Piping Materials of Construction

By William F. Harfst


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Selecting the proper materials of construction for water service can be a daunting task given the numerous options for metallic and plastic pipe. Cost is always a factor, but so is durability. Depending on its quality, water can be corrosive, so making sure the piping material is compatible with the water that it is conveying is important. As the saying goes, one must モdesign with corrosion in mind.ヤ This article will present some of the common piping materials and review appropriate applications for each. In addition, some of the potential problems or pitfalls will be identified. Iron. Various grades of iron pipe are available for use in water and steam service. Basically, steel is an alloy of iron and carbon. Wrought iron is a steel alloy with very low carbon (C) content (0.1 to 0.25% C) as compared to cast iron (2.1% to 4.0% C). Many grades of steel pipe are available, depending on the carbon content and other alloying materials. Cast iron pipe is used for underground or submerged applications, and for main steam lines and branches. It is also common practice to use cast iron pipe for municipal cold water lines greater than 4 inches in diameter. Ductile or malleable iron (DMI) is suitable for use in potable water service whereas carbon steel (CS) is not because it tends to corrode (rust) and contaminate the water with by-products of corrosion. Carbon steel can be welded, whereas ductile or malleable iron cannot. Cast iron can be threaded whereas ductile or malleable iron cannot. In general, steel pipe is not as corrosion resistant as other materials of construction. However, it is a lower cost option and therefore used in many common applications, including steam and condensate lines. It is often noted that carbon steel (CS) steam lines rarely corrode, whereas CS condensate lines often do. This is because of the formation of carbonic acid in the steam condensate, which works to attack the steel. Even so, because of the high temperature of steam and condensate lines, steel is the preferred material of choice.

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