Failure Analysis of Hydrogenation Unit Water Separator Inlet Flange and Tee

Overview and Failure Mode Identification

The paper by Xiong Jinping, Zuo Yu, and Tian Mei from Beijing University of Chemical Technology and Qilu Petrochemical Company presents a failure analysis of a tee and flange assembly at the inlet of a water separator in a hydrogenation unit. The failure manifested as cracking in both the tee and the flange, leading to loss of containment. The authors employed a comprehensive suite of analytical techniques including optical microscopy (OM), scanning electron microscopy (SEM), electron spectroscopy for chemical analysis (ESCA/XPS), microhardness testing, and ferrite content measurement to identify the root cause of the failure.

The hydrogenation unit operates under conditions that are particularly aggressive for carbon steel and low-alloy steel piping, involving the presence of hydrogen gas at elevated pressures and temperatures. This environment creates a high susceptibility to hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC), which are well-documented failure mechanisms in refinery and petrochemical service.

Detailed Failure Analysis

The analytical findings revealed two concurrent failure mechanisms:

Hydrogen-induced cracking (HIC): The SEM examination revealed characteristic HIC morphology, including stepwise cracking and blister formation. The presence of hydrogen atoms diffusing into the steel matrix and accumulating at microstructural traps (inclusions, laminations, grain boundaries) leads to the formation of microvoids and cracks. The hydrogenation unit environment provides a ready source of atomic hydrogen, which can enter the steel through surface reactions or through defects in the protective coating.
Chloride stress corrosion cracking (Cl-SCC): The ESCA/XPS analysis detected chloride contamination on the crack surfaces, indicating that chloride ions were present in the service environment. The combination of tensile stress (from residual welding stress, thermal stress, or mechanical loading) and chloride exposure led to stress corrosion cracking. The microhardness measurements revealed elevated hardness in the crack-affected regions, consistent with strain hardening and possible temper embrittlement.

Analysis Technique	Key Finding	Failure Mechanism Indicated
Optical Microscopy (OM)	Stepwise crack morphology	HIC
Scanning Electron Microscopy (SEM)	Blister formation and microvoid coalescence	HIC
ESCA/XPS	Chloride contamination on crack surfaces	Cl-SCC
Microhardness	Elevated hardness in crack-affected zones	Strain hardening / temper embrittlement
Ferrite measurement	Ferrite content within specification	Not the primary cause

Root Cause Analysis and Engineering Lessons

The failure analysis reveals a multifactorial failure scenario where hydrogen-induced cracking and chloride stress corrosion cracking acted synergistically. The root cause can be attributed to the following factors:

Material susceptibility: The base material of the tee and flange may have had microstructural features (inclusions, laminations, coarse grain structure) that facilitated hydrogen trapping and crack initiation. The ferrite content, while within specification limits, may have been at the upper end of the acceptable range, increasing susceptibility to HIC.
Residual stress: Welding residual stresses, if not adequately relieved through post-weld heat treatment (PWHT), provide the tensile stress component necessary for both HIC and Cl-SCC. The tee-to-flange connection, if welded in the field, may have been inadequately stress-relieved.
Environmental factors: The presence of hydrogen in the process stream and chloride contamination in the water separator inlet created a highly aggressive environment. The chloride source may have been from process water, cooling water, or atmospheric deposition.
Corrosion protection inadequacy: Any defects in the internal coating or cathodic protection system would have exposed bare metal to the aggressive environment, facilitating hydrogen entry and chloride attack.

Engineering Practice and Preventive Measures

Based on this failure analysis, the following preventive measures should be implemented in hydrogenation unit piping systems:

Material selection: For hydrogen service, low-sulfide-stress-cracking-resistant materials should be specified. API 5L X65 or X70 with HIC-resistant specifications (such as NACE MR0175/ISO 15156 compliance) should be used for piping in hydrogen-rich environments. For fittings, ASTM A234 WP11 or WP22 with appropriate tempering treatment should be specified.
Welding and PWHT: All welds in hydrogen service should be followed by PWHT to reduce residual stresses. The PWHT temperature and duration should be verified through hardness testing at the weld and HAZ. The maximum hardness at the HAZ should not exceed 22 HRC (approximately 300 HBW) per API 941.
Corrosion protection: Internal coatings or linings should be specified for piping in hydrogen service. The coating system should be designed to resist hydrogen permeation and chloride attack. Regular inspection of coating integrity should be performed.
Monitoring and inspection: Regular inspection programs should include:
Hydrogen blistering testing (per ASTM G123 or NACE TM0177) for material qualification
Ultrasonic testing for internal blister and crack detection
Eddy current testing for near-surface defect detection
Periodic hardness testing at welds and HAZ to monitor temper embrittlement
Environmental control: Chloride levels in process water and cooling water should be monitored and controlled. The water separator inlet piping should be designed to minimize stagnant water zones where chloride concentration could build up.

Key Reflections and Study Insights

This failure analysis case study is a powerful reminder of the complexity of failure mechanisms in petrochemical service. The concurrent action of HIC and Cl-SCC illustrates that failure rarely occurs due to a single mechanism, and that a comprehensive failure analysis must consider all potential contributing factors.

The most important lesson for pipeline and fitting engineers is the need for a holistic approach to material selection, welding quality, corrosion protection, and inspection. No single measure is sufficient; rather, a defense-in-depth strategy is required. The tee and flange assembly that failed in this case likely met all individual specification requirements but still failed because the combination of material susceptibility, residual stress, and environmental aggressiveness exceeded the system's tolerance.

From a quality control perspective, this case underscores the importance of verifying not only that materials meet specification requirements but also that they are appropriate for the specific service environment. The concept of "fitness for purpose" goes beyond mere compliance with standards and requires a thorough understanding of the operating conditions, failure mechanisms, and material behavior under those conditions.

For future hydrogenation unit projects, I recommend implementing a systematic approach to failure prevention that includes material characterization for HIC and SSC resistance, rigorous welding procedure qualification with PWHT verification, comprehensive corrosion protection design, and a structured inspection and monitoring program. The cost of preventing such failures is far less than the cost of the consequences, including production shutdown, environmental release, and potential safety incidents.