General Major Incident # 4: Silver Eagle Refinery Flash Fire and Explosion

 1. Incident Location:

On November 4, 2009, an explosion occurred in the Mobil Distillate Dewaxing Unit (MDDW) of the Silver Eagle refinery, located in Wood Cross, Utah.

 2. Incident Description:

A massive explosion and fire at the Silver Eagle Refinery on November 4, 2009, in Woods Cross, Utah, which damaged homes in a nearby neighborhood, which was caused by a rupture in a pipe that had become dangerously thin from corrosion.

A catastrophic failure of a 10-inch pipe happened which contained more than 600 lbs of pressurized hydrogen. Sulfur compounds in the process stream corroded a steel piping segment causing the pipe walls to become severely thin. A Texas-based engineering company showed the pipe segment that failed was put into service in 1993 and was at the bottom of a reactor in the distillate dewaxing unit. The rupture sent a tremendous blast across the refinery into a nearby subdivision but did not result in any serious injuries.

3. Links to the photos or videos: 

https://www.csb.gov/videos/silver-eagle-refinery-explosion-surveillance-footage-/

https://insights.globalspec.com/images/assets/826/12826/refinery.jpg

 4. Root Cause of Incident:

 The accident occurs because Sulfidation corrosion is a known problem in oil refinery process streams operating between 450 ° F and 1000 ° F where there is sulfur or H2S.

• Low-alloy carbon steel with low silicon, chromium and molybdenum levels are particularly vulnerable. In the process stream, the pipe metal surface reaction with sulfur compounds results in the formation of a sulfide scale.

• On susceptible components, this scale may be non-adherent and, particularly under turbulent flow conditions, leads to significant wall thinning.

• While additional analysis, such as characterization of the surface scale still present on the incident piping components, may further confirm the presence of sulfide scale, the evidence is currently consistent with sulfidation corrosion as the primary cause of the wall thinning in the 10-inch piping that resulted in the rupture.
• Turbulent flow at the elbow likely caused the most significant wall thinning at the rupture location just downstream of the elbow.

• Also, It is very difficult to inspect for HTHA because the damage might not be detected; it can be microscopic and may be present only in small localized areas of equipment. In addition, equipment must already be damaged by HTHA for equipment inspection to identify HTHA

 5. Financial Loss:

 The refinery was fined more than $1 million by Utah regulators for equipment and safety violations.

6. Injury or Fatality:

 Seriously Injured 4 people.

 7. Environmental Impact:

 Obnoxious odor has subsided after a lid blew on a tank at the Silver Eagle refinery.

• In the neighborhood just east of the refinery, pieces of insulation rained down on homeowners' yards

 8. Company Safety Program:

 Process Hazard Analyses (PHAs) at the refinery was present but repeatedly failed to ensure that these hazards were controlled.

 9. Was there any missing inspection or reliability practice that has led to incident? 

• Yes, did not monitor actual operating conditions of the B and E heat exchangers within the NHT heat exchanger banks, even though it would have been technically feasible to do so.

• External corrosion experts repeatedly and erroneously assumed that heat exchanger design conditions were representative of actual process operating conditions despite knowing that these heat exchangers experienced severe heat transfer performance deterioration and required frequent cleaning.

 10. How this incident could have been avoided?

If silver eagle performed rigorous analyses of mechanisms during the PHA process they could have identified the significant hazards of HTHA and sulfidation corrosion, respectively

11. What were the main recommendations of the CSB to solve the problem?

Main CSB recommendations include

• Clearly establish the minimum necessary “shall” requirements to prevent HTHA equipment failures using a format such as that used in ANSI/AIHA Z10-2012, Occupational Health and Safety Management Systems.

• Require verification of actual operating conditions to confirm that material of construction selection prevents HTHA equipment failure.

• Prohibit the use of carbon steel in processes that operate above 400 ºF and greater than 50 psia hydrogen partial pressure.