(Completed: 2009)
Abstract:
In one notable case involving a propane gas-fueled residential explosion, which resulted in a 500-foot diameter debris field, an in-depth investigation was conducted to determine the exact cause and conditions that resulted in the complete structure demolition. This case study delves into the forensic investigation of the catastrophic gas explosion that occurred in Michigan in 2005.
The incident was rooted in a complex sequence of events involving flammable gas leaks, corrosion, gas migration through soil, and ignition, resulting in the complete destruction of a two-story home. The investigation ultimately uncovered that the explosion’s cause traced back to issues during the original construction of the gas system, some 16 years prior to the incident.
Overview
Residential propane gas usage is widespread in rural areas of the United States, serving as a cost-effective alternative to natural gas distribution infrastructure. A typical setup includes a 500-gallon propane tank, which supplies a residence with vapor propane through an underground line. Safety measures like odorization, multiple regulators, and leak checks are employed to prevent and detect gas leaks.
The midwestern single-family dwelling was a typical two-story house for that rural area, with a full-poured wall basement and used propane to fuel its heat, hot water, and cooking. The catastrophic incident took place in early September around 11:30 p.m., with nine family members inhabiting the residence at the time of the deadly explosion. The case presented multiple complexities, while working to test the theories and uncover the facts of this deeply unfortunate event.
The Investigation & Findings
Due to the devastating nature of the loss of life and property, the first and likely theory suggested fugitive propane. During the discovery period, the full timeline of events pertaining to the propane system was 18 pages long, noting a 1989 installation, several “out-of-gas” events, and a last inspection in 2001 – four years prior to the explosion. Forensic experts engaged on the case included mechanical engineers and materials specialists.
The 500-gallon tank situated approximately 45 feet from the east side of the residence served to deliver fuel gas through a 3/8″ underground supply line, composed of three sections connected with two ferrule compression fittings. One of these fittings was positioned 64 inches from the residence’s foundation. Upon closer inspection, gas residue was detected on the interior of the residence’s east basement wall. Calculations indicated the resulting damage could have been produced with as little as one pound of propane.
The explosion resulted from a series of factors converging over 16 years since the gas system’s installation. Key findings from the forensic analysis included:
- Gas Leak: A flammable gas leak occurred due to a damaged ferrule fitting in the propane supply line, leading to gas escaping into the soil.
- Corrosion and Gas Migration: Over time, corrosion exacerbated by sulfur compounds reacting with local soils weakened the fittings and the supply line, allowing gas to migrate through the surrounding soil.
- Gas Accumulation: Fugitive gas seeped into the basement, gradually accumulating to an explosive concentration.
- Ignition: An ignition source, possibly from the furnace or the sump pump ignited the accumulated gas, resulting in a devastating explosion.
Contributing Factors:
- Improper Installation: The use of ferrule fitting for below grade connections was a violation of the Mechanical Codes in effect at the time.
- Improper Actions Following “Out of Gas” Conditions: On more than one occasion, gas out situations required a leak check of the system – checks which were not completed. This allowed the corrosion and gas leakage to go unnoticed.
- Odor Scrubbing: In addition to creating acidic compounds, the reaction of the sulfuric odor compounds with the soil scrubbed the odorant. The occupants of the residence had no warning of the leak.
The Conclusion
Analysis of all of the appliances and the residence gas distribution system showed no evidence of mechanical failures, other than the underground ferrule fitting. Detailed examination of the ferrule revealed corrosion and consequent leakage.
Under our thorough investigation, we concluded that the buried ferrule fitting likely leaked from initial installation and had been progressively increasing. Moreover, the leak location being underground, near the foundation, led to collection within the basement with mercaptan scrubbing. We concluded the main factors that led to the final tipping point were the installation (and shut down) of the natural vent water heater, combined with the closing of windows/reduced air flow due to it being the end of summer in the Midwest, allowing for collection of gas above flammable limits. So, the fugitive gas ignition resulting in complete structural demolition was found to be a perfect storm of events occurring about 16 years prior with its original construction.