Explosion at a vent stack

A large quantity of hydrogen was being vented to atmosphere during pre-chill operations. Normal pre-cooling time was about 45 minutes, but due to technical problems, the pre-cooling continued for about 80 minutes. The LH2 tank was pressurised and vented a number of times releasing additional hydrogen. The weather was calm so the hydrogen failed to disperse as quickly as normal. The accumulated gas ignited (unknown source) and the explosion caused damage to the test area.

[Ordin, NASA (1974)]

The INITATING CAUSE was the unplanned release of large quality of gas at the vent stack, which did not disperse at stack exit but formed an explosive mixture.
The large quantity vented was due to a pre-chill procedure which lasted much longer than planned, with consequent need to pressurise the liquid hydrogen tank more times.
The ROOT CAUSE could be related to a vent stack design unable to avoid explosions in all cases and/or to mitigate their occurrences (for example venting at the safe distances). An additional factor could be related to operative procedures unable to mitigate the consequences of unplanned events.

vent stack

DESCRIPTION OF THE SYSTEM
The report used as a source does not specify the system and the operation affected. Nevertheless, from similar reports it can be assumed that this was a liquid hydrogen propulsion engine test.
The pre-chill operation which was ongoing when the unplanned vent occurred was required to bring all the storage-to-engine components to the operative low temperature, avoiding temperature shocks which could have damaged them.
To this, a certain quantity of liquid hydrogen is made flowing by pressurising the on-board liquid hydrogen tank containing the propellant.
It is probable that this test was not a pre-launch test, but an engine test performed at a testing facility, so that the liquid hydrogen storage tank was not the one installed on board of the rocket.

Damage to the test area.

The formation of an explosive air-hydrogen mixture at he vent stack exit due to lack of wind is reported also in HIAD_081, see related lesson learnt there.