For small one off pressure vessel shells cut from seamless pipe we use this manual chain-driven pipe cutting machine – see Jim’s expert operation below:

it gives a smooth and accurate cut with a low gear ratio and smooth worm wheel drive.
Cuts 5-50mm thick wall on pipe with 102-600mm (4-24”) diameter.
Depending upon the weld procedure requirements the bevelling head angle can be set quickly and accurately by scale on a rack and pinion to give the angle required.