Code B31.3 of the American Society of Mechanical Engineers (ASME) specifies requirements for process piping. Process piping means pipes used to move materials from one location to another. The B31.3 code is a 384-page book that includes requirements for piping usually located in facilities such as petroleum refineries, chemical and pharmaceutical plants, textile and paper mills, semiconductor and cryogenic plants and similar processing facilities.
Industrial radiography is the use of X-rays to inspect pipes, walls and other structures. Code B31.3 includes radiography requirements to inspect process piping.
Code B31.3 specifies that the default inspection requirement for checking the butt welds of a piping system under construction is radiography of a random 5 percent of each lot of pipe. If the radiographs from that lot are approved, the entire lot is approved. According to "The Fabricator," a publication of the Fabricators and Manufacturers Association, the definition of what constitutes a lot of piping is open to interpretation.
If the inspector rejects a radiograph of one weld from a lot, radiographs of two more welds must be done. Rejection of one of these two radiographs requires radiographs of two more welds. If one of these two radiographs is rejected, radiographs of all the welds in the lot must be done.
The 5 percent random radiography rule refers to normal process piping that carries low-pressure, nonlethal materials. Piping is also classified into four other categories: Category M carries lethal fluids, High Pressure and Severe Cyclic piping carry materials above specified pressure limits and Category D carries harmless, low-pressure, low-temperature fluid. Category M requires 20 percent random radiography of butt welds. High Pressure and Severe Cyclic piping require 100 percent radiography of butt welds and branch connections. Category D piping requires only a visual examination.