From Pipe Stress Engineering by Peng and Peng.

"Pressure elongation is often mistreated as a sustained load because of its association with pressure, which is a sustained load. Just like thermal expansion, pressure elongation generates a displacement that is a self limiting load. Its effect on the piping system is determined by the potential axial displacement of each leg of the piping. Once the disaplcement reaches the potential elongation amount, it stops regardless of whether or not the yielding occurs in the piping. This pressure elongation is generally included in the flexibility analysis the same was as thermal expansion is. In general, pressure elongation is added to thermal expansion to become the total displacement load in the analysis.

Excellent book BTW, worth every cent.