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What Is Primary Stress? | Piping Engineering Glossary
What Is Primary Stress?
In piping engineering and process engineering, primary stress is stress produced by loads that are not relieved as the piping deforms, such as internal pressure, the weight of the pipe and its contents, wind, and earthquake forces. If primary stress becomes too high, the pipe can keep deforming until it collapses or bursts, which is why piping codes limit it with strict allowable values.
Primary vs. Secondary Stress
Piping codes separate stresses by how they behave. Primary stresses are not self-limiting: the load stays the same even as the pipe yields. Secondary stresses, mainly caused by thermal expansion, are self-limiting, because a small amount of local yielding relaxes them. Secondary stresses are controlled to prevent fatigue failure over many cycles, while primary stresses are controlled to prevent gross plastic deformation.
In ASME B31.3, primary stresses from pressure and weight are combined and checked as sustained load stresses against the hot allowable stress. Primary stresses from wind, earthquakes, and other short-term events are checked as occasional loads, with a higher allowable because they act only briefly.
Applications
Pipe Stress Analysis
Every pipe stress analysis includes a sustained case that sums pressure and weight stresses at each point in the system.
Support Design
Primary stress checks determine how far apart supports can be and where additional pipe supports are needed to carry heavy valves or vertical runs.
Occasional Load Design
Wind, earthquake, relief valve thrust, and water hammer are evaluated as primary loads to confirm the system survives short-term events. Each case is combined with the sustained loads as the code requires before the result is compared with the occasional allowable.
Benefits
Prevents Collapse
Limiting primary stress keeps the piping well away from loads that could cause it to fail outright. In high-temperature lines, it also limits long-term creep damage.
Clear Code Criteria
Codes provide well-defined allowable values, which makes compliance straightforward to check. Reviewers and owners can verify results against the same published limits.
Guides Support Layout
Primary stress results show exactly where the system needs more support or stronger components.
Limitations
Simplified Stress Model
Beam-based stress analysis does not capture all local effects at nozzles, trunnions, and attachments, which may need separate checks.
Depends on Accurate Loads
Errors in weights, insulation, or contents can lead to unconservative results. Hydrotest water weight, snow, ice, and future insulation upgrades should all be included where they apply.
Does Not Address Fatigue
Meeting primary stress limits says nothing about cyclic damage, which is assessed separately through displacement stress range checks.
FAQ
Is thermal expansion a primary stress?
No. Stress from restrained thermal expansion is normally treated as secondary because it is self-limiting, although some high-temperature situations need special consideration.
What happens if primary stress exceeds the allowable?
The design must change, typically by adding supports, shortening spans, increasing wall thickness, or reducing concentrated loads, until the stress falls within the code limit.
Does pressure count as a primary stress?
Yes. Longitudinal stress from internal pressure is included in the sustained stress calculation along with weight effects.
Why are occasional allowables higher?
Occasional loads act for short periods and are unlikely to occur together, so codes permit higher stresses. ASME B31.3 commonly allows up to 1.33 times the basic hot allowable stress for occasional loads.
Reference
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