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Portable Butane Stove Explosion # 4

A college coed was using a portable butane stove to boil water and prepare meat and vegetables for her boyfriend and herself. They were in a studio apartment that they shared together and the stove was on the wood floor at the end of a studio bed. She had been cooking for about 25 minutes when strong flames emerged from the stove, causing serious burns to the coed's legs and arms. The couple immediately headed for the bathroom where they doused water on the coed's burns. When they emerged from the bathroom all flames had gone out.


Apartment Floor after the Flash Fire


Coed's Leg Burns

The evidence collected and examined included the stove, the butane canister and the cooking pot. The stove and the canister dome are shown below.


Evidence Stove


Evidence Butane Canister

From the evidence it was immediately obvious that this was an RVR incident. In other words, the butane in the canister became hot, the internal butane pressure rose, and the canister dome expanded thereby opening the Rim-Vent-Release devices. When the RVRs opened, the emerging butane gas from the RVRs was ignited by the burner flame, thereby creating a flamethrower effect.

The main question was: Why did the butane canister get hot?

First, it is important to understand that lacking any misuse or defects in the stove or canister, evaporative cooling inside the butane canister actually lowers the temperature and pressure of the butane inside the canister. Temperatures have been measured to decrease to as low as 0°C (32°F). Therefore, something had to be wrong somewhere.

It was proven that the coed was not using an oversized pot. She had also used this very same stove about 20x before and always stored the stove with the grill in the upright position. Hence, it was unlikely that the grill had been upside down (which is a design defect in the stove itself).

On a hunch, the butane canister was x-rayed and the internal tube was discovered to be pointing 180-degrees in the wrong direction, as shown below:


X-Ray of Evidence Canister

In the x-ray above, the internal tube is pointing directly away from the outer rim notch. The white arrow is pointing to the outer rim notch. The yellow arrow is pointing to the internal tube. For evaporative cooling to operate, the internal tube must point toward the notch. Hence, this was a manufacturing defect in the canister that prevented evaporative cooling and caused the heating of the butane fuel to the temperature (100°F) and pressure needed to activate the safety feature in the regulator, described next.

Another safety mechanism common in portable butane stoves is the regulator that controls the pressure of butane gas going to the stove's burner. The butane canister is inserted directly into the regulator for normal use. When the regulator senses an increase in butane pressure, another safety mechanism of the regulator is to kick-back the butane canister and thereby separate the canister from the regulator and hence turn off the flow of butane gas to the burner. If all works properly, that regulator safety mechanism turns off the flame at the burner and everyone is safe.

In this particular case, when the butane in the canister heated up (due to the inverted internal tube), the regulator did attempt to kick-back the butane canister. However, due to a copper tube positioned under the regulator activation pin the kick-back mechanism failed to operate. There was also simultaneous interference from the front up-down lever that engages the canister. Rather than moving freely as it should, the lever was rubbing against the slot and that also stopped the kick-back mechanism from working properly.


Copper Tube Interference


Front Lever Interference

An unfortunate design defect in the regulator is that when the kick-back mechanism is activated, but is prevented from working properly (by the copper tube and the front lever friction), butane gas leaks from the bottom of the regulator directly into the butane canister compartment. The animation below is an illustration of this leak mechanism.

Since the flames at the burner are not extinguished (because the butane canister was never kicked-back), the butane gas coming out of the regulator was ignited and produced a fire inside the butane canister compartment. That fire raised the temperature and pressure of the butane inside the canister to the point (about 200°F) where the canister dome expanded and the RVRs were activated. The butane gas coming out of the RVRs created the flamethrower effect that burned the young coed. The animation below attempts to illustrate these events.

The investigation lasted about four years from start to finish. Many cooking simulations were performed to determine the root cause. For example, it was experimentally determined that, at least in this case, the incorrectly positioned internal canister tube would heat the butane in the canister, but only to the point (about 100°F) of activating the regulator. That was not a high enough temperature to expand the canister dome and activate the RVRs. Something more was needed, which turned out to be the butane leak and fire from the regulator. Of course, to get the butane leak from the regulator, other defects such as the copper tube and the front lever friction were also needed. Hence, this flamethrower event was caused by a number of synergistic defects, both manufacturing and design.

The discovery of the leaking butane gas from the regulator was a breakthrough that led to the final determination of the overall cause.

The case settled just prior to trial.

Dr. Fox is a fire expert, explosion expert and chemical expert with extensive experience in OSHA, EPA and DOT chemical regulations and chemical safety.