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ConAgra Natural Gas Explosion and Ammonia Release

Overview

On June 9, 2009, a natural gas explosion occurred at the ConAgra Slim Jim facility in Garner, North Carolina, during installation and purging of gas piping for a new gas-fired industrial water heater. Purged gas was vented indoors into a utility room, accumulated to a flammable concentration, and ignited. The explosion killed four workers, injured dozens more, and caused major structural damage and an ammonia release from the refrigeration system.

Incident Snapshot

Field Value
Facility / Company ConAgra Foods
Location Garner, NC
Incident Date 06/09/2009
Investigation Status Completed
Accident Type Flammable Vapor Investigation
Final Report Release Date 02/04/2010

What Happened

  • A new gas-fired industrial water heater was being installed and commissioned at the ConAgra Slim Jim facility in Garner, North Carolina.
  • A new three-inch steel gas line was tied into the existing six-inch natural gas supply line on the roof and ran to the utility room.
  • After installation, the new piping and existing gas supply line were pressure-tested with air.
  • On the day of the accident, an ESA worker attempted to purge the new gas piping by using natural gas to displace the air.
  • Purged gases were vented indoors into the centrally located utility room / vacuum pump room, intermittently over approximately two-and-a-half hours.
  • ConAgra did not have a uniform procedure for gas line purging and did not require the purged gases to be vented outdoors.
  • Nonessential personnel were not evacuated or instructed to leave the area, and over 200 people were in the building.
  • No appropriate combustible gas detectors were used, and personnel relied primarily on the sense of smell.
  • Natural gas accumulated in the building, exceeded the lower explosive limit, found an ignition source, and exploded at approximately 11:25 a.m.
  • The explosion caused major structural damage and ruptured ammonia refrigeration lines, releasing ammonia.

Facility and Process Context

  • ConAgra Slim Jim production facility / manufacturing plant in Garner, North Carolina.
  • Installation and commissioning of a new fuel gas-fired industrial water heater.
  • New gas piping tied into the building's natural gas distribution system and routed from the roof to an interior utility room / pump room.
  • The utility room was centrally located and contained numerous potential ignition sources, including unclassified electrical devices.
  • The plant's refrigeration system contained anhydrous ammonia.

Consequences

  • Fatalities: 4
  • Injuries: 67
  • Environmental release: Approximately 18,000 pounds of ammonia were released to the environment; ammonia was detected in discharged waters and in air above the release area.
  • Facility damage: Serious structural damage to the approximately 87,000 square foot south packaging and warehouse area, including wall and roof collapse; about 37% of the roof area collapsed and about 60% was collapsed or unstable.
  • Operational impact: Plant operations were suspended; the facility suffered major damage and several hundred employees were later laid off when operations resumed.

Key Findings

Immediate Causes

  • Purged fuel gas was vented indoors into the utility room.
  • A dangerous accumulation of natural gas occurred in the building, exceeding the lower explosive limit.
  • The accumulated natural gas found an ignition source and exploded.

Contributing Factors

  • No assessment was made of the adequacy of ventilation in comparison to the rate of gas release.
  • No appropriate combustible gas detectors were used to warn of a potential accumulation of gas in the building.
  • Personnel relied primarily on the sense of smell to determine whether an unsafe release of natural gas had occurred.
  • Some employees smelled gas while others did not.
  • The vicinity of the utility room contained numerous potential ignition sources, including unclassified electrical devices.
  • Nonessential personnel were not aware of the water heater start-up or instructed to leave the plant during the gas purging activity.
  • Purging occurred intermittently over a period of up to two-and-a-half hours.
  • Difficulties in lighting the water heater contributed to the prolonged purging.
  • Odor fade and odor fatigue made odor-based detection unreliable.

Organizational and Systemic Factors

  • ESA reported that it was the company's normal practice to purge fuel gas piping directly into the room or area when installing gas-fired equipment.
  • ConAgra did not have a uniform procedure for gas line purging.
  • ConAgra did not require ESA to vent the purged gases to the outdoors.
  • Current codes at the time did not explicitly require purged gases to be safely vented outdoors, define adequate ventilation or hazardous conditions, require evacuation of nonessential personnel, or require combustible gas detectors.
  • The National Fuel Gas Code language was described as leaving the technician responsible for determining when flammable gas had reached its lower explosive limit.
  • ConAgra established new gas purging procedures after the incident.

Failed Safeguards or Barrier Breakdowns

  • Purged gases were not directly vented to a safe location outdoors.
  • Nonessential personnel were not evacuated.
  • All ignition sources were not controlled or eliminated.
  • Ventilation was not assessed as adequate to maintain the gas concentration well below the lower explosive limit.
  • No appropriate combustible gas detectors were used.
  • Personnel relied on odor alone.
  • ConAgra did not have a uniform procedure for gas line purging.
  • ConAgra did not require ESA to vent the purged gases to the outdoors.

Recommendations

  1. 2009-12-I-NC-UR1 — Recipient: National Fire Protection Association (NFPA), the American Gas Association (AGA) and the Chair of the NFPA 54/ANSI Z223.1 Committee — Status: Not specified — Summary: Enact a Tentative Interim Amendment and permanent changes to the National Fuel Gas Code to require direct venting of purged fuel gases to a safe location outdoors; allow indoor purging only with approval of a documented risk evaluation and hazard control plan; require evacuation of nonessential personnel, adequate ventilation, control or elimination of ignition sources, continuous combustible gas monitoring, and training on odor fade, odor fatigue, and the danger of relying on odor alone.
  2. 2009-12-I-NC-UR2 — Recipient: International Code Council (ICC) and the Chair of the International Fuel Gas Code Committee — Status: Not specified — Summary: Incorporate the revised gas purging provisions of the National Fuel Gas Code into the International Fuel Gas Code.
  3. 1 — Recipient: National Fire Protection Association and the American Gas Association — Status: urgent recommendations — Summary: Enact a tentative interim amendment and permanent changes to the National Fuel Gas Code to require outdoor venting of purged gases, approval and safeguards for any indoor purging, continuous combustible gas monitoring, and training against reliance on odor alone.
  4. 2 — Recipient: International Code Council — Status: urgent recommendations — Summary: Adopt the provisions recommended to the NFPA.

Key Engineering Lessons

  • Purging fuel gas piping should be directly vented to a safe location outdoors whenever practicable.
  • If indoor purging is unavoidable, nonessential personnel should be evacuated and ignition sources controlled or eliminated.
  • Ventilation must be evaluated against the actual gas release rate and maintained at a safe level well below the lower explosive limit.
  • Combustible gas detectors should be used to continuously monitor gas concentration during purging.
  • Odor alone is not a reliable indicator of fuel gas release because odor fade and odor fatigue can occur.

Source Notes

  • Priority 1 final report facts were used to resolve conflicts where later supporting documents differed.
  • The final report states 3 workers were crushed to death and 71 people were hospitalized, while other documents state 4 fatalities and 67 injuries; the incident metadata and higher-priority final report support 4 fatalities and 67 injuries in the consolidated dataset.
  • Some supporting documents describe the utility room as a vacuum pump room or pump room; these are preserved as source terminology where relevant.
  • The final report and recommendation documents consistently identify the event as a natural gas explosion caused by indoor purging of gas piping and a subsequent ammonia release from the refrigeration system.

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