Piping comes in sizes that are designated by a nominal diameter, all piping is fabricated in accordance with ASTM material specifications and ASME dimensional standards, all diameters and wall thicknesses must conform to the applicable code.
The two most common piping codes are:
B31.1    Power Piping
B31.3    Process Piping.
The piping engineer in determining the size and material of the piping, uses these codes to perform the calculations, then will list all the pipe, components and valves to be used for a particular service e.g. steam, in the piping specifications which must be used by the Designer in completing the piping layout and detail drawings.

PIPE MANUFACTURE
Steel pipe is manufactured by one of two methods - extrusion or welded.
Seamless Pipe is produced by extruding a steel billet over a mandrel, because there is no weld seam it is suitable for high temperature/pressure and fluids which must not be contaminated by weld deposit i.e. food and beverages.
Welded Pipe has either a longitudinal or a spiral seam which is filled by either E.R.W. (Electric Resistance Weld) or by a Butt Weld. Early ERW pipe was subject to pressure reversals, a problem that results in failure in service at a lower stress than that seen in the pre-service pressure test. This problem is caused by crack growth during the pressure test hold period, which in the case of early ERW pipe was due to a combination of low weld line toughness and lack of fusion defects.
As a result of these early problems, ERW pipe was generally regarded as a second-grade pipe suitable only for low pressure applications. However, prompted by a shortage of seamless pipe and the lower cost of ERW pipe, suppliers and end users directed a major effort toward improving the pipe mill quality in the 1980s. In particular, accurate tracking of the weld line by the automatic ultrasonic inspection equipment was found to be crucial, since the weld line can rotate slightly as the pipe leaves the welding station. In addition, the standard of heat treatment of the weld line, which is necessary to ensure good toughness, was found to be important and some specifications call for local weld line heat treatment using induction coils followed by full body normalizing of the whole pipe in a furnace. As a result of these improvements, modern ERW/HFI pipe has much better performance than the traditional product and has been accepted by a number of operators for high pressure gas transmission.

Above is only an excerpt from the chapters of the book: Detail Engineering and Layout of Piping Systems 5th Edition.
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