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The Investigation of Explosions Involving Fuel Gases James H. Shanley, Jr. , CFEI, CFII
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James H. Shanley, Jr. , CFEI, CFII is a Fire Protection Engineer who specializes in the investigation of fires and explosions. He works for The Travelers Engineering Laboratory as a Senior Engineering Specialist. Mr. Shanley has 26 years of experience as a volunteer fire fighter and currently serves as the Fire Investigator for the Pingree Grove Fire Protection District, is the President of the Rural Kane Fire Investigation Unit, and is an Investigative Team Leader for the Kane County (IL) Fire Investigation Task Force. Mr. Shanley has been a member of NAFI since 1992 and a member of the Board of Directors since 1993. Fuel gases are used in many different occupancies, usually for the purpose of providing heat in some form. Fuel gases are generally either methane based which is called Natural Gas, or propane based which is called Liquid Petroleum (LP). Natural gas is usually provided as a gas to the end user via underground piping by a utility, whereas LP is usually stored on site as a liquid in aboveground tanks. The equipment, which utilizes these gases, includes everything from small space heaters to large industrial ovens. This equipment along with the supply apparatus to them has the potential to leak fuel gas to the environment, which in the presence of a competent ignition source can be ignited. As with all ignitable gases natural gas and LP are only ignitable in a specific concentration with air. The flammable limits for methane are 5% to 15%. The flammable limits for propane are 2.4% to 11%. These limits will change (widen) with increasing oxygen concentration and with increasing pressure. If there is a sufficient quantity of fuel gas and it is close to the middle of the flammable limits the combustion of the gas may be rapid enough to cause an explosion. An explosion is evidenced by a rapid rise in pressure, which can cause damage to structural elements, equipment and people greater than that expected from a fire. In certain circumstances (fuel rich) a following fire may occur after the initial explosion. In this case there was sufficient excess fuel after the explosion that a fire could continue. The release of fuel gas outside of the supply or utilization equipment (fugitive gas) may be the result of malfunction, material failure, excavation, poor or improper maintenance, or deliberate action. The goal of the investigation is to determine the mechanism for the release. In most cases it will be difficult to determine with certainty the ignition source of the fuel gas. This is because there are often several possible, competent ignition sources present, leaking gases may travel some distance before being ignited, and little or no evidence may remain at the ignition source to indicate its involvement. Ignitable gas may be produced by other sources at the scene. This includes organic decomposition, chemical reactions, fire, spilling of ignitable liquids, and heating of ignitable liquids. Methane gas generated by septic or sewer systems may enter a structure via plumbing or holes in the foundation. This is commonly called sewer gas or swamp gas. Because methane and propane are odorless, gas utilities are required to add chemical odorants to these fuels prior to distribution. These odorants may not be present or detected at the source of the leak for a variety of reasons. Just because a witness claims to not have smelled gas does not preclude a gas leak. KEY INVESTIGATIVE POINTS: • Evidence at an explosion scene may be widely dispersed as a result of the explosion. Take early and affirmative action to secure the scene to prevent movement, altering or removal of evidence. Hiring a local investigator to get to the scene as soon as possible is often fully justified. • Gas utility representatives and equipment often arrive at the scene of a fuel gas explosion while the fire department is still present. It is often their standard operating procedure to dig up gas lines, close valves and remove equipment. This is done primarily for public safety but also helps to protect the interests and public image of the utility. Make official contact with the utility as soon as possible and advise them of your need for them to preserve any and all evidence removed from the scene. This should be done in writing as well, preferably by an attorney. • Once on the scene evaluate the need for heavy equipment and extra resources. Explosion scenes often have large amounts of debris, are structurally compromised, and must be made reasonably safe. • Assessment of the damage may give an indication of where the fugitive gas was located when it was ignited. For example in the case of a 2-story house with a basement, if the ground and floor above are heavily damaged with debris scattered around them but the basement is intact with debris blown into it from above, then this indicates that the gas was likely not present in the basement. • Interviews of victims and witnesses may give an indication of where the fugitive gas was located when it was ignited. Take steps to canvas the area and neighborhood to identify and interview these people. If witnesses have smelled gas it is important to identify when, where and for how long they had done so. • Canvas the neighborhood to identify indicators of other gas leaks. This includes dead vegetation and excavated areas. Simultaneous but separate gas leaks are possible. • If it is suspected that the lack of odorant has a role in the cause of the explosion then take steps to secure a sample of the gas from as close as possible to the explosion scene. This requires specialized equipment and procedures. • Significant investigative effort should be directed at identifying the fuel for the explosion and its source. Other explosive fuels should be identified, considered and eliminated if possible. This includes explosives, fireworks, ammunition, chemicals, carbon monoxide (backdraft), pressurized cylinders, welding gases, etc. • Determine the source and equipment used for the supply of all fuel gas equipment in the building. Document this equipment and examine it for evidence of leakage. Documentation should include the position and status of all controls and valves. In most cases it will be necessary to preserve all or most of this equipment as evidence. Have adequate resources for this. • If it is necessary to cut fuel gas piping for purpose of preservation, do so away from fittings. • Fuel gas utilization equipment is often heavily damaged by an explosion and any resulting fire. As a result it is often difficult to examine this equipment at the scene. It should be preserved for later laboratory examination. Locating exemplar equipment is often very useful. • Interview people familiar with the building and its operation to determine the status of the gas and gas equipment at the time of the explosion. Note any discrepancies. • Determine the security of the building at the time of the incident and document all indicators of this status. Determine who has access to the building. • Obtain all security and fire alarm records for at least the previous 24 hours. • Continuously monitor the scene for conditions, which may make it unsafe. REFERENCES: NFPA 921, Guide for the Investigation of Fires and Explosions, 2001 edition
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