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What Is Longitudinal Stress? | Piping Engineering Glossary
What Is Longitudinal Stress?
In piping engineering and process engineering, longitudinal stress is the stress acting along the length of a pipe, parallel to its axis. It comes from internal pressure pushing on the pipe’s closed ends, from bending caused by weight and other loads, and from any axial forces in the line. Longitudinal stress is one of the main stresses checked in pipe stress analysis.
Sources of Longitudinal Stress
For a closed pipe under internal pressure, the pressure component of longitudinal stress is about half the hoop stress, commonly estimated as pressure times diameter divided by four times the wall thickness. Bending adds tension on one side of the pipe and compression on the other, with the largest values usually at supports, elbows, and branch connections. Axial forces from friction or restraint add a further component.
Codes treat longitudinal stress differently depending on its cause. Stresses from pressure and weight are combined as sustained load stresses, while bending caused by thermal expansion is evaluated separately as a displacement stress range because it is self-limiting.
Applications
Sustained Stress Checks
In ASME B31.3, the sustained longitudinal stress from pressure and weight must not exceed the hot allowable stress of the material. Stress intensification factors at fittings are applied to the bending part of this stress.
Occasional Load Checks
Wind, seismic, and other occasional loads add to longitudinal stress and are checked against a higher allowable for short-term events.
Support Design
High longitudinal bending stresses often point to spans that are too long or to pipe supports placed too far from heavy valves and fittings.
Benefits
Prevents Overstress and Collapse
Keeping longitudinal stress within limits protects the pipe from excessive bending and gross deformation. It also limits sag that could pocket liquid or misalign flanges.
Clear Design Guidance
Stress results show exactly where supports, restraints, or heavier pipe are needed. This helps designers fix problems before the layout is finalized.
Supports Code Compliance
Longitudinal stress checks are a required part of demonstrating that a piping system meets its design code. Results are documented in stress reports reviewed by the owner or a third party.
Limitations
Simplified Calculations
Beam theory used in stress software does not capture all local effects at attachments and nozzles. Critical locations may need separate local stress or finite element checks.
Depends on Accurate Inputs
Errors in weights, pressures, or support locations lead directly to errors in calculated stress. Support gaps, friction, and settlement can also change the real stress.
Interaction With Other Stresses
Longitudinal stress combines with hoop and shear stresses, and some situations require a more detailed combined stress check.
FAQ
How does longitudinal stress compare with hoop stress?
For pressure alone, longitudinal stress is about half the hoop stress. Once bending from weight is added, the longitudinal stress at some locations can approach or exceed the hoop stress.
Is thermal expansion included in sustained longitudinal stress?
No. Thermal bending is evaluated separately as a displacement stress range against its own allowable, because it relaxes as the pipe yields slightly.
Where is longitudinal stress usually highest?
At supports, elbows, tees, and connections where bending moments are largest and fittings concentrate stress.
How do engineers review longitudinal stress results?
Stress engineers check the reported values at every node of the pipe stress analysis model and mark critical lines and support changes on the stress isometric.
Reference
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