reply.
It really helpful for me.