Most of those associated with the design and construction of any building has likely assumed that the pipe wall thickness specified for different products equals what is defined by ASTM.  An ASTM, UL or FM stamp at a section of 12 in. schedule 40 pipe ready for installation means that the pipe wall is as defined by the stamp at 0.406 in.  Right?

Well not really.  Just like the incredible shrinking bag of potato chips or cereal box, pipe manufacturers seem to have followed a similar path.

Pipe can be produced +/- 12.5 % of its ASTM defined wall thickness and still be termed acceptable for sale and installation – a 25% tolerance which was appropriate in the early 1900’s when the code was first written, but not today.  Decades ago, almost all pipe was manufactured at or above ASTM specifications, and in fact it is not unusual to still document pipe at older and well maintained buildings still exceeding new pipe specifications after 65 or more years of service.  Today that trend has changed 180 degrees, with most new pipe now produced at or near its minimum allowable wall thickness.  This trend seems to have begun with foreign pipe sources; American manufactures quickly following given a highly competitive and quite unfair world pipe market.

The following examples document the wall thickness of new pipe not yet installed.  For 12 in. schedule 40 pipe, its beginning wall thickness is not likely to be the 0.406 in. stamped on its side, but closer to 0.355 in. – ironically less than the specification for thinner 0.375 in. standard pipe.  Many photographs show both the wall thickness as measured by ultrasound, against the ASTM, UL, or FM stamp indicating its specified wall thickness.  In contrast to common belief, stamped wall thickness dimensions only indicate what the pipe wall should be, as opposed to its manufactured dimension.

This issue is not only prevalent for steel pipe, but has been documented in our investigations of new galvanized steel, copper, cast iron, and ductile iron piping.  As we have used as a headline in one of our Technical Bulletins, “A Simple Dial Caliper May Reveal Surprising Results.”

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