Selected Projects

Butane Cook-stove Fuel Canister Explosion

Exponent has investigated explosions involving portable butane cook-stoves similar to the ones pictured below.  We have identified several defects that can be associated with these products and some ways the products can be misused.  In the worst-case scenario, the fuel canister overheats and ruptures, releasing a fireball that causes personal injury and property damage.  In the right-hand photo, a side-view of an incident cook-stove that suffered damage from such an explosion is shown adjacent to an undamaged exemplar.

Cook-stove Safeguards.  Although the cook-stoves are normally equipped with a number of safety devices that are intended to prevent dangerous increases in pressure inside the gas canisters, sometimes these safeguards fail.  When the fuel canister is installed into the chassis of the cook-stove, it must be “latched” into place for gas to begin flowing.  The latching process causes the mouth of the can to be “engaged” into a multi-purpose gas valve on the cook-stove that regulates the gas pressure and controls the amount of gas flowing to the burner.  The valve also contains a safety release mechanism that is intended to automatically release the chassis latch, retract the spring-loaded chassis, and disengage the mouth of the gas cartridge whenever the internal fuel pressure exceeds a safety threshold.  In testing Exponent performed on certain cook-stoves, we observed intermittent problems with the latching mechanism, and occasional failure of the chassis to retract.

Canister Safeguards.  The butane fuel canisters are also normally equipped with safety features that are intended to prevent or mitigate catastrophic releases of flammable gas.  Because the fuel canisters are installed horizontally into the cook-stove chassis, the withdrawal tube inside the can is equipped with a right-angle extension (see middle photo below) to ensure that butane vapor is withdrawn, rather than butane liquid.  To help prevent users from installing the can in the wrong orientation, the cans are equipped with a notch in the outer rim (see left and right photos below) that is intended to inhibit chassis travel and prevent can engagement if the can is rotated in the wrong orientation.  However, Exponent found that users could unknowingly defeat this “notch” safeguard and install the can in the wrong orientation, permitting butane liquid to be withdrawn instead of butane vapor.  In the United States, butane canisters of this type are required to contain an additional safeguard called “rim vent release” that is designed to relieve excessive internal pressures by opening small “vents” around the can rim allowing gas to bleed out slowly, thereby preventing a catastrophic rupture.  Exponent determined that some cans sold in the U.S. (see left and middle photos below) do not contain this safeguard and can fail catastrophically if the can is overheated.

Contact: Richard J. Martin, Ph.D., P.E. in the Los Angeles office for further information

Rotary Screw Air Compressor Explosion at Lumber Mill

Exponent was retained to investigate an explosion at a sawmill in the Pacific Northwest that caused five injuries and extensive damage. Based on damage patterns, eyewitness statements, analyses, and other evidence, Exponent concluded that the explosion occurred when a large amount of lubricating oil was expelled from a failed piping component in a 350 hp rotary screw air compressor system that had developed an internal fire.

Immediately prior to the explosion, employees observed heavy gray smoke with a strong chemical odor coming from the compressor room, but no obvious indications of fire or heat. The compressor, which was more than 30 years old, was examined after the incident and found to exhibit signs of heat damage from an internal fire. The oil reservoir, which normally contains approximately 70 gallons of lubricant, was found to be nearly empty upon inspection after the incident, and two breaches in the pressure containment were found – a blown-off oil filter-housing lid and a burned-up isolation hose. Initially, the compressor rotor assembly was immovable but, upon disassembly, the bearings were found to rotate freely and showed no major damage. The high temperature safety switch was found in an open-circuit condition, indicating that one or more safety logic elements failed to perform their intended function when the internal fire was first initiated.

Based on the evidence available, Exponent concluded that the explosion occurred as a result of the failure of fasteners on the oil filter-housing lid, which allowed a large amount of high temperature lubricating oil to escape into the compressor room and form a fuel mist, which later ignited explosively. Exponent did not analyze the role sawdust may have played as a possible contributing fuel, but the circumstances were not consistent with a “sawdust only” scenario.

At the time of this writing, there was insufficient evidence to determine whether the internal compressor fire caused the filter-housing fastener failure, or whether some undetermined defect associated with the design, manufacture, installation, or maintenance of the filter housing or fasteners led to its failure, irrespective of the internal fire. Several possible causes of the internal compressor fire were under consideration during the investigation. One scenario involved metal-to-metal contact and sparking inside the compressor’s “air end” igniting an oil reservoir fire, and another scenario involved autoignition of lubricant due to inadequate coolant flow or heat rejection. The evidence was nevertheless conclusive that the internal compressor fire preceded the oil filter housing failure, and that the internal fire was not caused by external flames being sucked into the compressor’s air intake.

Contact: Richard J. Martin, Ph.D., P.E. in the Los Angeles office for further information

Hair Care Product Injury Fire

Exponent was retained by a major insurance company to investigate an injury that occurred when a woman’s hair caught fire upon coming into contact with a candle flame, approximately one hour after she had applied a hair care product. We were asked to evaluate whether the product was flammable or combustible; whether the product’s flammability characteristics could change when applied to hair; whether warnings about flammability should have been included on the product’s packaging, and whether a chemical engineering consultant hired by the injured woman’s attorney had used an excessive amount of the product in videotaped burn tests he performed.

Exponent found that the flash point of the subject hair care product is greater than 230ºF and that application of the product to hair decreases its flash point by less than 1ºF. According to the Federal Hazardous Substances Act (FHSA), materials with flash points between 20ºF and 100ºF are considered flammable, and materials with flash points between 100ºF and 150ºF are considered combustible, so the product in question did not fall into a category wherein flammability or combustibility warnings were required.

Exponent also found that the plaintiff’s expert applied an excessive amount of the product to his hair samples, and his conclusions were skewed accordingly. The amount he applied was equivalent to the use of slightly less than one full 4 oz. bottle (approximately 550 drops) of the product on a full head of woman’s hair. When a moderate amount (the equivalent of 14 drops to a full head of hair) was applied to the hair samples used in Exponent’s tests, the treated hair released only a modestly greater amount of heat than the untreated hair. The untreated hair burned quite strongly without any product applied in both the tests conducted by both Exponent and the plaintiff’s expert (see figure above).

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