Analysis and Treatment of Saturated Hot Water Tower Top Pipe Elbow Cracking

Literature Overview

The technical report by Wang Qing, Zou Hong, Song Dongbao, and Liu Hongjia (CNOOC Shandong Chemical Engineering Co., Ltd., 2016, Fertilizer Technology, Vol. 37, Issue 2, pp. 30-32) documents a comprehensive failure analysis of a saturated hot water tower top gas outlet pipe elbow that experienced cracking in a full cryogenic sulfur-resistant shift conversion process. The study identifies the root cause as stress corrosion cracking in a wet hydrogen sulfide environment and proposes design modifications to eliminate the safety hazard.

Operating Conditions and Failure Description

The saturated hot water tower is a critical component in the full cryogenic sulfur-resistant shift conversion process used in natural gas processing and ammonia synthesis. The tower top gas outlet piping operates under conditions that create a highly aggressive corrosion environment:

Failure Mode Identification

The elbow cracking manifested as multiple fine cracks propagating from the outer surface (belly side) of the elbow. The crack morphology, combined with the operating environment, pointed to stress corrosion cracking (SCC) as the primary failure mechanism. The combination of tensile stress (from bending moments and thermal cycling) and the wet H2S environment created ideal conditions for sulfide stress cracking (SSC).

Root Cause Analysis

Multi-Factor Failure Mechanism

The failure analysis revealed that no single factor was solely responsible for the cracking. Instead, a combination of factors created the failure conditions:

1. Material susceptibility: The pipe material, while meeting general mechanical property requirements, lacked adequate resistance to sulfide stress cracking under the specific operating conditions.
2. Stress concentration: The elbow geometry inherently creates stress concentrations, particularly at the belly (outer radius) where bending moments produce maximum tensile stress.
3. Environmental aggressiveness: The saturated hot water environment with dissolved H2S creates a highly corrosive electrolyte that promotes crack initiation and propagation.
4. Thermal cycling: Temperature variations during operation and startup/shutdown cycles introduce additional fatigue stresses that lower the threshold for SCC initiation.
5. Residual stress: Welding residual stresses from elbow fabrication and installation contributed to the total stress state at the crack initiation sites.

Failure Analysis Methodology

The investigation employed a systematic approach consistent with ASME FFS-1 and API 579 practices:

Treatment and Design Optimization

Design Modification Approach

Based on the failure analysis conclusions, the following design modifications were implemented to eliminate the cracking hazard:

1. Piping layout optimization: Using stress calculation software (such as CAESAR II or AutoPIPE), the piping configuration was redesigned to reduce bending moments at the elbow location. This involved:

• Adding a support close to the elbow to reduce span length
• Modifying the route to reduce the number of bends in the critical section
• Incorporating an expansion joint to accommodate thermal movement

1. Material upgrade: The elbow material was upgraded to a grade with demonstrated resistance to sulfide stress cracking, meeting NACE MR0175/ISO 15156 requirements for sour service.
2. Stress reduction measures:

• Reduced bending moment at the elbow by optimizing support spacing
• Added a saddle support to distribute loads more evenly
• Modified nozzle orientation to minimize misalignment during installation

Quantitative Improvement

Engineering Practice Implications

Lessons for Similar Applications

This case study provides valuable lessons for engineers designing piping systems in sour service environments:

• Always verify material compliance with NACE MR0175/ISO 15156 for wet H2S service, even when general mechanical properties are adequate
• Conduct stress analysis specifically for SCC assessment, not just for general mechanical integrity
• Pay special attention to elbows and other geometric discontinuities in sour service piping
• Consider the combined effect of mechanical stress, thermal stress, and residual stress in SCC evaluation
• Implement a systematic failure analysis approach when cracking is observed, rather than applying quick fixes

PDCA Cycle Application

The response to this failure exemplifies the PDCA (Plan-Do-Check-Act) quality improvement cycle:

• Plan: Conduct comprehensive failure analysis to identify root causes
• Do: Implement design modifications and material upgrades
• Check: Verify through stress analysis that modifications eliminate the hazard
• Act: Update design standards and inspection procedures for similar installations

Study Insights and Reflections

This failure case underscores a fundamental principle in pressure vessel and piping engineering: meeting minimum mechanical design requirements does not guarantee serviceability in aggressive environments. The material selected for this elbow met standard tensile and toughness requirements but was not evaluated for its resistance to the specific corrosion mechanism present in the operating environment.

The systematic approach to failure analysis—combining visual examination, metallurgical investigation, stress analysis, and environmental assessment—demonstrates the value of a multidisciplinary approach to failure investigation. No single analysis method would have identified all contributing factors.

For piping engineers in the oil and gas industry, this case reinforces the importance of considering the full operating environment in design decisions, particularly for components in sour service. The cost of a design modification during the engineering phase is orders of magnitude lower than the cost of an unplanned shutdown for repair, and the safety implications of hydrogen sulfide exposure make prevention absolutely critical.