Recently when sizing a storage tank, found that the required thickness will be 51mm (2"), over the limit 45mm (1-3/4") thick as set in section 4.2.1.4 of API 650. The question is whether the maximum plate thickness 45mm is mandatory? If not, what additional requirements? If so, what is the rationale behind this limit?

Thanks guys in advance.

Your only choice is to use higher strength steel to stay within API 650.

Considering the low design metal temperature and cost, CSA G40.21 Grade 350 WT has been specified. ALso the allowable stress has been elevated according to section 5.6.2.1 and 5.6.2.2. That is why i am questioning the limit ...

Thanks.

This must be a huge tank - Appendix K illustrates tanks up to 1,321,200 barrels capacity under 1.75 inch thick. What are your diameter and height?

Yes, it is. For the given pump suction head and working capacity, the tank has been sized to 34m Dia X 55m H, with the high S.G and corrosion allowance, the wall thickness at the bottom course would be 1.96".

Considering the small ratio of D/H, the variable-Design-Point method would not give a better result but a little bit thicker. However, probably it would help refine the upper courses a little bit.

With requirements like that, one option would be to build two tanks side-by-side, and operate them in parallel. This might actually be cheaper than building the single larger tank.

Where shell limitations come in, the more common scenario is just building a huge tank, not a tall tank. The companies that have the capability to do this are also the companies that have been involved in writing the standards and have the engineering/vessel/welding background to know whether they can safely exceed the code thicknesses.

With pressure vessels, if you get above certain limits, stress-relief of welds is required, and this is impractical with large tanks.

Foundations can become a challenge with a tank of the dimensions you propose, also.

Thanks guys.

Yes, two tanks might be a better option under the limit. However, my point here is about the limit set in the API 650, is it a strength issue or toughness concern? Any insights?