Why does Sustained load case include pressure?

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

These sustained stress calculations are used to ensure that the system does not collapse. Force-based loads are used in this check. With sufficient wall thickness (provided by rule), hoop pressure need not be considereed in this check. Longitudinal and shear stresses caused by these loads are used to calculate the stress intensity (2 times maximum shear stress) and this stress is comapred to the basic allowable stress at temperature.
Don't confuse regular structural evaluation with Code-defined stress evaluation.

Thank you for the reply.