What's a Pressure Vessel?

Different countries have different laws governing pressure vessels and or vessels under pressure and hence different definitions of what constitutes a "pressure vessel".

Tell us where you are and maybe someone from your location could help.

This is correct defenition for pressure vessel...

A container designed to withstand internal or external pressure. This pressure may be imposed by an external source, by the application of heat from a direct or indirect source, or by any combination thereof. This definition includes heat exchangers, air-coolers, unfired steam generators and other vapor generating vessels which use heat from the operation of a processing system or other indirect heat source.

Nonsense. Every code uses a combination of pressure and volume to define when a vessel becomes a pressure vessel.

You are also mixing in fired equipment which is often treated differently.

Thank Durai

That's short and correct.