Explosion in a vent stack of a rocket testing facility

During general repair and equipment replacement on a liquid oxygen-liquid hydrogen Saturn test facility, a section of the 24-inch GH2 vent line ruptured. An explosion occurred causing extensive damage. The hydrogen vent system from the stage included the vent valve (being removed) which was connected through two flexible lines to the facility vent system. The facility system included a 20-inch vacuum jacketed line from the stage which connected to the 24-inch line leading to a catch tank. From the catch tank, a 36-inch line led to the flare stack. The LH2 tank was pressurized to about 4 psig with helium.

[Ordin, NASA (1974)]

Most probable cause of explosion was a mixture of hydrogen-air at a particular location in the duct. Hydrogen accumulated in the system due to a number of pressurisation and venting cycles with insufficient purging or from leakage of gaseous hydrogen system on stand (2500 psig, 170 bar). The vent system was not purged with high velocity gas and the periodic incremental purge with helium would not necessarily have removed the hydro­gen. Air could have entered the system when the stage vent valve was removed and from an open valve at the base of the catch tank.
Ignition was most likely caused by the flare stack. The combustible mixture was consid­ered to have reached the flare stack and ignited. The flame propagated through the catch tank into the 24-inch diameter vent line and upstream until a mixture and pres­sure were attained which resulted in a detonation.

vent, flare

The stage tanks were empty of fuels and filled with helium for the repair works