What is the difference between specification expression of seamless steel pipe and welded steel pipe

The specification of seamless steel pipe is the expression of outer diameter * wall thickness. Generally, the symbol of Φ is added in front of the specification, for example: Φ 89 * 6, which means that the specification is seamless steel pipe with outer diameter of 89mm and wall thickness of 6mm. Adding its steel grade and standard before the specification, can we fully understand the basic information of the seamless steel pipe.

The specification of welded steel pipe is generally expressed by DN, which means the nominal diameter of welded steel pipe. For example, DN 100 means the nominal diameter of the welded steel pipe is 100 mm. According to the comparison table of welded steel pipe specification, we can know that the actual diameter is 114mm.