Stainless Steel Tank Flat Spot / Deformation

[Ganolli]

Since some of you guys are very knowledgeable and experienced in the Storage Tank domain, I was wondering if your could provide me some guidelines/solutions to an issue we are presently having out on the field.

We are currently erecting a Stainless Steel Storage tank (mat SS316L). During construction, several problems came up, especially concerning the welding procedure and the foundation level. These extreme conditions, and the constant modifications/manipulations/repairs induced to the tank, resulted in several flat spots in our shell. Since we are bound to provide a Storage Tank designed and constructed as per API650 Standard, we have to make sure our flat spots size do not exceed what is accepted by the code. From art 7.5.4c, 7.5.2a and ASTM standards, we came up with a maximum deviation of the shell plate of 0.9in relative to the vertical plane. From what we read in the ASTM code and the API code, there is no precision in how to measure this deviation if we have a flat spot affecting more than one shell plate. Actually, even if the flat spot is in only one plate, there is no precision on the proper procedure to measurement it. Also, since we had previous experiences with regular steel tanks with thin shell plates, it seems to us that a 0.9in deviation appears to be a small value.

Would any of you guys be able to give us some guidance concerning the real method of measuring flat spots and where to get adequate values for flat spots maximum deviations ? The code isn’t precise enough in this area from our standpoint to make any valuable conclusions.

 

[JStephen]

Measuring along a vertical line, you have roundness measured near the base and out-of-plumb measured up the tank, with an out-of-plumb limitation for a single ring as well, not specifically a flat-spot measurement.

Check App. S and see if it allows for an increase in tolerances for stainless.