Hello sri_idea,

The longitudinal stresses due to pressure (P * Do) / (4 * t) are tensile stresses in the longitudinal direction caused by the impingement of the pressure loads upon the piping at the changes in direction at opposing ends of the pipe section at issue. This is a sustained static loading stress. As long as the pipe wall is uninterrupted in the section at issue, the forces are opposite and equal and equilibrium is maintained (the opposing forces "cancel" each other). If there were a bellows element in the section of pipe (that did not have “tie-bars”), the opposing forces would act to pull the bellow element apart from end to end. In this case, the bellows element would have to be protected from the pressure forces.

Regards, John