You may see pressure vessels or storage tanks listed as one of various numbered “Types.”
Pressure vessels are organized into five types:
Type I: All-metal construction, generally steel.
Type II: Mostly metal with some fiber overwrap in the hoop direction, mostly steel or aluminum with a glass fiber composite; the metal vessel and composite materials share about equal structural loading.
Type III: Metal liner with full composite overwrap, generally aluminum, with a carbon fiber composite; the composite materials carry the structural loads.
Type IV: An all-composite construction, polymer — typically polyamide (PA) or high-density polyethylene (HDPE) liner with carbon fiber or hybrid carbon/glass fiber composite; the composite materials carry all the structural loads.
Type V: Linerless, all-composite construction.
Source: Composites end markets: Pressure vessels