Hydrogen explosion in a LH2 storage tank

A combustion event occurred in a liquid hydrogen (LH2) tank at a testing site for space equipment, when a high-pressure test was being performed.
The liquid H2 tank was contanining 12 m³ (700 kg) of LH2, and was pressurised by a control device up to 250 bar in 30 sec. When the pressure in the tank reached 160 bar, a noise indicating a pressure drop was observed and the detectors recorded a pressure fluctuation of 10 bar. The sudden and significant fluctuations of the measured parameters (temperature of wall, pressure, flow) brought to the termination of the test. The stop did not have consequences.

The cause of the accident was the presence of combustion of frozen oxygen in an blind branch of the tank’s drain pipe. Its presence was probably due to the partial unsuccessful purging by helium, which had not been able to purge the air contained in the blind pipe, because placed in the lower part of the tank. The trapped air had re-solidified when while refilling the tank with LH2.

Several combustion spots were detected: in condensed phase between solid O2 and H2 gas, in less dense phase between H2 and O2 gas. The combustion was undoubtedly triggered by the friction of the pellets against the walls and the adiabatic compression of the H2/O2 gas mixture.

The INITIATING CAUSE INITIATING CAUSE was the presence of air/oxygen in a blind branch of a pipe, due to unsuccessful purging of the tank before refilling with LH2.

The ROOT CAUSE could b identified in an ineffective purging procedures, as well as in a design mishap due to the presence of blind section of pipes.

liquid hydrogen cryogenic tank

Few months before, the test tank had been emptied and purified (by injecting helium) as the hydrogen gas network was suspected of being polluted due to an intake of air from a compressor, one year before.

The event occurred at a testing centre for propulsion system aerospace (rockets). It was in an isolated area with no admittance to the public.

The cryogenic LH2 tank nominal capacity was 12 m3 correspomding to 700 kg of hydrogen, with a nominal working pressure of 250 bar (MAWP 400 bar).
The testing setup was complete with a liquid oxygen tank and a tank containing gaseous hydrogen up to 800 bar.

no consequence to person or equipment

4 months after the incident, the restarting was approved by local control authorities (Seveso II regulation control).

The police stopped circulation on the roads around the site, till the intervention of the site crews assisted by the fire brigades depressurised the tank. An evacuation was not necessary .

This was a near miss, and process control system worked as designed, able to detect on time an abnormal situation thanks to the signal sent by the sensors measuring process parameters.
On top of the need for improvement of tank design, this near miss highlighted also the need to understand better the behaviour of liquid hydrogen in presence of impurities.

Following an internal analysis and the improvement of processes, the system was put back in operation after 4 months of stop). Among the improvements introduced are:
1. a new valve allowing for the regular purge of the dead branch of the pipe.
2. An operational process to avoid the accumulation of impurities which come with the delivered hydrogen.
3. Better monitoring/instrumentation