I've always wondered over the years why Sustained Load Cases include both Weight and Pressure. As it was explained to me by my supervisor and as I've read here and elsewhere, the Sustained case is the "as-installed" case. Pipe is installed, fitted and welded, empty and shut-down. So why do we not calculate the sustained case as weight only?

I have looked in B31.3 and elsewhere online and this is never explained.

Consequently, why is one of the hanger sizing load cases Weight only [W (HGR)], instead of weight + pressure?

Thank you,
Dmitriy