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You need to calculate the hoop stress for the given pressure. This is the force balance between the walls (F=stress*t*L*2) and the projected area the pressure acts on, not the curved area (F=P*L*2*r).
I think you've quoted the ultimate tensile strength there (1.4 GPa) which would lead to true explosion-style failure. The yield stress would be a better target, you'd be able to ride the frame after that loading. For typical 4130 (Cro-moly) steel that's ~460MPa (ref) and gives a radius-thickness ratio of 15/460, or a wall thickness of ~0.46 mm for a 28.6 mm tube. Not dainty, but not a gas pipe by any stretch.
hamrack
@chiroshi
Sorry, I was out by a factor of 10 in my calculation, so wall thickness of 2mm would suffice for 20bar.
Yeah, 150Bar would need around a 1.4cm wall thickness. Not sure how that is possible. Paging @mdcc_tester to enlighten us all.