Failure: Failure of a structure is an event, the transition from a normal working state, where the structure meets its intended requirements, to a failed state, where it does not meet its requirements
Limit states: A limit state is a structural condition beyond which the design performance requirements of a component are not satisfied. Limit states are classified into ultimate limit states and serviceability limit states
Elastic limit states: An elastic limit state is a structural condition associated with the onset of plastic deformation. This term is usually used in connection with monotonic actions, and it relates to virtual structures, usually with zero initial stress distribution
OPERATING PRESSURE : The pressure which is required for the process, served by the vessel, at which the vessel is normally operated.
DESIGN PRESSURE : The pressure used in the design of a vessel. It is recommended to design a vessel and its parts for a higher pressure than the operating pressure. A design pressure higher than the operating pressure with 30 psi or 10 percent, whichever is the greater, will satisfy this requirement, The pressure of the fluid and other contents of the vessel should also be taken into consideration.
MAXIMUM ALLOWABLE WORKING PRESSURE : The internal pressure at which the weakest element of the vessel is loaded to the ultimate permissible point
HYDROSTATIC TEST PRESSURE : One and one-half times the maximum allowable working pressure or the design pressure to be marked on the vessel when calculations are not made to determine the maximum allowable working pressure.
There are many more terms than this… but to start with.. these are sufficient.