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 pressure vessels in wet H₂S environments. Refinery pressure vessels include items such as, but not limited to, columns, heat exchangers, drums, reboilers, and separators. All repairs should be conducted in accordance with API⁽¹⁾ 5103 or another applicable industry code or standard. Limited cracking has been noted in piping and therefore, piping is not included in the scope of this standard. Information on inspection practices for new pressure vessels (never in service) is given in NACE Publication 8x194.

For the purposes of this standard, wet H₂S service is defined as refinery process environments containing free water as a liquid phase and in which

(a) the concentration of H₂S is >50 mg/L (50 ppmw) in the free water, or

(b) free water pH is <4 with some dissolved H₂S, or

(c) free water pH is >7.6 with at least 20 mg/L (20 ppmw) dissolved hydrogen cyanide (HCN) in the water with some dissolved H₂S, or

(d) >0.0003 MPa abs (0.05 psia) partial pressure of H₂S is present in the gas in processes with a gas phase.

However, the threshold concentration of H₂S in the aqueous phase required for cracking to occur has not been clearly established. Therefore, selective application of this standard may be appropriate when past experience has indicated the presence of cracking or blistering in comparable service, regardless of H₂S concentration.

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 fa~ility.~ In response to this incident, NACE Task Group T-8-14 on 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. Appendix B presents a summary of the survey findings. The results of the T-8-14 effort have been reported separately.

In 1988, some new results on vessel inspections and the cracking found were reported to the industry. 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 on Cracking in Wet H₂S Environments was formed in the spring of 1988.

⁽¹⁾American Petroleum Institute (API), 1220 L St. NW, Washington, DC 20005