There are three main tubular piping systems used for describing dimensions; ANSI, ISO and Metric. The North American ANSI system is a national system applied all over the world, particularly within the petrochemical and offshore industry. The outside diameter is constant and expressed in inches, the wall thickness is expressed in schedules (Sch.), designed to withstand different inside pressures classes. The European ISO system is also OD based; it provides almost the same OD as the ANSI system, but in millimetres, and with a lot more choices for different wall thickness. The Metric system also use millimetres and also offers a wide range of different gauges, it has constant ID and has its roots within the Scandinavian pulp & paper industry.

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