A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambientpressure.
The pressure differential is dangerous, and fatal accidents have occurred in the history of pressure vessel development and operation. Consequently, pressure vessel design, manufacture, and operation are regulated by engineering authorities backed by legislation. For these reasons, the definition of a pressure vessel varies from country to country, but involves parameters such as maximum safe operating pressure and temperature, and are engineered with a safety factor, corrosion allowance, minimum design temperature (for brittle fracture), and involve nondestructive testing, such as ultrasonic testing,radiography, and pressure tests, usually involving water, also known as a hydrotest, but could be pneumatically tested involving air or another gas. The preferred test is hydrostatic testing because it’s a much safer method of testing as it releases much less energy if fracture were to occur (water does not rapidly increase its volume while rapid depressurization occurs, unlike gases like air, i.e. gasses fail explosively). In many countries, it is the law that vessels over a certain size and pressure (15 PSIg) be built to Code, Boiler and Pressure Vessel Code (BPVC), these vessels also require an Authorized Inspector to sign off on every new vessel constructed and each vessel has a nameplate with pertinent information about the vessel such as maximum allowable working pressure, maximum temperature, minimum design metal temperature, what company manufactured it, the date, its registration number (through the Concern authority), making the vessel traceable and officially Coded vessel.