A high pressure gas cooler located in an offshore platform have been operating for more than 10 years. Throughout that period the gas cooler have been subjected to several tube failures, the failures have caused gas leak from the tube side. Several materials upgrades have been used to contain the tube failure. The last choice was to use a more corrosion resistant material Hastelloy C22. Crevice corrosion has been reported as the primary failure mechanism. The tube and tube plate joined surfaces have been exposed to high temperature which is relatively higher than the critical crevice temperature of Hastelloy C22. There was a poor heat transfer between the shell side fluid and tube side due to a small heat transfer area and low fluid velocity in the affected zone. Stress corrosion and fatigue corrosion accounted for the secondary failure mechanism which ultimately caused a crack in the tubes. Other possible reason identified to cause crevice corrosion was the mechanical rolling expansion technique. Oftentimes it creates rear crevices on the tubes with enough geometry to develop crevice chemistry. Mitigation methods such as hydraulic expansion technique, heat treatment for residual stresses and baffle design enhancements have been proposed in this report.
Key words: shell and tube heat exchanger failure; Hastelloy C22 material; tube failure; localized corrosion; crevice corrosion; internal pitting corrosion
Table of Contents
1. Introduction
1.1 Methodology
2. Failure Investigation
2.1 Material description
2.2 Service condition
2.3 Localized corrosion
2.4 Effect of rolling expansion
2.5 Stress corrosion
2.6 Corrosion fatigue
2.7 Effects of temperature
3. Discussion
3.1 Causes of crevice corrosion
3.2 Crevice corrosion mechanism
3.3 Stream temperature effects
3.4 Causes of pitting
3.5 Crack initiation
4. Conclusion
5. Recommendation to mitigate failure
5.1 Tube assembly
5.2 Residual stresses
5.1 Temperature and baffle design
Objectives & Core Themes
This technical report investigates the recurring tube leak failures of a high-pressure gas cooler on an offshore platform, aiming to identify the root causes of material degradation in Hastelloy C22 tubes and propose effective mitigation strategies.
- Analysis of material failure mechanisms in offshore heat exchangers.
- Evaluation of the impact of mechanical rolling expansion techniques on crevice formation.
- Assessment of temperature effects and thermal stress on corrosion initiation.
- Investigation into the synergistic effects of crevice, stress, and fatigue corrosion.
- Development of engineering recommendations for improved tube assembly and design.
Extract from the Book
3.2. Crevice corrosion mechanism
Crevice corrosion occurs within the occluded regions of a metal to metal surface (tubes to tube plate), and requires the presence of a continuous liquid film connecting the occluded region to the external bulk material. Within this region there is a volume of stagnant liquid inside a crevice with enough geometry to allow liquid entry but narrow enough to maintain a stagnant layer. The initiation of crevice corrosion involves the local depletion of oxygen in the crevice, a spatial separation of the anode from the cathode, a metal dissolution and the hydrolysis of dissolved metal ions and the accompanying acidification of the creviced region (Mon, Gordon and Rebak, 2005). Within the crevice the pH is low whereas there is a high concentration of chloride ions that neutralizes the charge imbalance promoting a local breakdown of passive films which leads to an increased metal dissolution rate.
Summary of Chapters
1. Introduction: Presents the operational context of the high-pressure gas cooler and outlines the methodology used to analyze the tube failures.
2. Failure Investigation: Details the material specifications, service conditions, and identifies the primary corrosion mechanisms including localized corrosion, stress corrosion, and fatigue.
3. Discussion: Examines the technical root causes, specifically linking the rolling expansion method and temperature differentials to crevice formation and crack initiation.
4. Conclusion: Synthesizes the findings, confirming that crevice corrosion exacerbated by design and manufacturing techniques is the primary cause of failure.
5. Recommendation to mitigate failure: Proposes engineering solutions such as hydraulic expansion, stress relief heat treatments, and baffle design enhancements to prevent future failures.
Keywords
Shell and tube heat exchanger, Hastelloy C22, tube failure, localized corrosion, crevice corrosion, internal pitting corrosion, mechanical rolling expansion, stress corrosion cracking, corrosion fatigue, offshore platform, residual stresses, baffle design, hydraulic expansion, heat transfer, metallurgical analysis.
Frequently Asked Questions
What is the primary scope of this investigation?
This report focuses on analyzing the recurring tube leak failures of a high-pressure shell and tube heat exchanger operating on an offshore platform, specifically addressing material degradation over a ten-year service life.
What are the central themes of this technical report?
The report centers on material science and corrosion engineering, specifically examining localized corrosion, the effects of tube expansion methods, thermal stress, and the performance of Hastelloy C22 in offshore environments.
What is the main objective of the study?
The primary goal is to determine the underlying causes of the tube failures and to provide actionable engineering recommendations to prevent future corrosion and structural integrity loss.
Which scientific methods are applied in the report?
The study utilizes defect assessment through eddy current testing, analysis of photographic evidence from the field, and a literature-based comparative analysis of corrosion mechanisms and metallurgical properties.
What topics are covered in the main body of the work?
The main body covers the detailed failure investigation including service conditions, the impact of rolling expansion, crevice corrosion mechanisms, stream temperature effects, and the initiation of cracks.
Which keywords define this report?
Key terms include Hastelloy C22, crevice corrosion, shell and tube heat exchanger, tube failure, mechanical rolling expansion, and stress corrosion.
Why is Hastelloy C22 susceptible to failure in this specific setup?
While Hastelloy C22 is highly corrosion-resistant, the report identifies that the operating environment's temperatures exceeded the critical crevice temperature (CCT) of the material, which, combined with geometry-induced stagnation, promoted crevice chemistry.
What role does the "mechanical rolling expansion" technique play?
The investigation concludes that the rolling expansion process creates physical gaps and residual stresses that serve as ideal sites for crevice corrosion, thereby acting as a root cause for the tube failures.
What does the author suggest to mitigate these issues?
The author recommends transitioning to hydraulic expansion for better joint tightness, implementing solution annealing to relieve residual stresses, and replacing single segmental baffles with helical designs to reduce dead zones and vibration.
- Arbeit zitieren
- Ataliba Miguel (Autor:in), 2013, Heat exchanger failure investigation report, München, GRIN Verlag, https://www.grin.com/document/269681
