Piping and Pressure Vessel Failures Associated with Secondary Stresses

Why It Matters

Unaddressed secondary stresses can trigger premature failures, driving up maintenance costs and downtime for critical infrastructure. Recognizing and mitigating them improves safety and asset longevity.

Key Takeaways

• Secondary stresses arise from constraints and welding distortions
• Non‑PWHT welds can yield under hydrostatic testing pressure
• Residual compression reduces stress‑corrosion crack propagation rates
• Overlooked secondary stresses cause costly pipe and vessel failures
• Proper heat treatment mitigates secondary stress‑related failures

Pulse Analysis

Secondary stresses develop when a component is constrained by adjacent parts or its own geometry, often emerging from welding distortion, thermal expansion, or uneven cooling. Unlike primary loads such as internal pressure, these stresses are less intuitive and can be difficult to detect without detailed analysis. In the process industry, where pipelines and pressure vessels operate under cyclic loads, overlooking secondary stresses can compromise structural integrity, leading to unexpected leaks or ruptures that jeopardize safety and operational continuity.

The Inspectioneering article presents three real‑world failures that illustrate the hidden danger of secondary stresses. A post‑weld heat‑treated carbon steel pipe in caustic service failed because non‑PWHT welds yielded during a high hydrostatic test, leaving compressive residual stresses that altered crack propagation. Similarly, high‑temperature austenitic tube assemblies and boiler feed‑water components suffered environmental stress cracking, directly linked to thermal gradients and inadequate stress relief. Laboratory strain‑gauge data confirmed that compressive fields generated during testing can both suppress and accelerate crack growth, depending on the residual stress state, highlighting the nuanced role of secondary stresses in failure mechanisms.

Mitigating secondary stress‑related failures requires a proactive approach: comprehensive finite‑element stress modeling, stringent welding procedures, and consistent post‑weld heat treatment. Industry standards now emphasize residual stress assessments and the use of hydrostatic testing to introduce beneficial compressive stresses that can retard stress‑corrosion cracking. By integrating advanced inspection techniques—such as ultrasonic residual stress measurement—and adopting design practices that minimize constraint, operators can extend asset life, reduce unplanned shutdowns, and uphold regulatory compliance. As the energy sector modernizes, the focus on secondary stress management will become a critical component of reliability engineering.

Piping and Pressure Vessel Failures Associated with Secondary Stresses

By Ana Benz, Chief Engineer at IRISNDT, and Glenn Roemer, Senior Materials Engineer at Apave Canada · Published in the November/December 2025 issue of Inspectioneering Journal

Keeping pressure equipment operating reliably requires a clear understanding of its mechanical operating conditions, such as loads and load cycles. Primary loads/stresses, such as those associated with dead and live weights, and internal pressure, are easy to envisage. However, secondary stresses, often related to welding, displacements, changes in operational temperature, and deformation, are not as easily visualized. Yet these easily overlooked stresses result in costly failures of piping and pressure vessels. This article illustrates the importance of secondary stresses by presenting several failures resulting from them:

• A post‑weld heat‑treated (PWHT) caustic service NPS 1 carbon steel pipe failure.

• A high‑temperature austenitic radiant tube assembly failure.

• A high‑temperature boiler feed water service, environmental stress cracking failure.

Introductory Secondary Stress Example

A secondary stress is “developed by the constraint of adjacent parts or by self‑constraint of a structure.” [1] The concept is abstract and illustrated below.

While testing the pressure capacity of a pipe fitting, strain gauges were instrumented along the spool, which included the fitting welded to the pipe and two non‑PWHT welds between the spool and the sealing flanges (Figure 1). The welds between the fitting and the pipe had been PWHT. The applied hydrostatic pressure was high because the objective was to assess the fitting’s yielding pressure. Once the hydrostatic test was completed, the strain on the two non‑PWHT welds was negative. The pressure had made the non‑PWHT welds yield, and once it was removed, the elongated welds were compressed within the spool.

Figure 1. Spool tested during a hydrostatic test.

This compressive field is known in the pipeline industry, where hydrotests have been reported to reduce the stress‑corrosion crack propagation rates. The rate reduction is associated with compressive residual stresses in front of the crack tips [2].

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References

1. Definition of secondary stress.

2. Study on hydrotest‑induced compressive residual stresses and SCC rate reduction.