PIPELINE

line of pipe equipped with pumps and valves and other control devices for moving liquids, gases, and slurries (fine particles suspended in liquid). Pipeline sizes vary from the 2-inch- (5-centimetre-) diameter lines used in oil-well gathering systems to lines 30 feet (9 m) across in high-volume water and sewage networks. Pipelines usually consist of sections of pipe made of metal (e.g., steel, cast iron, and aluminum), though some are constructed of concrete, clay products, and occasionally plastics. The sections are welded together and, in most cases, laid underground. Various cleaning measures are undertaken to reduce the corrosion that often occurs in pipelines. These include maintaining a negative electrical charge throughout the pipe (cathodic protection); lining the interior with paints and coatings of plastic and rubber; fibreglass, asphalt, and felt wrappings; the injection of corrosion-inhibiting chemicals into the cargo; and periodic cleaning. Pipelines are also cleaned by passing devices called pigs through them. A pig may be simply a ball of the same size as the pipe's diameter, which scrapes the pipe's interior clean as it is propelled along by the flowing cargo. It may also be a complex scrubbing machine inserted through a special opening. line of pipe equipped with pumps and valves and other control devices for moving liquids, gases, and slurries (fine particles suspended in liquid). Pipeline sizes vary from the 2-inch- (5-centimetre-) diameter lines used in oil-well gathering systems to lines 30 feet (9 metres) across in high-volume water and sewage networks. Pipelines usually consist of sections of pipe made of metal (e.g., steel, cast iron, and aluminum), though some are constructed of concrete, clay products, and occasionally plastics. The sections are welded together and, in most cases, laid underground. Most countries have an extensive network of pipelines. Because they are usually out of sight, their contribution to freight transport and their importance to the economy are often unrecognized by the general public. Yet, virtually all the water transported from treatment plants to individual households, all the natural gas from wellheads to individual users, and practically all the long-distance transportation of oil overland goes by pipeline. Pipelines have been the preferred mode of transportation for liquid and gas over competing modes such as truck and rail for several reasons: they are less damaging to the environment, less susceptible to theft, and more economical, safe, convenient, and reliable than other modes. Although transporting solids by pipeline is more difficult and more costly than transporting liquid and gas by pipeline, in many situations pipelines have been chosen to transport solids ranging from coal and other minerals over long distances or to transport grain, rocks, cement, concrete, solid wastes, pulp, machine parts, books, and hundreds of other products over short distances. The list of solid cargoes transported by pipelines has been expanding steadily. Additional reading John L. Kennedy, Oil and Gas Pipeline Fundamentals (1984), offers a general account of the history and development of oil and gas pipelines. Various applications of pneumatic pipelines are discussed in Milton N. Kraus, Pneumatic Conveying Systems for Bulk Materials, 3rd ed. (1991). Henri Liu, Hydraulic Capsule Pipeline, Journal of Pipelines, 1(1):1123 (January 1981), describes the history and concept of the pipeline in which cargo in capsules is moved through the pipeline by liquid. T.L. Thompson and T.C. Aude, Slurry Pipelines: Design, Research, and Experience, Journal of Pipelines, 1(1):2544 (January 1981), provides a general treatment of pipeline types and a list of the world's major slurry pipelines. I. Zandi et al., Transport of Solid Commodities via Freight Pipeline, vol. 2, Freight Pipeline Technology (1976), is a report from the U.S. Department of Transportation, providing detailed discussions of various types of pipelines and surveying their history and extent of usage. Henry Liu