That is outside the range of API-620, and inside the range of ASME Section VIII. In a lot of areas, local boiler/ vessel laws would mandate construction per ASME. But from a mechanical/structural standpoint, there's not that much difference between designing for 15 psi vs 18 psi.

It would be informative to run through the calculations based on API-620. I think you'll have a gazillion anchor bolts and a compression ring that looks like something on a battleship by the time you're done. Perhaps I exaggerate. Also check foundation sizing, a ringwall can be massive under those conditions.

In the past, I have found that in most cases where a "high" pressure was specified for a large tank, the specifiers were able to reduce that pressure once they understood there was a considerable financial incentive to do so. For people used to working with pressure vessels, 15 psi is nothing and 18 psi is nothing, and it's easy to throw that number out there, not realizing you're adding a half million dollars to the cost of a tank.

"In the past, I have found that in most cases where a "high" pressure was specified for a large tank, the specifiers were able to reduce that pressure once they understood there was a considerable financial incentive to do so."

Amen to that one.... ! I agree completely ! Wise advice

I would also like to add that a cost comparison should be made between a single, huge, battleship reinforced API-620 design versus multiple smaller tanks.

Perhaps multiple ASME-VIII "propane bullets" designs should be considered.

The multiple tanks can also be added to (or subtracted from) the plant configuaration as sections of the process come on line. The finance guys also like the multiple tank idea in many cases also....

Thanks. If it could be done, it will be done. All of your recommendations have been asked before I posted this thread. Client is very aware of the operating pressure is at 15 psig, and with pressure fluctuation that the design pressure has to be set higher. Multiple tanks also involve more foot print taken, and may not have any saving on construction cost, with additonal piping, compressor/pump involved that make it more complicate. It all comes down to to process requirements, and client will need to accept Div 1 design if no other option.