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).
One particular standard, API Specification 5L, defines requirements for pipe made to transport natural gas, oil and water. This specification includes standards for the dimensional, physical, mechanical, and chemical properties of the carbon steel. Several pipe mills in North America and around the world manufacture API 5L line pipe for the natural gas industry. 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.
Pipeline Size and Design
The size of interstate pipelines varies, but in most cases a mainline, the principal pipeline that delivers natural gas, ranges from 16 to 48 inches in diameter. Other smaller pipelines called laterals deliver gas to the mainline or take gas from the mainline and range from six to 16 inches in diameter.
The volume of gas to be delivered and the pressure at which the pipeline will be operated determines the pipeline’s ultimate diameter. In order to meet customer delivery requirements most interstate gas pipelines operate at a pressure of at least 600 pounds per square inch (psi), but typically at about 1,000 psi.
The thickness of the pipeline is determined by the maximum operating pressure (MAOP), and is based on published industry standards and federal regulations. The pipeline incorporates a design safety factor, prescribed by U.S. Department of Transportation (DOT) regulations, that is related to the type of construction and population density along the pipeline route.