Summary
A faulty modification to a multiple-gas piping manifold allowed mixing of hydrogen and oxygen that resulted in a storage tube explosion. Several employees suffered severe burn injuries from the incident.

Incident Synopsis
An employee, without authorization, fabricated and installed an adapter to connect a hydrogen tube trailer manifold to an oxygen tube trailer manifold at a facility for filling compressed-gas cylinders for a variety of gases, including hydrogen, oxygen, nitrogen, and helium. A subsequent improper purging procedure allowed oxygen gas to flow into a partially filled hydrogen tube on a hydrogen tube trailer. An ignition occurred in the manifold piping system and a combustion front traveled into the hydrogen tube where, after traveling about a meter, the deflagration apparently transitioned to a detonation that ruptured the tube.

Discussion
At the time of ignition, the composition of the gas in the tube was estimated to be about 42 vol% hydrogen with the remainder being oxygen. Although not stoichiometric, this mixture is well within the detonable range for a hydrogen-oxygen mixture. The pressure of a deflagration would not have been sufficient to rupture the tube. The initial pressure in the tube was about 4 MPa. A pressure ratio of approximately 13 can be expected for a stoichiometric mixture of hydrogen and oxygen. Thus, the maximum pressure that could have been produced in the tube by a deflagration was about 52 MPa. The tube had previously been hydrostatically tested to 90 MPa. Considering the hydrostatic test pressure, a pressure ratio in excess of 22 would have been required to rupture the tube. The close approach to a stoichiometric mixture and the high degree of confinement appears to have been sufficient to permit a transition to detonation. The far end of the tube was folded out like the petals of a flower. With the reflection of the shock wave at that point, a pressure ratio of over 40 times initial pressure would have been possible. Tube fragments (some of considerable size) were thrown a substantial distance. A 20-kg piece of the hydrogen tube was found about 425 m from the site of the explosion. The fragments did not result in any damage; however, several employees suffered severe burns.

Causes
The immediate causes of the accident include: an unauthorized piping modification, an unapproved, improper purging procedure, and a complete disregard for applicable policies and procedures. Insufficient involvement of management in the operation of the facility was identified as a contributing factor to the incident.

Contributing Factors
A lack of proper supervision permitted an employee to act on his own in the modification and operation of a piping system.