Failure Analysis of Steam Pipeline Elbow Burst Caused by Internal Fold Defect

Literature Overview

This study, published by Jiang Daomin in 2014 in the journal Refining and Chemical Engineering, documents a failure investigation conducted at the First Chemical Plant of Daqing Petrochemical Company. The subject is a steam pipeline elbow that suffered a catastrophic burst during service. The investigation employed fractography, chemical composition analysis, and metallographic examination to identify the root cause. The finding was that a large fold defect existed on the inner wall side of the elbow, which progressively bulged under operational stress until final rupture occurred. This case is instructive because fold defects in hot-formed or rolled elbows are among the most insidious manufacturing imperfections, often escaping detection at the mill and only revealing themselves under sustained cyclic or steady-state loading.

Core Technical Findings

The investigation followed a systematic failure analysis methodology that can be summarized through the 5W2H framework. The what was a burst failure of a steam elbow; the where was the inner curvature surface where the fold defect was located; the why traced back to an incomplete weld or roll closure during elbow forming. The fold defect acted as a stress concentrator, and under the combined action of internal steam pressure and thermal cycling, the material at the fold gradually thinned and bulged outward. Eventually, the remaining ligament could no longer sustain the hoop stress, leading to a ductile overload rupture.

The metallographic examination revealed that the microstructure in the region of the fold was severely deformed, with elongated grains and possible partial recrystallization depending on the peak temperature reached during forming. The fracture surface showed a typical ductile overload morphology with microvoid coalescence, confirming that the failure was not brittle but rather a progressive loss of load-bearing cross-section.

Engineering Implications and Preventive Measures

The key lesson from this case is that elbow forming processes—whether hot-rolling, hydraulic expansion, or bending from pipe stock—must be accompanied by rigorous internal defect detection. ASME B16.9 specifies radiographic examination for seamless and welded fittings, but compliance is not always enforced in lower-specification procurement. Engineers should insist on RT or phased-array ultrasonic testing (PAUT) for elbows used in high-temperature steam service, particularly where the operating pressure exceeds 1.0 MPa and the temperature exceeds 200 °C.

Reflections and Study Insights

This failure case underscores a fundamental principle in pressure vessel and piping integrity: a manufacturing defect that reduces the effective wall thickness can initiate a progressive failure mechanism that is difficult to predict from design calculations alone. The fold defect did not cause immediate failure; instead, it created a slow-burning integrity issue that could have been detected by routine ultrasonic thickness gauging at the inner curvature. The study reinforces the importance of in-service inspection programs that specifically target the inner curvature of elbows, where stress concentrations are highest and where forming defects are most likely to manifest.

From a materials engineering perspective, the case also highlights the need for forming process qualification. Elbow manufacturers should document the forming temperature, number of passes, and inter-pass cooling conditions. Any deviation from the qualified forming window increases the probability of fold formation. For engineers involved in procurement and inspection, this case serves as a reminder that visual examination alone is insufficient for detecting internal folds, and volumetric NDT methods are essential for ensuring fitness for service in critical steam systems.