pipe, hollow structure, usually cylindrical, for conducting materials. It is used primarily to convey liquids, gases, or solids suspended in a liquid, e.g., a slurry. It is also used as a conduit for electric wires.
The earliest pipes were probably made of bamboo, used by the Chinese to carry water c.5000 B.C. The Egyptians made the first metal pipe of copper c.3000 B.C. Until cast iron became relatively cheap in the 18th cent. most pipes were made of bored stone or wood, clay, lead, and, occasionally, copper or bronze. Modern materials include cast iron, wrought iron, steel, copper, brass, lead, concrete, wood, glass, and plastic. Welded steel pipe is made by bending strips of steel into the form of a tube and welding the longitudinal seam either by electric resistance, by fusion welding, or by heating the tube and pressing the edges together. Seamless pipe is made from a solid length of metal pierced lengthwise by a mandrel with a rounded nose.
Steel pipe, introduced in the early 20th cent., is widely used for conducting substances at extremely high pressures and temperatures. Cast-iron pipes, which came into common use in the 1840s, resist corrosion better than steel pipes and are therefore frequently used underground. Clay and concrete pipes usually carry sewage, and concrete pipes are also used to carry irrigation water at low pressures; for moderate pressures, the concrete is reinforced with steel or mixed with asbestos. Seamless copper and brass pipes are used for plumbing and boilers. Because of its softness and resistance to corrosion, lead is used for flexible connections and for plumbing that does not carry drinking water. The chemical and food industries use glass pipes. During World War II manufacturers developed plastic pipe to replace metals that were in short supply. Today PVC pipe is widely used to carry waste water as well as certain corrosive liquids.
A pipeline carries water, gas, petroleum, and many other fluids long distances. In laying an oil pipeline, 40-ft (12-m) sections of seamless steel pipe are electrically welded together while held over a trench. Before being lowered into place the pipe is coated with a protective paint and wrapped with a substance composed of treated asbestos felt and fiberglass. Pumping stations located 50 to 75 mi (80–120 km) apart boost the dwindling pressure back up to as much as 1,500 lb per sq. in. The piping must be kept clean, either by applying a negative electrical charge to the pipe or by regular use of a "pig," or scrubbing ball, inserted at one end and carried along by the current. An oil pipeline 6 in (15 cm) to 24 in (60 cm) in diameter will move its contents at about 3 to 6 mi (5–10 km) per hr.
Water has been moved since ancient times in pipelines called aqueducts. The first natural-gas and petroleum pipelines in the United States were built during the 19th cent. Today in many parts of the world pipelines are an extremely important means of transporting diverse fluids. The Trans-Arabian Pipeline, which carries oil from the Persian Gulf to the Mediterranean, is over 1,000 mi (1,600 km) long. There are more than 180,000 mi (288,000 km) of pipeline in the United States alone.