Liquid hydrogen tank explosion

A 9000 gal (34 m3) LH2 storage tank exploded during repair of the vent stack.
Few days before the explosion, the tank had been filled with LH2, but later the hydrogen released and ignited at the tank pressure control system, damaging the vent stack. To perform the required repairs, the tank had to be emptied. To this purpose the vessel was purged with nitrogen gas to display the LH2 by letting it boiling. However, the pressure inside the vacuum jacket increased because of the opening the vacuum valve. This pressure increase eventually led to the catastrophic tank rupture. One end of the tank blew off.


The detailed events sequence was:
Thursday : Tank filled with 9000 gal LH2.
Friday : Pressure in the vacuum jacket was 30 microns (0.03 Tor = 40 10-6 bar).
Saturday, 12:05 p.m. Fire erupted on the controls end of the tank. The fire folded the vent stack over and it was finally cut off just below the fold. The tank needed repair.
Saturday, 7:30 p.m. : Nitrogen gas was injected into the tank to boil away the LH2.
Sunday, 9:00 a.m. The nitrogen purge was stopped since it was too slow. The pressure in the vacuum jacket was 300-350 microns. The valve to the vacuum jacket was slightly opened.
Sunday, 10:30 p.m. : The pressure in the vacuum jacket had risen to about 850 microns (0.85 Tor = 1 10-3 bar).
Sunday,11:30 a.m. The vacuum valve was opened completely and water was sprayed onto the tank to prevent a subsequent fire.
Sunday, 12:05 p.m.: The tank exploded.

The INITIATING CAUSE was an increase of internal pressure which the venting system wa not able to work out.

According to Mires (1985) and Lodhi and Mires (1989), the calculations clearly show that the incident was initiated by the opening of the vacuum valve, which resulted in the establishment of a good thermal exchange between the internal vessel and the external jacket.
The various references do not explain the reason for the opening of the vacuum valve. Considering the need to empty the tank, it can be assumed that this was a measure to allow for a quicker boil-off.
In this sense, the ROOT CAUSE could be attributed to a human (assessment) error and/or to management shortcoming, due to the absence of a procedure based on risk assessment.

storage tank

Repair works were ongoing at the vent stack

DESCRIPTION OF THE UNIT
The vacuum jacket of the cryogenic tank was filled with Pearlite insulation. The outer tank was 9 ft in diameter and 38 ft long. The inner tank was 6 ft 10 in. in diameter and 35 ft long. The tank was mounted horizontally on two concrete footings. The aluminium tank was an alloy of type SB-209-5083-0 having a wall thickness of 0.693 in. and a tensile strength of 40,000-51,000 psi. The steel tank, which was type SA-36, had a wall thickness of 5//16 in. and a tensile strength of 36,000 psi.

Nobody was seriously hurt (number of possible light injuries unknown), but the pressure wave caused significant property damage.
The worker reported burns of unknown grade.
The 650 kg bulkhead was propelled above a 2 m fence and hit the ground at 60 m, skidding for additional 35 m.

In absence of effective pressure release measures, and under accidental increased heat exchange, the LH2 boil-off can cause the tank internal pressure to reach values well beyond those allowed by the structural integrity of the tank.