Failure Analysis of Synthesis Tower Gas Outlet Elbow Due to Hydrogen Attack

Incident Description and Background

This 2007 paper by Li Chao from Hebei Qian'an Fertilizer Co., Ltd. reports on the failure analysis of an elbow in the gas outlet line of a synthesis tower No. 2. Published in Petroleum and Chemical Equipment, Volume 10, Issue 3, pages 54 to 55, the study investigates the cause of an elbow rupture that occurred during normal operation. The synthesis tower operates at temperatures exceeding 300 degrees Celsius with hydrogen-rich gas flowing through the piping system.

Failure Investigation and Root Cause

The investigation revealed that the elbow had been in continuous service at temperatures above 300 degrees Celsius for an extended period. Under these conditions, hydrogen in the gas stream reacted chemically with carbon and iron carbide (Fe3C) in the pipe material, producing methane gas. This reaction caused decarburization of the pipe wall and the formation of microcracks. The decarburized material experienced a significant reduction in strength and an increase in brittleness, ultimately leading to catastrophic failure by bursting.

Hydrogen Attack Mechanism and Metallurgical Analysis

The mechanism of hydrogen attack in carbon steel is well documented in the literature. At temperatures above approximately 200 degrees Celsius, atomic hydrogen from the gas phase diffuses into the steel and reacts with carbide phases, particularly Fe3C, to form methane molecules. The methane molecules cannot diffuse back out of the steel and accumulate at grain boundaries and inclusion sites, creating internal pressure that drives microcrack formation. Over time, these microcracks coalesce into larger cracks that reduce the effective load-bearing cross-section of the pipe wall.

The decarburization process removes carbon from the steel matrix, transforming the microstructure from a pearlite-ferrite mixture to a nearly pure ferrite structure. This transformation eliminates the hard and strong pearlite phase, resulting in a material with significantly reduced yield strength and tensile strength. The combination of microcracking and decarburization leads to a progressive loss of structural integrity that is particularly severe at locations of high stress concentration, such as the inner fiber of an elbow bend.

Preventive Measures and Material Selection

Prevention of hydrogen attack requires a combination of material selection, process monitoring, and operational controls. Materials resistant to hydrogen attack include low-alloy steels with controlled carbon content, hydrogen-resistant steels such as 2.25Cr-1Mo, and certain austenitic stainless steels. The Nelson Curve, which plots hydrogen partial pressure against temperature for different material grades, provides guidance for material selection in hydrogen service environments.

Operational controls include monitoring hydrogen partial pressure and temperature to ensure they remain below the threshold for hydrogen attack for the selected material. Regular inspection programs using ultrasonic testing or radiographic testing can detect early stages of hydrogen damage before catastrophic failure occurs. For existing equipment, replacement with hydrogen-resistant materials or application of protective coatings may be necessary.

Study Insights and Engineering Lessons

This failure case underscores the critical importance of considering hydrogen service conditions in the design and material selection of piping systems. The synthesis tower application represents a classic scenario where hydrogen attack can occur, and the failure of the elbow demonstrates that even components that appear sound can be progressively weakened by hydrogen damage. Engineers must always consult the Nelson Curve during material selection for hydrogen service and must implement rigorous inspection programs for existing equipment operating in hydrogen environments. The case also highlights the vulnerability of elbows and other formed components, where the combination of hydrogen attack and stress concentration can accelerate failure.