Session: MF-05-01 Fitness-For-Service and Failure Assessment

Paper Number: 63030

Start Time: Wednesday, July 14, 2021, 08:00 PM

63030 - Fracture Toughness Behavior of Carbon Steels in Mildly Sour Waters 

Corrosion and cracking of typical materials used in refinery equipment exposed to sour water has long been a concern for the industry. The failure of an amine absorber tower in 1984 heightened the need to understand the performance of these materials in sour water encountered by the industry. In the years following the absorber tower failure numerous studies were undertaken to understand the performance of C-Mn steels exposed to sour water including fracture toughness (FT) type testing in NACE A or B solutions.

 

NACE A and B solutions are significantly more aggressive solutions (lower pH and higher Hydrogen Sulphide [H2S] concentration) than most common refinery sour waters. Due to the severity of the testing solution, the results are likely overly conservative and are not ideal for integrity assessments of refinery vessels by application of API 579 Fitness for Service (FFS) assessments.

 

Following the 1984 incident, most of the research carried out into sour water environments was targeted to material susceptibility and effect of sour water on new steel. There is currently very little research or baseline data on the effect of sour water on the mechanical properties on service aged or vintage material specifications.

 

When carrying out API 579 assessment on wet H2S defects such as Hydrogen Induced Cracking (HIC), API 579 Part 7 generally passes a level 2 assessment. However, the FFS is limited by small crack lengths from API 579 Part 9 crack like flaw assessments due to the application of the lower bound crack arrest fracture toughness curve providing a potentially conservative KIR values which are a key input parameter into a crack like flaw assessment.

 

Furthermore, the presence of H2S typically will enhance the crack growth properties by promoting hydrogen generation and ingress into the steel. In the downstream industry, the application of crack growth modelling is not normally applied and to prevent further degradation a protective barrier is nominally advised to prevent known tolerable flaw sizes from growing during future operation.

 

To address these issues a testing program was conducted to evaluate the fracture properties of ex-service material and modern material in mildly sour waters that is more applicable to the downstream industry. The environmental severity was changed by adjusting the pH and H2S concentration. The effect of cyanide content and ammonia concentrations have not been addressed and the test methodology along with the results are discussed in this paper.

Presenting Author: Brandon Rollins DNV GL

Authors:

Brandon Rollins DNV GL
Oliver Tarbard ExxonMobil Research and Engineering Company
Kenneth Bagnoli ExxonMobil Research and Engineering Company
Ramgopal Thodla DNV GL