Failure Analysis of Elbow Leakage at a Natural Gas Station

Literature Overview

The paper by Xu Yan, Liu Yinglai, Nie Xianghui, Feng Zhenjun, Li Liang, and Li Jike from the Petroleum Pipe Engineering Technology Research Institute of PetroChina Group and Beijing Longshengtaike Petroleum Pipe Technology Co., Ltd., published in Pressure Vessel (2020, Vol. 37, No. 4, pp. 61-67), presents a detailed failure analysis of an elbow that leaked during the hydrostatic pressure test prior to commissioning at a natural gas station. The investigation employed mechanical property testing, metallographic analysis, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) to identify the root cause of the failure.

Core Technical Content

Failure Description and Initial Observations

During the pre-commissioning hydrostatic pressure test at a natural gas station, a longitudinal crack was discovered on the outer arc side of an elbow, resulting in a pressure leak. The crack originated at the inner surface of the elbow and propagated through the entire wall thickness, eventually breaching the outer surface. The location of the crack on the outer arc is consistent with the tensile stress state experienced by the outer arc during elbow forming, where the metal is stretched and thinned.

Metallurgical Investigation

The investigation revealed that the root cause of the failure was porosity defects present in the elbow material. The porosity originated from inclusions (slag) in the raw material during the smelting process. These inclusions were not detected or removed during subsequent processing steps, and they propagated and worsened during the elbow forming operation, ultimately leading to crack initiation and propagation.

The failure mechanism can be summarized as follows:

Origin: Inclusions (slag) in the raw steel during smelting created initial porosity defects in the base material.
Propagation: During elbow forming, the porosity defects were elongated and concentrated along the deformation direction, creating stress concentrators.
Crack initiation: Under the hydrostatic test pressure, stress concentration at the porosity defects initiated cracks at the inner surface of the elbow.
Crack propagation: The main crack propagated through the wall thickness along the intergranular porosity defects, with secondary intergranular cracks forming on both sides of the main crack and around the crack tip.
Final failure: The cracks eventually penetrated the entire wall thickness, resulting in a through-wall leak.

Microstructural Analysis

The metallographic examination and SEM analysis revealed the following microstructural features:

Feature	Observation	Significance
Main crack	Longitudinal, through-wall	Initiated at inner surface, propagated through thickness
Porosity defects	Intergranular, elongated along forming direction	Originated from slag inclusions in raw material
Secondary cracks	Intergranular, along both sides of main crack and around crack tip	Indicate intergranular cracking mechanism
Inclusions	Present in the matrix, associated with porosity	Root cause of the failure

The EDS analysis confirmed the presence of non-metallic inclusions in the matrix, providing chemical evidence for the slag inclusion origin of the porosity defects. The intergranular nature of the cracks and secondary cracks indicates that the porosity defects acted as preferential crack paths, weakening the intergranular cohesion of the material.

Root Cause Analysis Using 5W2H Framework

Applying the 5W2H framework to this failure case:

Question	Answer
What failed?	Elbow with longitudinal through-wall crack
Where did it fail?	Outer arc side, crack originated at inner surface
When did it fail?	During hydrostatic pressure test before commissioning
Why did it fail?	Porosity defects from slag inclusions in raw material
Who is responsible?	Raw material supplier (inclusions), manufacturer (failure to detect and remove defects)
How did it fail?	Stress concentration at porosity under test pressure caused crack initiation and propagation
How many were affected?	Potentially all elbows from the same material batch

FMEA Analysis

A Failure Modes and Effects Analysis (FMEA) for this type of elbow manufacturing process would identify the following critical failure modes:

Failure Mode	Potential Cause	Potential Effect	Severity	Occurrence	Detection	RPN
Through-wall crack	Slag inclusions in raw material	Pressure leak, safety hazard	10	3	4	120
Internal porosity	Incomplete degassing during smelting	Reduced pressure capacity	8	4	3	96
Surface crack	Improper forming parameters	Pressure leak	9	3	5	135
Wall thinning	Excessive forming strain	Reduced pressure capacity	7	4	3	84

The high Risk Priority Number (RPN) values for through-wall cracks and surface cracks indicate that these failure modes require immediate attention in the manufacturing process.