H2S RELEASE DUE TO SULFIDATION CORROSION

On April 8, 2023, at about 7:45 a.m., hydrogen sulfide and sulfur dioxide were released from the sulfur recovery unit at the a Refinery in Minnesota . This event resulted in $2.235 million in property damage.

The accidental release resulted from a ruptured tube within the waste heat boiler of the sulfur recovery reaction furnace. The tube rupture was caused by a damage mechanism known as sulfidation corrosion (also known as high-temperature sulfidic corrosion). Under certain temperature conditions, sulfidation corrosion causes thinning in iron-containing materials over time due to a reaction between sulfur compounds and iron.

The company investigation also found fouling (hydrocarbon solids), which had formed on the outside of some tube wall surfaces, resulted in higher tube wall temperatures and areas of increased metal loss from sulfidation corrosion. The investigation also found that the hydrocarbons were in the boiler feed water. This indicates that the boiler feed water was not being effectively monitored, which allowed the hydrocarbon contamination to remain undetected.

The company reported that approximately 68 pounds of hydrogen sulfide and 999 pounds of sulfur dioxide were released.The company now monitors and controls boiler feedwater quality with in-line analyzers. The analyzers continuously sample boiler feedwater and strip any hydrocarbons from the sample. A carrier air then sweeps the hydrocarbons to a metal-oxide sensor for quantitative analysis. The in-line sample results provide the opportunity to evaluate hydrocarbon presence in near real-time. This allows for identification when boiler feedwater does not achieve American Society of Mechanical Engineers (“ASME”) recommendations based on hydrocarbon measurement and implementation of prompt corrective measures as necessary.
Probable Cause
Based on the company investigation, the CSB determined that the probable cause of the accidental hydrogen sulfide and sulfur dioxide release was a tube rupture in the waste heat boiler. The tube had thinned over time from sulfidation corrosion. Contributing to this event was hydrocarbon contamination in the boiler feed water that was not being effectively monitored and controlled, resulting in localized tube fouling that led to increased tube wall temperatures and thinning from the sulfidation corrosion damage mechanism.

Source:CSB.gov