“ANSI class” is the designation for pressure-temperature ratings of pipe flanges. In its denotation, the word Class is followed a by dimensionless number. This includes Class 150, 300, 400, 600, 900, 1500, and 2500.
Unsurprisingly, the phrase ANSI class, does, in fact, derive from the American National Standards Institute. However, it should be noted that this usage is more colloquial than official. Much like other objects and processes that have found themselves called ANSI something (e.g. ANSI C or ANSI A Paper), the ANSI portion of the name comes from being specified in an American National Standard.
In this instance, the American National Standard is ASME B16.5-2017: Pipe Flanges and Flanged Fittings, a document developed by the American Society of Mechanical Engineers (ASME) that covers pressure-temperature ratings, materials, dimensions, tolerances, marking, testing, and methods of designating openings for pipe flanges and flanged fittings.
This standard dates back almost one hundred years, when ANSI (then the American Engineering Standards Committee, AESC), was in its infancy. In 1920, AESC organized Sectional Committee B16 to unify and further develop standards for pipe flanges and fittings. After being approved as an American Tentative Standard, and after AESC became the American Standards Association (ASA), the flanges and fittings standard was released as ASA B16E-1932.
After seeing numerous revisions throughout its long and varied history—one incredibly noteworthy event was the appointment of a special War Committee of B16 in 1942 to conserve vital materials in piping components—the standard was released as ASA B16.5 in 1957. A rating basis for Class 150 flanges was developed in the ANSI B16.5-1973 publication, as ANSI’s current name had been established by then. This birthed what is known as ANSI class.
In 1982, American National Standards Committee B16 was reorganized as an ASME Committee operating under ANSI-accredited procedures. The B16.5 document was released in 1988 as the American National Standard designated ASME/ANSI B16.5-1988. Subsequent revisions have dropped the “ANSI” part of the title.
That’s how ANSI class ratings came to be. As for what class values represent, they designate the pressure rating of the flange. Often “lb,” “class,” and “#” are used interchangeably to designate this. However, 150 lb has no relation to 150 psi. The service temperature of flanges decreases as the temperature increases, but this has no real correlation with pressure class. In fact, different materials rated in specific classes react differently as the temperature changes. In general, ANSI class is similar to PSI and the pressure rating can be expressed as either an ANSI class or psi.
You can learn more about ANSI class in the above video.
Those who need to better comprehend ANSI classes or have to incorporate them into their work should acquire and consult the ASME B16.5-2017 standard. In the American National Standard, users can find the dimensions, figures, and other information regarding ANSI class ratings.
ASME B16.5-2017: Pipe Flanges and Flanged Fittings is available on the ANSI Webstore.