Pipe Stress Analysis: Basic Concepts

Introduction

 It is common practice worldwide for piping designers to route piping by considering mainly space, process and flow constraints (such as pressure drop) and other requirements arising from constructability, operability and reparability. Unfortunately, pipe stress analysis requirements are often not sufficiently considered while routing and supporting piping systems, especially in providing adequate flexibility to absorb expansion/contraction of pipes due to thermal loads. So, when “as designed” piping systems are handed-off to pipe stress engineers for detailed analysis, they soon realize that the systems are “stiff” and suggest routing changes to make the systems more flexible. The piping designers, in turn, make changes to routing and send the revised layout to the pipe stress engineers to check for compliance again.

Such “back and forth” design iterations between layout and stress departments continue until a suitable layout and support scheme is arrived at, resulting in significant increase in project execution time, which, in turn, increases project costs.

This delay in project execution is further worsened in recent years by increased operating pressures and temperatures in order to increase plant output; increased operating pressures increase pipe wall thicknesses, which, in turn, increase piping stiffnesses further. Such increased operating temperatures applied on “stiffer” systems increase pipe thermal stresses and support loads. So, it is all the more important to make the piping layout flexible at the time of routing. (CheckStress, SST’s add-on product for PDMS and Cadmatic directly addresses and solves this problem).

Piping systems experience different types of loadings, categorized into three basic loading types: Sustained, Thermal and Occasional loads.

Sustained Loads

These mainly consist of internal pressure and dead-weight. Dead-weight is from the weight of pipes, fittings, components such as valves, operating fluid or test fluid, insulation, cladding, lining etc.

Internal design or operating pressure causes uniform circumferential stresses in the pipe wall, based on which a pipe wall thickness is determined during the process/P&ID stage of plant design. Additionally, internal pressure gives rise to axial stresses in the pipe wall. Since these axial pressure stresses vary only with pressure, pipe diameter and wall thickness (all three of which are preset at the P&ID stage), these stresses cannot be altered by changing the piping layout or the support scheme.

A pipe’s deadweight causes it to bend (generally downward) between supports and nozzles, producing axial stresses in the pipe wall (also called “bending stresses”) which vary linearly across the pipe cross-section, being tensile at either the top or bottom surface and compressive at the other surface. If the piping system is not supported in the vertical direction (i.e., in the gravity direction) excepting equipment nozzles, bending of the pipe due to deadweight may develop excessive stresses in the pipe and impose large loads on equipment nozzles, thereby increasing its susceptibility to “failure by collapse.”

Various international piping standards/codes impose stress limits, also called “allowable stresses for sustained loads,” on these axial stresses generated by deadweight and pressure in order to avoid “failure by collapse.”

For the calculated actual stresses to be below such allowable stresses for sustained loads, it may be necessary to support the piping system vertically. Typical vertical supports to carry deadweight are:

Variable spring hangers

• Constant support hangers
• Rod hangers
• Resting steel supports. Rod hangers and resting steel supports fully restrain downward pipe movement but permit pipe to lift up.

A couple of examples are presented in this tutorial to illustrate how piping can be supported by spring hangers and resting steel supports to comply with the code requirements for sustained loads.

Thermal Loads (Expansion Loads)

These refer to the “cyclic” thermal expansion or contraction of piping as it goes from one thermal state to another (for example, from “shutdown” to “normal operation” and then back to “shut-down”). If the piping system is not restrained in the thermal growth/contraction directions (for example, in the axial direction of pipe), then, for such cyclic thermal load, the pipe expands/contracts freely; in this case, no internal forces, moments and resulting stresses and strains are generated in the piping system. If, on the other hand, the pipe is “restrained” in the directions it wants to thermally deform (such as at equipment nozzles and pipe supports), such constraint on free thermal deformation generates cyclic thermal stresses and strains throughout the system as the system goes from one thermal state to another. When such calculated thermal stress ranges exceed the “allowable thermal stress range” specified by various international piping standards/codes, then the system is susceptible to “failure by fatigue.” So, in order to avoid “fatigue” failure due to cyclic thermal loads, the piping system should be made flexible (and not stiff).

Occasional Loads

This third type of loads is imposed on piping systems by occasional events such as earthquake, wind or a fluid hammer. To protect piping from wind and/or earthquake (which normally occur in a horizontal plane), it is normal practice to attach “lateral supports” to piping systems (instead of “axial restraints”). On the other hand, to protect piping for water/steam hammer loads, both “lateral supports” and “axial restraints” may be required.

Source: basic_pipe_stress_analysis_tutorial.pdf

https://www.sstusa.com/pdfs/basic_pipe_stress_analysis_tutorial.pdf