These 3 terms are very important terms from ASME B31.3, Piping Engineers shall understand each one properly:
– Fabrication: the preparation of piping for assembly, including cutting, threading, grooving, forming, bending, and joining of components into sub-assemblies. Fabrication may be performed in the shop or in the field.
– Assembly: the joining together of two or more piping components by bolting, welding, bonding, screwing, brazing, soldering, cementing, or use of packing devices as specified by the engineering design.
– Erection: the complete installation of a piping system in the locations and on the supports designated by the engineering design including any field assembly, fabrication, examination, inspection, and testing of the system as required by this Code.