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Didion Milling Company Explosion and Fire

Overview

On May 31, 2017, multiple combustible dust explosions occurred at the Didion Milling dry corn milling facility in Cambria, Wisconsin. Five employees were fatally injured. Fourteen employees were injured. The CSB determined that ignition of combustible corn dust inside process equipment transitioned to multiple explosions and propagated through interconnected equipment and buildings.

Incident Snapshot

Field Value
Facility / Company Didion Milling, Inc.
Location Cambria, WI
Incident Date 05/31/2017
Investigation Status The CSB's final report was released on December 6, 2023.
Accident Type Combustible Dust Explosion and Fire
Final Report Release Date 12/06/2023

What Happened

  • On May 3, 2017, a mill employee submitted change request documentation to modify the Bran process to hook up North Bauermeister blower line after cyclone airlock to South Bauermeister cyclone airlock to use both Bauermeisters for bran production.
  • The change was implemented and reversed four times throughout May 2017, and was implemented for a fifth time at approximately 11:00 a.m. on May 31, 2017.
  • On the night of the incident, the two receiving cyclones downstream of the North BM and South BM were connected together such that both were sharing a transport blower and transfer line to a single receiving cyclone on 4B that fed the Six-Section Sifter.
  • On May 31, 2017, around 10:30 p.m., multiple employees throughout the mill facility noticed an unusual smell or saw smoke.
  • Smoke was first observed just outside B Mill, wafting out the northeast door of 1B, and in the center loading bay, just east of A Mill.
  • After searching for approximately 15 to 25 minutes, five employees converged in 1B and together continued to search for the smoke source.
  • Eventually, only two employees, Employee A and Employee B, remained in 1B continuing their search.
  • A smoldering nest likely developed in equipment downstream of the Bauermeister gap mills in 1B, and likely initiated the incident.
  • Either an oscillating flame front or a series of small explosions spread burning material throughout the Bran process piping on 1B and accelerated a localized smoldering nest into a deflagration.
  • An explosion downstream of the South Gap Mill in 1B occurred, which propagated through the North and South BM Cyclones and continued to propagate throughout the connected process.
  • The deflagration that began in 1B propagated to the Dry Grit Filter.
  • A deflagration in the Dry Grit Filter propagated to other previously uninvolved parts of Didion’s processes, which allowed explosions and fire to continue to spread throughout the mill processes.
  • The primary explosion in 1B and the ensuing propagations lofted fugitive dust and spread secondary explosions throughout the mill facility.
  • A primary explosion inside process equipment located in 1B initiated a secondary explosion inside the building on 1B.
  • The primary explosion inside the equipment propagated through other connected equipment, and the secondary explosion propagated through the connected air supply shafts and other openings to cause fire and structural damage to equipment and buildings in areas not associated with the equipment in 1B.
  • Immediately after the loud noise (Explosion 1), Employee A noticed dust and smoke coming out of the South BM area.
  • Employee B observed the inlet air filter pop off the product conveyance piping associated with the South BM discharge and then observed flames coming out of the South transfer line to the cyclone.
  • Employees A and B decided to leave 1B immediately.
  • Just as Employee B stepped outside, a significant explosion occurred (Explosion 2).
  • The explosion was felt throughout the facility.
  • Employee A felt fire coming from 1B up toward him from the stairwell and heard and felt multiple subsequent explosions.
  • Employee B described the Dry Grit Filter exploding as he was running past it outside the Mill buildings.
  • Before either Employee A or B could radio all channels for evacuation, multiple explosions had begun within the mill facility, which immediately led to collapse of multiple mill buildings as the explosions continued to propagate throughout the mill facility.
  • Explosion 2 is estimated to have occurred at 11:00 p.m.
  • The incident scene was found to contain several mill buildings partially or completely collapsed, evidence of prolonged fire in several parts of the facility, and several connected pieces of equipment with internal fire or explosion damage.
  • The pressure generated during the dust explosions throughout the facility resulted in the collapse of several buildings and significant damage to the remaining buildings.
  • Didion’s add-on building design employing shared walls between connected mill buildings caused multiple buildings to collapse and significant structural damage throughout the mill facility, which caused multiple fatalities and injuries.

Facility and Process Context

  • Didion Milling Inc. is a family-owned agricultural processing company.
  • Didion operates a dry corn mill facility in Cambria, Wisconsin.
  • The mill facility was initially constructed in 1991, and expansions of the facility occurred until the final building was constructed in 2012.
  • Prior to the 2017 incident, the Didion mill facility was comprised of several buildings, which were built as the facility expanded.
  • The processes were interconnected to allow for the milling of different types of materials and to meet different customer specifications.
  • A Mill and B Mill each had an air makeup shaft that ran vertically through each mill to supply air to the equipment.
  • The mill buildings were interconnected to other structures as the facility expanded, or “add-on construction.”
  • The facility handled combustible dust in milling, grinding, pneumatic conveying, dust collection, and bran production processes.
  • OSHA’s Grain Handling Facilities Standard, 29 CFR § 1910.272, applied to the facility.
  • Didion defined the milling operations as Class II, Div. 2.
  • Class II locations are defined as those that are hazardous because of the presence of combustible dust.
  • The milling operations were hazardous due to the presence of combustible dust and can present a flammable or explosible atmosphere during an upset condition.
  • Didion relied on its food safety programs to act as its combustible dust management system.
  • The requirements of the food safety programs did not address the potential hazards presented by combustible dust in the facility, nor were these programs intended to address combustible dust hazards.

Consequences

  • Five employees were fatally injured.
  • Fourteen employees were injured.
  • No off-site damage or consequences were reported following the incident.
  • The Multipurpose Building, F Mill, and the Boiler House suffered complete collapses.
  • D Mill suffered significant structural damage, rendering it inaccessible after the incident.
  • Portions of B Mill were significantly damaged, which prevented access to portions of the building.
  • A, B, and D Mill buildings were severely damaged.
  • The Multipurpose Building (including C Mill), F Mill, and the Boiler Room building were destroyed.
  • The Dry Grit Filter and its ductwork sustained fire and explosion damage.
  • The Torit Filter inlet duct ruptured at the flanged connections, indicating evidence of internal overpressure.
  • The A Mill area and Bulk Loadout were severely blackened and fire damaged.
  • Collapse of several buildings.
  • Significant damage to the remaining buildings.
  • Rupture damage to the South BM Cyclone.
  • Rupture of ducting.
  • Damage to dust collector ducting.
  • Equipment damage from fires.
  • Building collapses and widespread damage throughout the mill.
  • Multiple buildings collapsed.
  • Significant structural damage throughout the mill facility.
  • Fire and structural damage to equipment and buildings.
  • Severe damage to the baghouse.
  • Property damage.
  • Following the incident in 2017, the mill facility was reconstructed, and production started in 2019.
  • Fires, explosions, evacuations, shutdowns, and prolonged emergency response occurred.
  • The facility restarted approximately six hours after the May 29 fire before completing the incident investigation.
  • In 2019 the plant shutdown was delayed and evacuation took 20–35 minutes.
  • The mill facility experienced dust explosions and fire that spread throughout the mill processes.
  • Didion continued to operate despite knowledge of combustible dust hazards.

Key Findings

Immediate Causes

  • The CSB determined the cause of the dust explosions and collapsed buildings was the ignition of combustible corn dust inside process equipment, which transitioned to multiple explosions.
  • Smoldering material downstream of the Bauermeister gap mills caught fire.
  • The fire traveled through the process.
  • A dust collector explosion occurred.
  • Secondary dust explosions and flash fires occurred throughout the mill facility.

Contributing Factors

  • Didion’s lack of engineering controls allowed the fire and explosions to propagate through the facility uncontrolled.
  • The uncontrolled propagation of fire and explosions subsequently caused secondary explosions due to inadequate fugitive dust management.
  • Didion’s inadequate emergency preparedness failed to inform or train its employees to safely respond to a smoldering fire.
  • Didion’s lack of flame-resistant personal protective equipment could have protected employees from exposure to the flash fires.
  • Didion management’s failure to abate combustible dust hazards identified during external inspections resulted in Didion continuing to operate despite knowledge of these hazards.
  • Didion did not follow industry guidance such as NFPA 61, NFPA 91, or NFPA 654 when making modifications to pneumatic transport or dust collection system ductwork and did not ensure adequate transport velocity throughout the facility.
  • Didion had no cleaning or inspection program to remove combustible dust accumulations from dust collectors or pneumatic conveying systems.
  • Didion’s delayed implementation of Dust Hazard Analyses (DHAs) prevented the assessment of the mill processes for potential combustible dust hazards in a timely manner and implementation of safeguards that could have prevented or mitigated the severity of the incident.
  • Didion performed dust hazard analyses on the reconstructed mill, Didion did not implement deflagration controls that could have prevented the 2017 incident.
  • Didion did not provide explosion protection in accordance with the OSHA Grain Handling Facilities Standard by not installing venting that directed outside of the building or implementing explosion suppression systems on the dust collection systems.
  • Didion did not design the mill buildings to withstand overpressure events, construct its mill buildings in accordance with industry guidance to mitigate the damage during a combustible dust explosion, or install deflagration venting on the mill buildings to relieve excessive pressure from secondary explosions.
  • Didion’s Master Sanitation Schedule did not adequately identify or require maintenance of all areas at the facility to minimize the hazard of fugitive combustible dust accumulations.
  • Didion’s MOC procedure was not conducive to detecting or mitigating pneumatic conveying or dust collector hazards, or material accumulation inside ductwork, which are significant and well-recognized combustible dust hazards.
  • Didion’s incident investigation management system did not thoroughly analyze hazards, accurately determine what happened, accurately identify root causes and causal factors, properly identify preventive measures, ensure that follow-up actions were taken, or evaluate corrective action effectiveness, and therefore was ineffective.
  • Didion did not maintain adequate Process Safety Information, which contributed to Didion’s failure to identify, evaluate, and adequately address process hazards and implement adequate engineering controls and building design that could have prevented the incident or minimized its consequences.
  • Didion did not have an adequate audit and inspection program to address the findings and recommendations that were generated from these actions.
  • Didion did not provide specific instructions to employees for abnormal conditions in its emergency response plan, such as the smoldering material and unidentified smoke sources.
  • Didion did not require its employees to use flame-resistant garments for protection against potential thermal injuries from combustible dust flash fire events that contributed to the severity of the injuries in the incident.
  • The temporary change connecting the two gap mills’ transfer systems together likely contributed to the incident in part by confounding the operators’ search for the smoldering material when they were unaware the two mills’ discharges were tied together.
  • Significant combustible material deposits inside ductwork systems were present.
  • Didion did not have active deflagration isolation systems within any of the vent systems.
  • Didion did not have adequate deflagration venting systems to prevent equipment rupture that experienced overpressure.
  • Didion did not adequately design its dust collection system ducting to withstand deflagration pressures.
  • Didion’s delayed implementation of Dust Hazard Analyses (DHAs) prevented the assessment of the mill processes for potential combustible dust hazards in a timely manner.
  • Didion did not have a communication method to immediately notify all employees of upset conditions or emergency situations to trigger an evacuation.
  • Didion’s training program was inadequate to ensure the employees performed the tasks as directed by the training documentation.
  • Didion did not utilize or defer to expertise prior to the incident regarding the hazards of combustible dust within the process.
  • Didion normalized deviance regarding smoldering fires.
  • Prior to the incident, Didion had a poor safety culture and weak safety programs due to inadequate process safety leadership.
  • Didion’s inadequate safety management systems for combustible dust failed to mitigate the potential hazards of combustible dust in the process.
  • Didion inadequately managed changes to process equipment.
  • Didion failed to maintain critical safety information.
  • Didion failed to incorporate lessons learned from prior incidents.

Organizational and Systemic Factors

  • Didion’s poor safety culture and inadequate leadership on safety issues
  • Normalization of deviance regarding smoldering fires
  • A lack of deference to expertise
  • A lack of a sense of vulnerability
  • Inadequate understanding of the combustible dust and deflagration hazards present at Didion
  • Turnover of responsible management staff
  • Use of food safety standards to address combustible dust hazards
  • Didion used the changes in the deadline to the implementation of the DHAs to delay compliance with the NFPA standards.
  • Didion’s MOC program did not effectively address known combustible dust hazards and was insufficiently designed to identify or address them.
  • Didion’s MOC procedure did not contain any provision for subject matter expert reviews, such as a reviewer for combustible dust hazards.
  • Didion’s Incident Management procedure did not include tracking incident action items that resulted from the investigations.
  • Didion’s policies and procedures at the time of the incident did not appear to include a tracking system or guidance for developing incident corrective actions, or for evaluating their effectiveness.
  • Didion’s housekeeping program focused on food safety hazards and did not mention dust collectors or associated ductwork.
  • Didion’s HACCP did not contain any analyses of combustible dust hazards or scenarios.
  • Didion’s emergency response plan relied on radio communications and did not have a facility-wide alarm system.
  • Didion’s training and practices normalized process fires and smoldering events.
  • Didion did not disseminate findings and recommendations from external audits and inspections to new management employees.
  • Didion did not have an audit program that assigned responsibility to employees for follow-up on the action items.
  • Didion’s PPE procedure did not account for the potential hazards of combustible dust.

Failed Safeguards or Barrier Breakdowns

  • Didion did not use pre-deflagration detection systems that could have detected a smoldering nest or smoke.
  • Didion did not recognize that interconnecting equipment through dust collectors could present a deflagration propagation hazard and as a result did not mitigate this hazard.
  • Didion incorrectly concluded that there were no combustible dust hazards to be mitigated in its dust collection systems.
  • The mill buildings lacked deflagration vents that could relieve pressure and mitigate the damage that occurred due to the propagation of the pressure wave through the mill from secondary dust explosions.
  • The rotary airlocks beneath the North and South BM Cyclones were not designed or maintained as propagation isolation devices.
  • Didion’s dust collector calculations were incorrect and the Dry Grit Filter did contain an explosive dust concentration on the night of the incident.
  • Didion did not ensure adequate transport velocity throughout the facility.
  • Didion did not evaluate pneumatic transport or dust collection systems design after subsequent process changes.
  • Didion’s emergency response plan failed to adequately prescribe actions to be taken by employees and management during fire incidents.
  • Didion lacked a facility-wide notification system.
  • Didion did not require the use of fire-resistant clothing within the milling facility.
  • Didion did not thoroughly investigate prior incidents involving fires and smoldering events.
  • Didion did not maintain key process safety information.
  • Didion did not abate combustible dust hazards identified during external inspections.
  • Inspection and maintenance of industrial exhaust systems.
  • Periodic inspection and cleaning of dust collectors and associated ductwork.
  • Cleaning on a set frequency.
  • Measuring transport velocities on a routine basis.
  • Dust Hazard Analyses (DHAs).
  • Pre-deflagration spark detection systems.
  • Deflagration suppression systems.
  • Deflagration venting systems.
  • Deflagration isolation systems.
  • Deflagration containment.
  • Active deflagration isolation systems.
  • Rotary airlocks meeting the design and maintenance requirements of NFPA 69, chapter 12.
  • Deflagration venting directed to a safe, outside location away from platforms, means of egress, or other potentially occupied areas.
  • Pressure-resistant construction.
  • Written housekeeping program that establishes the frequency and method(s) determined best to reduce accumulations of fugitive agricultural dust.
  • Unscheduled housekeeping.
  • Management of change review for combustible dust hazards.
  • Incident investigation corrective action tracking.
  • Process Safety Information system.
  • Facility-wide alarm system.
  • Emergency shutdown procedures for the process.
  • Flame-resistant garments.

Recommendations

  1. 2017-07-I-WI-R1 | Recipient: Didion Milling, Inc. (Didion) | Status: Closed – Acceptable Action | Summary: Contract a competent third party to develop a comprehensive combustible dust process safety management system, such as OSHA’s Process Safety Management standard or the requirements in the 2019 edition of NFPA 652, Standard on the Fundamentals of Combustible Dust, Chapter 8, which includes, at a minimum, Management of Change for combustible dust; Process Safety Information management; Management of Audits and Inspections; Fugitive Dust Management; Incident Investigation; Dust Hazard Analyses; Management of Engineering Controls for combustible dust; Personal Protective Equipment; and Emergency Preparedness.
  2. 2017-07-I-WI-R2 | Recipient: Didion Milling, Inc. (Didion) | Status: Open – Acceptable Response or Alternate Response | Summary: Contract a competent third party to develop and implement modifications to the pneumatic conveying and dust collector ductwork systems in accordance with guidance such as NFPA 61, NFPA 652, and NFPA 654, to include ensuring minimum required transport velocity is maintained throughout the system and implementing a periodic inspection and testing program for pneumatic conveying and dust collector ductwork systems, following industry guidance such as NFPA 91 and FM Global guidance.
  3. 2017-07-I-WI-R3 | Recipient: Didion Milling, Inc. (Didion) | Status: Open – Acceptable Response or Alternate Response | Summary: Contract a competent third party to perform dust hazard analyses (DHAs) on all buildings and units that process combustible dust and ensure that the DHAs are revalidated at least every five years. Implement pre-deflagration containment engineering controls identified in the initial and revalidation DHA in accordance with NFPA 61.
  4. 2017-07-I-WI-R4 | Recipient: Didion Milling, Inc. (Didion) | Status: Closed – Acceptable Action | Summary: Contract a competent third party to assess and implement engineering controls for the structural design and venting requirements of the reconstructed facility to ensure they meet the requirements and guidance in NFPA 68, Standard on Explosion Protection by Deflagration Venting, for adequacy of venting capacity.
  5. 2017-07-I-WI-R5 | Recipient: Didion Milling, Inc. (Didion) | Status: Closed – Acceptable Action | Summary: Incorporate recording any paper-based process safety information into Didion’s existing electronic records management system so that the information can be reliably retained, retrieved, and analyzed in the event of a catastrophic incident.
  6. 2017-07-I-WI-R6 | Recipient: Didion Milling, Inc. (Didion) | Status: Closed – Acceptable Action | Summary: Contract a competent third party to perform personal protective equipment hazard analyses, such as those prescribed by NFPA 2113, and require appropriate flame-resistant garments for all operations that handle combustible dusts during normal and upset conditions.
  7. 2017-07-I-WI-R7 | Recipient: Didion Milling, Inc. (Didion) | Status: Open – Acceptable Response or Alternate Response | Summary: Contract a competent third party to update the facility emergency response plan and train all employees on updated emergency response plan, including a signal or alarm system; emergency shutdown procedures; instructions for when and how to trigger emergency evacuations; instructions for when to notify emergency responders for need of assistance; response to potential fire scenarios, such as smoldering fires inside equipment; and preventing firefighting of process fires inside equipment.
  8. 2017-07-I-WI-R8 | Recipient: Didion Milling, Inc. (Didion) | Status: Open – Acceptable Response or Alternate Response | Summary: Contract a competent third party to assess and update the pre-deflagration detection and suppression engineering controls, such as those discussed in Chapter 9 of the 2019 edition of NFPA 69, for adequacy to detect and alarm employees of an emergency situation, such as a smoldering fire, and trigger an evacuation.
  9. 2017-07-I-WI-R9 | Recipient: Didion Milling, Inc. (Didion) | Status: Closed – Acceptable Alternative Action | Summary: Contract a competent third party to develop and implement a process safety leadership and culture program, based on the guidance of the CCPS’s Guidelines for Auditing Process Safety Management Systems and Process Safety: Leadership from the Boardroom to the Frontline. The program should include, at a minimum, the following elements: a. A process safety policy; b. A process safety leadership and culture committee; c. Appropriate goals for process safety; d. A commitment to process safety culture; e. Leading and lagging process safety metrics; f. Process Safety Culture Assessments; and, g. Engagement with external process safety leadership and culture experts.
  10. 2017-07-I-WI-R10 | Recipient: Occupational Safety and Health Administration (OSHA) | Status: Open – Awaiting Response or Evaluation/Approval of Response | Summary: Promulgate a standard for all industries that handle combustible dust, which should be based on the requirements of current NFPA combustible dust standards, including NFPA 61, Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities, NFPA 484, Standard for Combustible Metals, NFPA 652, Standard on the Fundamentals of Combustible Dust, NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, NFPA 655, Standard for Prevention of Sulfur Fires and Explosions, NFPA 664, Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities, or a successor standard. At a minimum, the standard should include the following elements: Hazard Recognition; Dust Hazard Analysis; Management of Change; Incident Investigation; Engineering Controls; Building Design; Fugitive Dust Management; Operating Procedures; Process Safety Information; External Audit Management; Training; Emergency Response; and Personal Protective Equipment.
  11. 2017-07-I-WI-R11 | Recipient: Occupational Safety and Health Administration (OSHA) | Status: Open – Awaiting Response or Evaluation/Approval of Response | Summary: Following implementation of CSB Recommendation No. 2017-07-I-WI-R10, update the Grain Handling Facilities Standard to clarify grain handling facilities with combustible dust are covered by the new Combustible Dust Standard.
  12. 2017-07-I-WI-R12 | Recipient: Occupational Safety and Health Administration (OSHA) | Status: Open – Awaiting Response or Evaluation/Approval of Response | Summary: Develop a program to trigger follow-up inspections when hazard alert letters are issued for combustible dust hazards and there is insufficient evidence to demonstrate that those hazards have been abated.
  13. 2017-07-I-WI-R13 | Recipient: National Fire Protection Association (NFPA) | Status: Closed – Exceeds Recommended Action | Summary: Update NFPA 61, Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities, or a successor standard, to incorporate, at a minimum, the following elements: 1. Unify the requirements for performing dust hazard analyses to remove equipment exemptions and require the assessment of all processes, such as cyclones, as required in NFPA 652 and CCPS guidance. 2. Incorporate additional guidance for Management of Change, including temporary changes, operating and maintenance procedures, employee training, and dust testing results. 3. Update the requirements for incident investigation management systems to incorporate the optional guidance of NFPA 652 and CCPS incident investigation guidance.

Key Engineering Lessons

  • Ignition of combustible dust inside process equipment can transition to multiple explosions and propagate through interconnected process equipment and buildings.
  • Engineering controls for deflagration isolation, venting, suppression, and containment are needed to prevent propagation and equipment rupture.
  • Pneumatic conveying and dust collector ductwork must be designed and maintained to preserve minimum transport velocity and to prevent dust accumulation inside ductwork and collectors.
  • Dust Hazard Analyses must include all combustible dust process equipment and be revalidated periodically so safeguards can be identified and implemented before incidents occur.
  • Building structural design and venting must account for overpressure from secondary dust explosions, especially in interconnected or add-on construction.
  • Pre-deflagration detection and alarm systems are needed to warn employees of smoldering fires or other upset conditions and trigger evacuation.
  • Flame-resistant garments are needed for operations handling combustible dusts during normal and upset conditions to reduce thermal injury severity.
  • Process safety information, management of change, audit follow-up, and incident investigation systems must be robust enough to identify combustible dust hazards, track corrective actions, and prevent recurrence.

Source Notes

  • Primary source used: FINAL REPORT: Fatal Combustible Dust Explosions at Didion Milling Inc. (source_priority 1).
  • Recommendation status change summaries were used to update recommendation statuses for R1, R4, R5, R6, R9, and R13.
  • Supporting documents were used only to corroborate and enrich event sequence, process context, and engineering findings where explicitly stated.
  • Where source documents used different wording for the same fact, the highest-priority source wording was preferred.

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