Our company doesn't do any head forming, but we have bought a number of crown/petal hemispherical heads from vendors.
I would imagine a lot of the design is based on material availability and forming capacity at the shops. A couple of ideas...
[ul]
[li]Crown diameter is probably limited by two factors, available plate width and forming capacity. Depending on your material (CS vs. SS), typically 8ft or 10ft wide plates are available, so one limit on maximum crown diameter is likely the largest diameter that can be formed from the available plate width. Another limit on maximum diameter is likely the capacity of the machinery being used by the supplier.[/li]
[li]For petal design, it's probably a similar decision basis. You know the circumference of your vessel, and you know the available plate widths. So one way of selecting the number of petals could be: Number of petals = (Circumference) / (Available plate width)[/li]
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