They differ from pipelines and pipework in that, while they work at the same pressure, the volume of product within them is often much greater and so the stresses in the shell are higher and the design and construction must take these into account. There are usually large openings in the vessels and the compensation for this necessitates careful design. The vessels are frequently placed in areas where operators regularly work and so the consequences of failure are large. For these reasons, the design and construction of vessels must include high quality materials and high quality of construction.
Pressure vessels are often manufactured by specialists in their own factories and then shipped to site, preferably complete, or in component form for site assembly. As they are constructed off site, automatic welding is often used, particularly submerged arc welding[SAW].
The most common standard for pressure vessels is ASME Boiler and Pressure Vessel code which is published in 11 parts and is usually referred to by the shorthand of say, ASME IX which is part 9 and covers the welding and brazing aspects. Most countries have their own standards for the manufacture of pressure vessels in UK this is BS 5135. However in the oil and gas industry ASME is often used regardless of the national specification.