12F is a 'relaxed' 650 for small, truck transportable & shop-fabricated 'production' tanks that sit out in pastures by wellheads. The 12F design rules do make an acceptable product, that I feel is better than one designed under UL-143. Nothing wrong with using 12F tanks in a process unit inside a refinery or chemical plant.
I use an upgraded 12F design -- manway rounded at top, not square as 12F allwos -- for small tanks. If you look in the back of 12F, there is a limit to how large, both height & diameter, something built under the 12F rules can be.
I use a strict API-650 design at 40 to 50-ft and above, and go to an annular ring above 100-ft diameter.
In the area in between, it depends on the product and location [risk exposure] as to how strict or relaxed the design rules I follow are. API-650 is a 'perverted' standard IMHO. There are a number of design details forbidden under 650 & atmosphereic pressure that are fully acceptable on a ASME VIII design at 100 psig. Just doesn't make good engineering sense, and if you conservatively use good engineering design, you will build a good tank.
Conversly, if the calc's 'just don't look right', you are about to make a big mistake.
