An "Unfired Pressure Vessel" (UPV) means any enclosed vessel under pressure greater than atmospheric pressure by any gas or mixture or combination of gases and includes any vessel under pressure of steam external to steam boiler and any vessel which is under pressure of a liquid or gas or both, and any vessel subject to internally to a pressure less than atmospheric pressure. Example of UPV include air receivers, autoclave, sterilizers, petrochemical treatment, LPG storage tank and water treatment vessels.

An Unfired Pressure Vessel is a potentially hazardous piece of equipment as its content is under pressure. Any design or fabrication fault, or misuse could lead to an explosion and failure of an unfired pressure vessel. A number of such explosions and failures have occurred causing devastating damages to buildings and loss of human lives. Therefore, it is essential that these unfired pressure vessels to be stringently controlled in order to safeguard the lives of people and damage to property. The Department of Occupational Safety and Health (DOSH) carries out such control by approving, registering and inspecting periodically these unfired pressure vessels.

The design, fabrication, testing, installation, operation, inspection and maintenance of UPVs are mainly governed by the Factories and Machinery Act 1967 and applicable regulations made under the act. One such regulation is the Factories and Machinery (Steam Boiler and Unfired Pressure Vessel) Regulation, 1970.

In respect of imported unfired pressure vessel, if the product of the internal diameter in inches (inch) and the design pressure in pounds per square inch (psi) of which is more than three thousand (3000), it shall have been under the supervision of an authorized member of the Inspecting Authorities set out in the Fourth Schedule of the Factories and Machinery (Steam Boiler and Unfired Pressure Vessel) Regulation, 1970