Leakage Cause Analysis and Improvement of Seamless Elbows in an Ethylene Glycol Atmospheric Tank Farm

Literature Overview

The paper by Wang Bumei (2014), published in Chemical Engineering and Machinery, presents a failure analysis of seamless elbows made of austenitic stainless steel that experienced frequent leakage in an ethylene glycol atmospheric tank farm pipeline at a chemical plant. The study applies classical materials failure analysis methodology, examining chemical composition, hardness, microstructure, crack morphology, and corrosion products, and then correlates these findings with operating conditions to diagnose the root cause of the leakage.

Failure Analysis Methodology

The failure analysis follows a systematic approach consistent with established standards such as ASTM E1922 (Standard Guide for Conducting a Failure Analysis) and API 579-1/ASME FFS-1 principles. The investigation proceeds through the following stages:

1. Chemical composition verification — confirming that the material meets the specified grade requirements (typically 304 or 316L austenitic stainless steel for glycol service).
2. Hardness testing — assessing whether the material has undergone unintended hardening or softening due to service exposure or manufacturing defects.
3. Metallographic examination — evaluating the grain structure, presence of carbide precipitation, sensitization, and microstructural anomalies.
4. Fracture surface analysis — examining crack morphology under optical and scanning electron microscopy to determine the crack initiation mechanism (stress corrosion cracking, fatigue, mechanical damage, etc.).
5. Corrosion product analysis — identifying the chemical composition of deposits and corrosion products to infer the electrochemical environment at the failure site.

Root Cause Diagnosis

The critical finding of this analysis is the identification of stress corrosion cracking (SCC) as the primary failure mechanism. Austenitic stainless steels, particularly grades 304 and 304L, are susceptible to chloride-induced SCC when exposed to environments containing chloride ions at temperatures above approximately 50°C. In the ethylene glycol tank farm environment, several factors contribute to the SCC susceptibility:

• Chloride contamination — ethylene glycol solutions can contain chloride ions introduced through raw materials, process additives, or environmental ingress.
• Temperature cycling — atmospheric tank farm piping experiences diurnal and seasonal temperature variations, with summer temperatures potentially exceeding the SCC threshold for austenitic stainless steels.
• Residual stress — the seamless elbow, produced by hot bending or seamless tube bending processes, retains significant residual tensile stresses in the outer wall of the bend, providing the mechanical driving force for SCC initiation.
• Sensitization — if the elbow was exposed to elevated temperatures during manufacturing or service, chromium carbide precipitation at grain boundaries can deplete the boundary regions of chromium, creating preferential corrosion paths.

Improvement Measures

Based on the failure analysis findings, several improvement measures are proposed:

1. Material upgrade — replacing 304/304L with 316L or 316LN stainless steel, which has superior chloride SCC resistance due to the addition of molybdenum. For more severe chloride environments, duplex stainless steels (e.g., 2205) or super duplex grades may be considered.
2. Stress relief — implementing a post-bending stress relief heat treatment at 425–450°C for a duration sufficient to relieve residual stresses below the SCC threshold. The stress relief temperature must be carefully controlled to avoid sensitization; for 304L, the upper limit should not exceed 450°C.
3. Residual chloride control — implementing rigorous controls on the chloride content of the ethylene glycol solution, including periodic water quality testing and chloride scavenger addition.
4. Temperature management — providing insulation or cooling measures to maintain the piping temperature below the SCC susceptibility threshold for the selected material grade.
5. Enhanced inspection — implementing a periodic inspection program using eddy current testing (ECT) or ultrasonic testing (UT) specifically designed to detect intergranular SCC, which is particularly challenging to detect with conventional inspection methods.

Engineering Practice Implications

This case study underscores a fundamental principle in piping engineering: the selection of a corrosion-resistant alloy is necessary but not sufficient to ensure long-term integrity in a corrosive environment. The interaction between material microstructure, residual stress state, and environmental factors must be holistically evaluated. In this case, the seamless elbow was specified in austenitic stainless steel, which is generally considered appropriate for glycol service, but the combination of chloride contamination, temperature exposure, and manufacturing residual stresses created the conditions for SCC.

The failure also highlights the importance of failure analysis in engineering practice. Without the systematic investigation described in this paper, the root cause would likely have been misattributed to a manufacturing defect or material non-conformance, leading to inappropriate corrective actions such as simple replacement with the same material and specification, which would inevitably result in recurrence.

Study Insights

The paper is a textbook example of the application of materials failure analysis methodology to a practical engineering problem. The systematic approach — from chemical analysis to microstructural examination to fracture surface evaluation — demonstrates the value of a structured investigation process. The key insight for practicing engineers is that SCC is a multi-factorial failure mechanism, and effective prevention requires addressing all contributing factors simultaneously rather than focusing on any single variable.

From a standards perspective, this case reinforces the importance of complying with the requirements of ASME B31.3 for process piping, particularly the provisions regarding material selection for corrosive environments and the requirements for stress relief of formed components. It also highlights the relevance of NACE SP0472 (now AMPP SP0472) for the prevention of chloride-induced stress corrosion cracking in austenitic stainless steels.

In conclusion, this failure analysis provides a valuable case study for engineers working with austenitic stainless steel piping in chemical processing environments, demonstrating that a rigorous, systematic approach to failure investigation is essential for developing effective and lasting corrective measures.