API 5L is a standard specification for seamless and welded steel pipes used in the transportation of oil, gas, and other fluids. The American Petroleum Institute (API) developed this standard to ensure safe and efficient transportation of petroleum products and other materials over long distances through pipelines.

API 5L covers both seamless and welded pipes that are commonly used in the oil and gas industry. The specification defines the requirements for the manufacture of two product levels (PSL 1 and PSL 2) of seamless and welded steel pipes.

PSL stands for Product Specification Level and defines the technical requirements for the products manufactured to the standard. The difference between PSL 1 and PSL 2 is mainly in the chemical composition and mechanical properties of the steel.

The API 5L specification covers a range of pipe sizes from 1/8" to 48" in outside diameter (OD) and wall thicknesses from Schedule 10 to Schedule 160. The standard also covers various grades of steel, including grade A25, A, B, X42, X46, X52, X56, X60, X65, X70, X80, and X100.

In addition to the technical requirements for the manufacturing of the pipes, API 5L also specifies testing requirements to ensure the quality of the product. The testing requirements include hydrostatic testing, nondestructive testing, and mechanical testing.

API 5L is widely used in the oil and gas industry, as well as in other industries such as construction and water transportation. The standard has been adopted by many countries worldwide and is recognized as one of the most widely used specifications for steel pipes used in pipeline transportation systems.