Chapter 6
Design for Sustained and Occasional Loads


The wall thickness of pipe is nearly always selected based on the thickness required for internal pressure and allowances. The piping is then supported sufficiently such that the longitudinal stress (the stress in the axial direction of the pipe) is within Code limits and deflection is within acceptable limits.

Deflection limits are not Code requirements, but are generally accepted practices; a 13-mm (1∕2-in.) deflection is a generally accepted guideline for general process plant piping. More stringent limits may be required for lines that must avoid pockets caused by sagging of the line; greater deflection is generally acceptable from a mechanical integrity standpoint, if not an operator confidence standpoint.

It is fortunate that the longitudinal pressure stress is one-half of the hoop stress in a cylinder. What this means is that if the pipe is designed for pressure, at least one-half of the strength in the longitudinal direction remains available for weight and other sustained loads.

  • 6.1 Primary Longitudinal Stress
  • 6.2 Sustained Longitudinal Stress
  • Insert 6.1 Span Limits for Elevated Temperature Piping
  • 6.3 Limits of Calculated Stresses Due to Occasional Loads

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