Excerpt from PD 5500:
"Construction categories and design stresses
3.4.1 Construction categories
For each pressure-containing component of the vessel, the manufacturer shall select a construction category in accordance with Table 3.4-1. The purchaser may require a minimum construction category in which case it shall be specified in the purchase specification. A component is defined as a part of pressure equipment which can be considered as an individual item for the purpose of calculation e.g. flange, head, cylindrical strake.
NOTE 1 Any one of the three construction categories in Table 3.4-1 will provide adequate integrity for normal purposes within the material and temperature limitations specified therein. The justification for any special precautions (e.g. additional inspection and/or test requirements, secondary containment) to reduce external risks in the postulated event of an escape of hazardous vessel contents involves consideration of matters by the purchaser (and Regulating Authority) which are beyond the scope of this specification. Any modifications to the requirements of this specification which are required for the purpose should be covered in the purchase
specification.
NOTE 2 Construction categories, as defined in Table 3.4-1 are intended to apply to components of a vessel and not necessarily only to complete vessels which may therefore comprise components in two or more categories. Category 3, however, is commonly applied to complete vessels so that design stresses and inspection requirements are consistent throughout the vessel."
Cheers,
gr2vessels
