Explosion in a microbiological laboratory

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.

The INITIATING CAUSE was the erroneous injection of hydrogen in the chamber instead of nitrogen when purging.

In these cases, the ROOT CAUSE is usually attributed to a human error. However, the human error was enabled by inadequate preventive measures. A proper risk assessment should be based on the worst-case scenario and foresee that fully manual operations using of explosive gases without supervision would soon or later brought to mishaps, worsened by inadequate knowledge of the consequences of error. Adequate preventive measures should have been put in place to automatically exclude that scenario.

Hydrogen gas cylinder
, microbiological anaerobic chamber.

The set-up of the anaerobic chamber had been modified in the past. Due to unavailability of the correct spare parts, the original "T" connection with a toggle switch for different gases, had been replaced with e "T" connection without that feature.

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.

This incident was characterised by several manual operations executed without independent controls, based on human supervision (the principle of the ‘four-eyes’) and/or automatic preventive measures. It is known that soon or later, a too high confidence in the proper executions of the manual procedures would cause distractions.

The investigation arrived at the following conclusions:

(1) One of the first steps of the procedure was a tightness check. To ensure that the nitrogen and hydrogen lines from the compressed gas cylinders to the chamber were not leaking, the technicians we reopening opening the on-tank valve to pressurise the lines, keeping the end valve in the "off" position. After the leak test, the in-tank valves had to be closed again. This did not happened o the day of the incident: the technicians forgot to turn off the hydrogen in-tank valve back before beginning to purge the chamber with nitrogen.
(2) Both compressed gas cylinders were connected to the chamber through a common line and a "T" connector. A valve on the end had to be held open manually to introduce gas into the plastic bag. Originally, a toggle switch inside the "T" connection had the function to enable nitrogen or hydrogen flow, excluding the possibility of both flows not at the same time into the chamber. At some point in the past, because of the unavailability of the correct spare parts, a different "T" connection was mounted, which did not have a toggle switch. Had the toggle switch been present, the explosion would have been unlikely to have occurred.
(3) There was no need to use pure hydrogen just to eliminate residual oxygen. Therefore, the use of a 100% hydrogen compressed bottle was not necessary. A mixture of 95% nitrogen and 5% hydrogen would have done the job. This would have excluded the possibility to create an explosive atmosphere even in case of a mix-up of the gas flows.
(4) The laboratory had previously tried to use hydrogen and oxygen sensors as a precaution to warn of an explosive atmosphere in the chamber. However, the atmosphere proved to be corrosive, which led to an inaccurate sensor readout. Had adequate safety sensors been available while the chamber was brought online, again it is unlikely the explosion would have occurred.

Several procedural and design recommendations were issued:

(1) Replace the use of pure hydrogen with a 95:5 mixture of nitrogen and hydrogen to reduce the possibility of an explosive atmosphere occurring. Laboratory personnel should check each tank that is delivered to ensure that the gases are present in the proper ratio.
(2) Adhere to the manufacturer's recommendations for operation of the anaerobic chamber.
(3) Develop a checklist to ensure the proper execution of each step of the procedure (i.e. all connections tight, all gas cylinders closed; only activation of the required gas line, etc.).
(4) The safety sensors market should be explored, looking for of hydrogen and/or oxygen sensors able to withstand the corrosive atmospheric environment.
(5) All laboratory personnel should receive refresher training that includes standard safety precautions (including wearing personal protection equipment) as well as a more detailed review of the hazards of working with hydrogen.