Sch. 40 black steel also comes bundled in 25 ft. lengths. Not sure of the history, but as a fitter I've used both 21s and 25s.
You can order pipe in 25' lengths. But the reason 21' has become the standard is due to the production of steel pipe. A heated length of steel is pulled through a hole that will shape it in a tube. 21' was a good length to maintain strength of the steel pipe.
I actually had this same curious question last year and here is the response I got from a contact at Wheatland:
Lots of theories I have heard over the years but what seemed to be the most consistent and reasonable answer was this.not too exciting of answer for sure
The 21' length became the standard because of the old way pipe was produced. Pipe was produced in the past using the bell and tongue method. This method of pipe making started with a heated ( red hot - close to melting temperature) length of steel strip and then the strip was manually pulled with a pair of tongues through a bell. Hence the bell and tongue method. The bell was used as both the forming roll and the welding roll. The 21 foot length was the length that could be produced and still keep enough heat in the strip to produce a good weld. A longer length would not have produced suitable weld quality over the entire length. Shorter pipe would not have as good a yield.]
What is the term tor that length of pipe? Isn't there a name for it?
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