The H2Incidents database contains three incidents in which excess hydrogen was unintentionally introduced into an anaerobic chamber in a laboratory setting. The combination of hydrogen, air, and an ignition source proved to be extremely hazardous; two of the incidents resulted in explosions, and the third came very close to an explosion. The incidents were all caused by human error, which highlights the importance of 1) proper training and 2) not becoming complacent when working with hydrogen. Just because you think you have an anaerobic system environment, it doesn't mean that air or oxygen will be far away, and special care must be taken at all times.
Setting: Laboratory
Description: A hydrogen explosion occurred in a university biochemistry laboratory. Four persons were taken to the hospital for injuries. Three of these were treated and released shortly thereafter; the fourth was kept overnight and released the following evening. All of the exterior windows in the laboratory were blown out and there was significant damage within the laboratory. One sprinkler was activated that controlled a fire associated with a compressed hydrogen gas cylinder.
First responders from the local community and the university campus were quickly on the scene. Once the injured were attended to and the site secured, response efforts focused first on assessing potential hazards (electrical, fire, hazardous materials, etc). Campus personnel worked into the night to board up windows, isolate utility services, clean up debris, and otherwise secure the affected laboratories. The building reopened to occupants the following morning. There was minor water damage from sprinkler water in the laboratory below the explosion, but no equipment was damaged.
The laboratory works with soil bacteria that cannot survive in the presence of oxygen. As a result, research work is conducted inside a plastic chamber in which the chemical constituents in the air can be controlled. The explosion occurred during the set up of one of these chambers. The chamber is essentially a plastic bag with a volume of approximately two cubic meters. The setup procedure calls for using nitrogen to purge normal atmospheric air out of the bag three times, leaving a very small amount of residual oxygen present. The remaining small amount of oxygen is then removed by reaction with hydrogen in the presence of a palladium catalyst to form water.
Hydrogen was mistakenly introduced into the plastic bag as part of the first purge. As a result, the hydrogen concentration reached an explosive level inside the bag due to the relatively large presence of oxygen. The ignition source was most likely an electrical source inside the chamber or the palladium catalyst becoming too hot. The burn pattern observed after the explosion suggests that the fire ignited at the position of the catalyst, but that doesn't rule out the possibility that a spark was involved. The amount of hydrogen involved could not have exceeded one pound, which is the capacity of the compressed gas cylinder when full.
Lessons Learned:
Setting: Laboratory
Description: An explosion occurred in a Microbiological Anaerobic Chamber of approximately 2 m3 capacity that contained an explosive mixture of hydrogen and air. A fire followed the explosion, but was rapidly extinguished by staff using fire extinguishers prior to the arrival of fire service personnel. The pressure wave from the explosion blew windows out of the laboratory, with glass hitting a passerby on a path outside and glass shards landing up to 30 m away. Ceiling panels were dislodged in the laboratory and adjacent rooms, and a worker using the apparatus at the time was taken to the hospital by ambulance for burn treatment. The worker subsequently fully recovered. Another worker in the lab at the time required medical observation, but was otherwise unharmed.
Mixtures of inert gases and hydrogen are routinely used in the type of anaerobic chamber involved in the incident. The mixtures used in the chamber involved were produced locally in the laboratory using nitrogen, carbon dioxide, and hydrogen. The hydrogen in the mixture reacts with any oxygen present in the chamber, on a heated catalyst, to eliminate oxygen and keep the chamber anaerobic. The local operating procedures used in the lab allowed high concentrations of hydrogen to be introduced into the chamber. A worker inadvertently admitted air to the chamber while doing maintenance, allowing the hydrogen-enriched atmosphere in the chamber to mix with air and subsequently ignite, most likely on contact with the oxidation catalyst in the chamber, resulting in the explosion and fire.
Hydrogen gas has a very wide range of flammability when mixed with air (4–75%). Oxidation catalysts can ignite explosive gas mixes without heating, spark, or flame. Local operating procedures and practices varied from the manufacturer’s advice. An unknown concentration of hydrogen was present in the chamber, resulting in a significant fire and explosion risk.
Lessons Learned: The practice of making gas mixtures in the laboratory should be eliminated, and gas mixtures with a known low hydrogen concentration should be purchased for use. The concentration of hydrogen used should be such that it is not possible to form an explosive mixture on dilution with air (i.e., hydrogen concentration less than 4% after mixing with air from a leak, damage to the chamber, or inadvertent admission of air to the chamber). A gas monitor with a built-in alarm should be purchased and installed to continuously monitor both hydrogen and oxygen concentrations in the chamber, and to provide visible and audible indication of any problems. The manufacturer’s instructions and manuals should be closely followed. All users should be fully trained in the use of the equipment, and should be fully conversant with the potential hazards and how to manage the associated risks.
Setting: Laboratory
Description: A laboratory research technician entered a lab to begin preparing samples that were to ultimately be purged in an anaerobic chamber (glove box) located in that room. As the technician walked into the lab, she looked at the chamber to see if it was adequately inflated. This chamber is equipped with a gas concentration meter, capable of simultaneously displaying the oxygen and hydrogen concentrations of the chamber atmosphere. Under normal operating conditions, the atmosphere inside the chamber is comprised of 0% oxygen (as intended/desired for an anaerobic atmosphere), approximately 2-3% hydrogen, and with the remaining balance being nitrogen (approximately 98-97%). Under such normal operating conditions, the hydrogen concentration inside the chamber is less than the lower explosive limit (LEL) of hydrogen to prevent an explosion in case air or oxygen were ever inadvertently introduced into the chamber. When the technician observed the meter's digital readouts, the oxygen concentration in the chamber was still 0% but the hydrogen concentration was at 43%. (The explosive range of hydrogen in air is 4% to 75%) An alarm light was also flashing on the meter, but an audible alarm was not heard. (At first, the technician turned the meter off and back on again believing something was wrong with the meter. She then checked the placarding of the compressed gas cylinders that were hooked up to the chamber manifold system and discovered that one of the cylinders was a mixture of 95% hydrogen and 5% nitrogen, instead of the 5% hydrogen and 95% nitrogen cylinder that was supposed to be there and immediately summoned assistance. The face of the manifold onto which the erroneous cylinder was connected is labeled as "5% H2/95% N2". The cylinder was last changed out 30 days prior to the incident, by the facility's maintenance personnel. And equipment user log indicates that the chamber had been used a couple of times since the cylinder change-out. In addition to the manifold labeling, there are also job aid instructions posted in the work area pertaining to the operation of the chamber by the laboratory researchers.
Evidence is present to show that the research staff actually requested a mixture 5% H2 and 95% N2 through the onsite acquisition system, but the vendor failed to supply the customer with the requested mixture; instead, a mixture of 95% H2 and 5% N2 was supplied. The investigation team determined that a mixture of 95% H2 and 5% N2 would have never been in the facility (for maintenance staff and end users to select from) had the vendor error not occurred.
Lessons Learned: Personnel should be aware that items requiring special receiving inspections should still be verified/examined by the end user prior to use.
Given that the anaerobic chamber and associated nearby electrical equipment were not designed or expected to handle a potentially explosive atmosphere, nearly all conditions necessary to have a much more serious event were present if the problem had not been discovered and the bag was not successfully purged of the hydrogen mixture.
Verify the contents of all gas cylinders to make sure that the vendor did not make an error and supply the wrong composition.
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