Please see the definition of the pressure vessel:
Pressure vessel, refers to the gas or liquid, carrying a certain pressure of the closed equipment, the scope of its provisions for the maximum working pressure is greater than or equal to 0.1MPa (gauge pressure) of gases, liquefied gases and liquids with a maximum operating temperature higher than or equal to the standard boiling point, volume greater than or equal to 30L and the inner diameter (non-circular cross-section) Refers to the maximum geometric size of the inner boundary of the cross-section) greater than or equal to 150mm of fixed containers and mobile containers; containing nominal working pressure greater than or equal to 0.2MPa (gauge pressure), and the product of the pressure and volume is greater than or equal to 1.0MPa?L of gases, liquefied gases and liquids with a standard boiling point of equal to or less than 60 ℃ cylinders; oxygen tanks.
From:
A General Administration of Quality Supervision, Inspection and Quarantine Announcement on Revision of Catalog of Special Equipment
No.114 of 2014