Exclusion of 22" and 26" carbon steel pipes

Hi everybody,
I've seen numerous piping spec in oil & gas works. In most of them, some sizes are excluded from usage in the design, despite of their availability in manufacturers catalog. I understand exclusion of 5", 7" or 9" but can't get reason for 22" and 26". Does anybody know?
Regards,
Masoud

This is real simple actually! For whatever reason, these sizes (like your 5", 7", & 9") are considered "odd-ball" ...Something that is there for extremely project specific purposes, but not to be used for general construction. One of the first rules of construction economic is "odd-ball" equals cost! Material supply companies are there to have normal construction materials on hand that fabricators can quickly access and finish project construction, NOT something they seldom use! SO IF you specify these "odd-ball" sizes, two things will happen, first your material cost is going to be more! AND next the lead time to get these materials is going to be longer, adding to the cost! SO bottom-line, if some engineer is using a computer program that indicates that the most efficient flow of a commodity is thru a 22" or 26" dia. pipe, ask him to re-calculate to see how it would be through a 24" or 30" pipe! And if they still balk, add in the extra cost and see what they think then. Its not rocket science.

Thanks a lot for your help!

Really nice and informative post.