Explosion at a vent stack

The incident occurred at end of a hydrogen fuel pump test, when large volume of gaseous hydrogen was vented off. An explosion occurred at or near the top of the vent stack, with damage to stack segment, vent systems, ducting, and valves. The top section of vent stack was missing (it consisted in a covering "hat" section,15 ft. length, on top of the vent stack and was housing a double reversing turning vane assembly).

The INITIATING CAUSE was the formation and ignition of an explosive air-hydrogen mixture at the end of the venting stack, where a new ''hat" section of the vent stack had been installed. Failures of internal structures had been observed prior to this incident. Therefore, the vent system had a slightly higher than normal catch tank pressures due to partial restriction of the vent stack outlet. Ignition was by static electricity or burning gas in vicinity of stack outlet.

The ROOT CAUSE could be identified in an inadequate design (and possibly manufacturing) of the stack ‘hat’.

LH2 pump, vent stack

Losses probably limited to the vent end.

This and similar events occurred when knowledge of hydrogen fluid-dynamic and ignition behaviour was still in its infancy. The NASA was learning-by-doing from these mishaps, which were causing only some property damage.

In the specific case of this event, it could be further considered if the vent ‘hat’ caused an overpressure due to some failed internal element, or just because of its geometry, forcing the vented hydrogen to reverse the flow direction. We do not know, unfortunately, the reason for the need to change the direction of the venting. It could have been the need to ensure better protections for neighbouring components.