If the tanks are simple tanks, it'd probably be quicker to go the spreadsheet route.  If they are pressurized vacuumized stainless tanks at elevated temperatures, etc., canned software would be more advantageous.

API-650 has changed the way that pressure, wind, and seismic overturning and uplift are evalatuated with each edition.  If you anticipate evaluating the tanks per the original design standard, you would need to investigate whether the software in question could do so.

I don't know the current state of things, but I recall in the past seeing that certain software was running about 6 months behind API in making changes to the standard, so that would be another issue to question.

We are also looking at ETank2000 which is currently merging with AMTank to form AMETank which has more 3D capabilities. It is nice that you can separate out the geometry and estimating capabilities from the software licensing to keep the cost down.

I don't think that either does external nozzle loads yet under Appendix P. Things like conical bottoms, legs and skirts, and square manways might also need a separate FEA package or custom spreadsheet. I know there are plans to expand AMETank to include these.

You might call ETank. Jeff and Adel have been fairly accessible during our review but perhaps they are going through some growing pains at the moment as they merge the software.