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.
Personnel should be properly supervised, and supervisors should be aware of the activities of their personnel. Personnel must be motivated to adhere to established policies and procedures. All personnel associated with potentially hazardous work should receive necessary safety training.