The University of Arizona


Risk Management and Safety

Chemical Safety Bulletins


Incident: Potential Fire or Explosion


What happened? (1991) On a routine Risk Management & Safety (RM&S) laboratory safety inspection a particularly dangerous situation was discovered. A compressed gas cylinder, originally containing 600 grams of silane (SiH4), was found next to an oxygen cylinder in a laboratory recently vacated by a faculty member who had accepted a position at another institution. The cylinder was 7 years old and was tagged with a label which read "Danger - Do Not Remove This Regulator."


The main valve of the silane cylinder was open and the cylinder pressure gauge on the regulator showed the cylinder was pressurized at about 250 psi. Because of the age and type of gas, it was questionable whether the main valve would close. Silane will oxidize to a particulate which may cause valve blockage or seizure. The delivery pressure adjusting screw was also open and the delivery pressure gauge was pressurized past the maximum gauge pressure of 200 psi. The only mechanism containing the silane was the flow control valve (see regulator photo below).


This situation was considered significant because silane is an extremely hazardous gas. It is either pyrophoric (spontaneously flammable) in air between 1 and 100 percent or explosive. Because much is still unknown about its exact ignition characteristics, it is always unpredictable.


Because the regulator was pressurized with silane and was not fitted to be purged with an inert gas, it could not easily be removed from the cylinder in a safe manner. With the regulator in place, the cylinder could not be returned to the manufacturer (the Department of Transportation prohibits the transportation of compressed gas cylinders with regulators in place). Fortunately, a serious accident did not occur when this cylinder was initially used or attempted to be used, or before the subsequent condition of the cylinder was discovered. With considerable effort, RM&S was able to safely defuse the predicament.


After consulting with those knowledgeable in the handling of this type of gas, it was decided to detonate the cylinder instead of attempting to close the main cylinder valve and remove the pressurized regulator. Explosive experts from the Arizona Department of Public Safety (ADPS) were summoned to perform the detonation. Under a special permit, the cylinder was legally transported to a remote location where it was placed in a four foot deep trench. An explosive, shaped charge was placed along the length of the cylinder and it was detonated. The cylinder was split open and the silane was released forming a large flaming mushroom cloud.


Had ADPS not been unable to execute the detonation at no cost to the University, the department in which this professor worked would have had to absorb the costs associated with hiring a specialized firm. Undoubtedly, this would have been very expensive.


Why did it happen?

  1. Silane had been acquired without institutional prior approval. It was used or attempted to be used, without taking any apparent safety precautions.
  2. The situation was made even worse by storing the silane with an incompatible, oxidizing gas (oxygen).
  3. An irresponsible researcher was allowed to abandon an exceptionally unsafe situation.

How can a similar occurrence be avoided?

  1. Silane is primarily used in the semiconductor industry where it has been responsible for many serious explosions and fires even though state-of-the-art precautions have been employed. Most of these incidences resulted in extensive property loss, injuries, and even deaths. As a result, silane incites a significant amount of respect even from those who are knowledgeable in its hazards.

    Before deciding to use an extremely hazardous chemical such as silane, consideration first must be given to a suitable and safer alternative. Liquid chemical substitutes are becoming more available for replacing metal hydride gases similar to silane. For instance, tetra ethyl-o-silicate (TEOS) can be used as a substitute for silane in some applications. Also, silane/non-flammable gas mixtures in concentrations less than 0.8 – 2.2% (depending on the non-flammable gas) are not pyrophoric. Should no appropriate substitute compound or safe concentration be acceptable and there is still a need to use silane, extensive and extremely restrictive safeguards must be met.

    Silane is an extremely hazardous chemical that requires prior approval (Chemical Safety Level 3 or CSL-3) by the Institutional Chemical Safety Committee (ICSC). For approval, the Minimum Requirements for Toxic/Corrosive Gas Use (PDF format) will at least be required by the ICSC.

  2. Gases, like any other chemical, must be stored according to compatibility. If there had been a silane release in the lab, the nearby oxygen cylinder would have undoubtedly been involved and would have made a serious accident even more catastrophic (see Chemical Management Best Practices – Storage, Section 4.2).

  3. Each department must initiate a check-out procedure for the permanent departure of any lab personnel to ensure that all hazardous materials have been properly disposed of or donated to another research group. This step must take place prior to the individual leaving the University and before the lab space is re-assigned. Failure to do so may result in a dangerous and costly occurrence similar to that described above (see the UA Laboratory Chemical Safety Manual, Section 1.14).
regulator explosion

Top Left – Typical automatic pressure regulator. Top Right – Silane cylinder detonation. Bottom – Silane cylinder after the detonation.


silane cylinder