Iron pipe size
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The iron pipe size standard (IPS) was initiated in [1801 - 1810]. It remained in effect until after World War II. Under IPS, the pipes were cast in two halves, and welded together. The outside diameters (ODs) of the pipes under IPS were roughly as we know them today under the Ductile Iron Pipe Standard (DIPS) and Nominal Pipe Size (NPS) Standards, and some of the wall thicknesses were also retained with a different designator. The IPS system was primarily used in the US and the United Kingdom.
The IPS is an old system still used by some manufacturers and legacy drawings and equipment. The IPS number (reference to an OD) is the same as the NPS number, but the schedules were limited to Standard Wall (STD), Extra Strong, (XS) and Double Extra Strong (XXS). STD is identical to SCH 40 for NPS 1/8 to NPS 10, inclusive, and indicates .375" wall thickness for NPS 12 and larger. XS is identical to SCH 80 for NPS 1/8 to NPS 8, inclusive, and indicates .500" wall thickness for NPS 8 and larger. Different definitions exist for XXS, but it is generally thicker than schedule 160.
In the 1920s, the Copper Tube Size (CTS) standard was combined with the IPS standard. CTS diameter always specifies the OD of a tube, where pipe diameter specifications only approximate the pipe ID for sizes of 12 inch or less, and STD wall thickness.
In 1948, the DIPS came into effect, when greater control of a pipe's wall thickness was possible.
