Fire during LH2 transfer from tanker

During the preparation for a liquid hydrogen transfer from a tanker to a stationary storage system, the safety valve opened and the gas ignited.
The incident occurred when a pressure build-up was initiated by opening the liquid hydrogen valve to the build-up coil. Flames filled the control cabinet. The fire appeared at three pipe connections to the vent line, all on the vent side of the relief valve. The fires were terminated after all the valves were closed. However, the release continued through the high pressure relief valve. The fire was put out by terminating the supply to all the leaks. The gas was allowed to burn out and also slowed by using helium in the gas phase.

The INITIATING CAUSE the build-up of an overpressure in the liquid hydrogen tanker the preparatory pressurisation step, with consequent correct activation of the pressure relief valve.
IGNITION occurred most likely by static charges.

A few FACTORS contributed to an escalation:

(1) The blow down probably caused a pressure rise in the vent line manifold into which all the relief and vent lines were connected. The pressure rise broke a pipe on the vent line and a large amount of high velocity gas was vented through that crack.
(2) Hydrogen leaks and fires developed at the threaded sections at the ruptured pipe nipple. The reason for this was road vibrations and limited pipe support, which caused blockage at the threaded sections and rupture of the pipe nipple.
(3) The pressure relief valve did not reclose, because impeded by a metallic piece, and the hydrogen flow from the tanker continued.

The ROOT CAUSE could be identified as a combination of shortcomings in design (vent pipe structure, vibrations and overpressure in the vent), and in liquid hydrogen transfer procedures.

safety valve, vent, LH2 storage

The preparation of the tanker for LH2 transfer had just started.

Probably the whole content of the tanker was lost. Assuming a cost of LH2 of approximately 0.2 US$/kg (in 1968: https://ntrs.nasa.gov/api/citations/19680018755/downloads/19680018755.pdf), and a tanker load of approximately 3 t, result in a property loss of only 600 US$.
The LH2 cost provided corresponds to large scale production, it doe not cosnider conditioning and transport. Assuming one order of magnitude higher, 2 US$/kg would result in 6000 US$ of losses.

The gas was allowed to burn out and its flow was slowed by using helium in the gas phase.

At the time of this event, the LH2 transfer technology was still in its infancy. This and similar mishaps reported by Ordin in the ears 1950-70 have been critical in reaching technological maturity of LH2 production, handling and transport, thanks also to mishaps of relatively very small consequences.