Does anyone know of a pressure vessel designer and fabricator that could coil 40NB SCH160 S31803 pipe to ~Ø500?
(And delivery to New Zealand)
Also looking for general advice on whether or not such a tight coil on that pipe size is even possible.
I don't have experience with coils specifically but I know you can bend reasonably heavy-walled UNS S31803 tubing to a radius of of 1.5X the OD without issues. This is done routinely to make 2205 duplex U-tube bundles for heat exchangers with good results, although the tube material is typically thinner than what you're proposing.
This is likely at the very limit of what can done with this wall.
If it is a small number of turns maybe, but I can't see doing a lot of turns this tight.
Are your pressures really that high?
This is 10,000psi pipe.
Lighter wall will improve heat transfer and make fabrication easier.