In this episode of the DustSafetyScience Podcast, we go over a case study of a sawdust and wood chip storage silo explosion, where insufficient venting resulted in a fatality.
This study was taken from a paper called Silo explosion from smoldering combustion: A case study by Dr. Paola Russo and her colleagues. It was also covered by Enrico Danzi during his presentation at the 2020 Digital Dust Safety Conference. In this episode, we’ll talk about:
- The type of facility
- The silo layout
- The incident sequence
- The analysis completed by Dr. Russo and her colleagues
- Some key lessons from the disaster
Facility Overview and Silo Layout
The incident occurred at a carpentry facility. It had two storage silos, each one with a lower section that was 12.8 meters tall and five meters in diameter and an upper part that was 9.3 meters tall and six meters in diameter. The silos were metal shell construction and bolted in place around the outside. A bag house and filter system installed on top of the silos. Inside the silos, a large vertical screw drew sawdust and wood chips down into the rotary valve and discharged these materials for use at an onsite thermal power plant.
The Incident Sequence
Around 7:00 a.m. on the day of the explosion, workers noticed smoke and flames emanating from the bottom of a silo and called the fire brigade. They switched off the ventilation system, applied water jets to the silo exterior to cool it down, and attempted to douse the fire through a manhole at the top.
When a manhole at the bottom was opened to discharge the burned material, it allegedly caused a chimney effect, which drew oxygen through the top manhole down to the bottom and encouraged the smoldering combustion inside. The result was an explosion that ripped open the bolted plates and blew off the silo roof.
Four firefighters were injured. One of them fell off of a four-meter-high access platform and later died in the hospital from his injuries.
The Incident Analysis
Russo and her colleagues used computer modelling of four different scenarios to try to determine what happened in terms of how strong the explosion was and what the likely cause had been. Their conclusion was that pyrolysis gases from the smoldering combustion built up in the upper section of the silo, ignited, and caused the explosion.
In his presentation at the Dust Safety Conference, Enrico Danzi said that while a dust explosion was possible, it was not likely because the material was really compact at the bottom two meters of the silo. He also mentioned that it could have been a hybrid explosion consisting of pyrolysis gases and dust.
The ignition source was believed to be the vertical screw inside the silo. It snapped and heated up during operation and caused the fire spotted by the workers. Later once the pyrolysis gases had built up in the headspace, they were ignited causing the explosion.
Deflagration vents were located in a ring around the upper part of the silo, but bag filters had been installed in front of them, reducing their venting efficiency. In addition to that, the metal bolts on the bolted sections of the tank had quite a bit of corrosion on them, making them fracture under the stress of the explosion. The end result was that the entire top-section of the silo blew off instead of the deflagration venting through the relief panels.
Lessons Learned
There are four key lessons learned from this explosion.
The first is that there was a lack of prevention and protection measures in the silos. In other words, no temperature sensor cable, gas detector, water foam sprinkler systems, or gas inerting systems. All of these could have potentially stopped the fire or deflagration.
The second issue involved equipment maintenance. In particular, the tank bolts and plates had not been inspected for corrosion. A similar event occurred in March 2018, when a coal silo in New Mexico collapsed due to corrosion and a faulty weld, causing an explosion and $20 million in damage.
The third issue deals with firefighting efforts. This area was discussed at length in Episode #44. In his report Silo Fires: Fire extinguishing and preventive and preparatory measures, Henry Persson described the recommended way to attack a silo fire, which consists of 13 steps:
- Identify the silo and fire types.
- Carry out an initial risk assessment.
- Consider the risk of a gas or dust explosion.
- Close the silo to minimize any air entrainment.
- Requisition nitrogen gas and equipment.
- Inject nitrogen from the bottom of the silo.
- Assemble gas-measuring equipment.
- Apply foam to the headspace if necessary.
- Inert the silo.
- Discharge the material once the fire is under control.
- Go slowly.
- Sort discharged material as it comes out.
- Keep injecting the gas during discharge.
This strategy minimizes the risk of a devastating dust or gas explosion and can help ensure that everyone goes home safely afterward.
The fourth lesson that should be taken from this tragedy, is that deflagration venting must be designed appropriately in order to function safely. Obstructions in front of the vent increases the area needed to relieve the explosion pressure. Furthermore, reduced strength of the vessel containing the explosion can cause it to rupture instead of deflagration being safely vented. In this case, that ended up being a deadly mistake.
Conclusion
If a fire occurs at a silo in your facility, it’s critical to know the best way to approach it and review those steps with incoming firefighters. This way, you stand the best chance of saving your product, keeping the silo intact and, most important of all, preventing injury to workers and first responders.
If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.
Resources Mentioned
DustSafetyScience
Combustible Dust Incident Database
DustSafetyScience Podcast
Questions from the Community
2020 Digital Dust Safety Conference
Publications
Persson, Henry. Silo Fires: Fire extinguishing and preventive and preparatory measures
Russo, Paola, Armando De Rosa Michele Mazzaro. Silo explosion from smoldering combustion: A case study.
Events
Faulty Weld Cause of Power Plant Explosion in Waterflow, New Mexico
Previous Podcasts
DSS044: Preventing Storage Silo Explosions During Firefighting Efforts
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Download the Episode
DSS074: Case Study – Insufficient Venting During Sawdust Silo Explosion Leads to Fatality