NACE SP0296

Abstract

Scope: General  This standard is intended to be a primary source of information on cracking in wet H2S petroleum refinery environments and provides guidelines on the detection, repair, and mitigation of cracking of existing carbon steel refinery equipment in wet H2S environments.  For the purposes of this standard, the term equipment refers to pressure vessels and piping made of carbon steel plate material. Refinery pressure vessels include items such as, but not limited to, columns or towers, heat exchangers, drums, reboilers, and separators.  Limited cracking has been noted in seamless piping; therefore, the information in this standard concentrates on longitudinally seam-welded pipe fabricated from plate.  Information on fabrication and inspection practices for new pressure vessels (never in service) is in NACE Publication 8X194.  For the purposes of this standard, the term wet H2S environments includes, but is not limited to, refinery process environments known to cause wet H2S cracking resulting from hydrogen entry into the steel, as defined in NACE Standard MR0103.4 Some environmental conditions known to cause wet H2S cracking are those containing an aqueous phase and: (a) > 50 ppmw total sulfide content in the aqueous phase; or (b) ≥ 1 ppmw total sulfide content in the aqueous phase and pH < 4; or (c) ≥ 1 ppmw total sulfide content and ≥ 20 ppmw free cyanide in the aqueous phase and pH > 7.6; or (d) > 0.3 kPa absolute (0.05 psia) partial pressure H2S in the gas phase associated with the aqueous phase of a process. However, the threshold total sulfide content in the aqueous phase required for cracking to occur has not been clearly established. Therefore, selective application of this standard may be appropriate when experience has indicated the presence of cracking or blistering in comparable service, regardless of total sulfide content. Alkaline environments such as alkanolamine solutions that contain sulfides and carbonate-containing sour waters also are included in the term wet H2S environments and thus are within the scope of this standard. Two forms of alkaline stress corrosion cracking (ASCC) are commonly found in these alkaline wet H2S environments. Amine stress corrosion cracking (commonly referred to as amine cracking) can occur in amine service under certain conditions, which are discussed in API(1) RP 945.5 Alkaline carbonate stress corrosion cracking (commonly referred to as carbonate cracking) can occur in alkaline carbonate-containing sour waters under certain conditions. NACE Publication 341086 describes where carbonate cracking has occurred in process equipment in petroleum refining service, the refining community’s current theory(ies) on the conditions and mitigation techniques that may have an impact on this type of cracking, and analytical and inspection techniques that have been used to address the issue.  Increased industry attention to the potential for cracking of carbon steel pressure vessels began in 1984 with the rupture of a monoethanolamine (MEA) absorber tower at a Lemont, Illinois refinery. The ensuing explosion and fire resulted in fatalities and extensive damage to the facility.7 In response to this incident, NACE Task Group T-8-14, “Stress Corrosion Cracking of Carbon Steel in Amine Solutions,” was formed in the fall of 1984. An industry survey to determine the nature and extent of the cracking problem was conducted by T-8-14. The results of the T-8-14 effort have been reported separately.8 In 1988, some new results on vessel inspections and the cracking found were reported to the industry.9 Among the significant findings was the observation that cracking problems were occurring in other wet H2S environments, not just in MEA. It was further reported that inspection techniques commonly used at the time (visual, liquid penetrant, and dry magnetic particle testing) were not sensitive enough to find these cracks. In response to this new information, NACE Task Group T-8-16, “Cracking in Wet H2S Environments,” was formed in the spring of 1988. Work Group T-8-16a conducted a survey of cracking experiences in wet H2S environments to better identify the extent of the problem. Appendix B (nonmandatory) summarizes the 1990 T-8-16a survey findings