Line pipe is manufactured from high-strength carbon steel, and is made to strict engineering and metallurgical specifications developed by the American Petroleum Institute (API). Here we will introduce the classification of line pipe.

Pipe mills produce two types of line pipe: seamless and welded. Seamless pipe is formed from a cylindrical bar of steel that is heated to a very high temperature and then is pierced with a probe to create the hole through the cylinder.  Rollers size the cylinder to produce the proper diameter and wall thickness. This technique is used to make small diameter pipe, from 0.5 inches to 24 inches in diameter.

Most pipe produced for interstate natural gas pipelines is the welded variety, because interstate systems require larger diameter pipe.  Pipe mills manufacture line pipe by forming a steel plate or coil into a cylindrical shape, and closing the seam using a welding process.  The mill evaluates the quality of the weld seam using ultrasonic and/or radiological inspection methods and pressure tests each joint of pipe to levels significantly higher than the eventual operating pressure of the pipeline.  

The pipe is further tested to ensure that it meets all requirements of steel chemistry, strength and toughness, and dimensional characteristics.  Mills that produce line pipe to API specifications meet the most stringent criteria for steel making and pipe production technologies to ensure safe, reliable pipeline service.  The gas pipeline industry maintains the manufacturing and test records of the pipe for the life of the pipeline.