LH2 release during transfer at an aerospace company

A liquid hydrogen leak occurred at the aerospace company (high Tier Seveso), specialised in testing rocket engines. 
A 53 m³ liquid hydrogen road tanker had just been unloaded, its bottom valve closed, and the transferring hose was being purged, when the portable gas detectors of the personnel on site triggered an alarm, indicating a hydrogen concentration in the air above 10% of the lower explosive limit (LEL). At the same time, operators observed a flow of liquid from the hose and white plumes of vapor emanating from the rear of the tank. 

The bottom valve of the tanker was not completely closed, resulting in an internal leak of liquid hydrogen downstream of the valve into the flexible hose. This caused a liquefaction of liquefaction of ambient air in contact with the uninsulated, cold surface of the hose. 
A second effect was a higher than usual release rate of hydrogen through the vent stack, which was connected to the hose being purged. This led to the detection of hydrogen by portable detectors. 

The emergency stop was activated, the facility shut down for safety and the area sprinkled with water. The operator released the pressure in the tanker by performing a manual release and re-injected helium through the hose to warm up the valve and dilute the leaked hydrogen. 

Nobody was injured.

The INITIATING CAUSE was the only partial closure of the valve at the bottom of the tanker. The formation of an ice plug of unknown composition had probably caused a mechanical blockage in the valve seat, impeding its complete closure. This valve, operated pneumatically, was the main valve for the unloading of the tanker. 

The formation of ice plug was possibly caused by unexpected low temperatures during the unloading process, and/or by a loss of vacuum in the double-walled enclosure of the valve or the piping to which it was attached. The failure to close completely could not be detected by the operators, who started the purge of the hose. The purging gas, helium, flowing through the hose, carried the leaking hydrogen up to vent stack. 
 

Hydrogen detection near the ground was facilitated by the prevailing meteorological conditions: the wind was blowing the hydrogen emissions from the stack towards the operators. The white plumes observed behind the tank were caused as well by the atmospheric conditions of the day, which was cold and humid with a low dew point, favouring the vaporisation of liquid air and the condensation of air moisture.
 

The ROOT CAUSE is unknown. However, the investigation found out that operators of the hydrogen delivery company had previously experienced the blocking of the valve. Moreover, the plant risk assessment considered the possibility of the failure to close of the valve, but did not foresee any mitigating action. 

LH2 road tanker

Only damage was the loss of hydrogen and the loss of time during the emergency (7 hours)

The emergency stop was executed, and the facility was shut down for safety. This prevented the bottom valve from being reopened and triggered the automatic sprinkler system in the unloading area. This was supplemented by a fire truck from the internal emergency services. The emergency plan was activated, and the 127 employees on site were evacuated. A 900 m safety perimeter was established, and the road was closed. 

To bring the tank back to a safe state, the operator: 

1. Opened the automatic valves of the tanker to direct the hydrogen to the main safety flare. This reduced the pressure in the road tanker and thus decreased the leak flow rate upstream / downstream through the incorrectly closed bottom valve. 
2. Injected helium downstream of the bottom valve to dilute the hydrogen release and to warm the valve. This allowed the bottom valve to be properly closed and the facility to be returned to a safe configuration. Once the operations were completed, the emergency plan was deactivated approximately 7 hours later. 

 Clogging of valves working with liquid hydrogen is not an uncommon phenomenon, cause by solidification of air or other gases at temperatures approaching the liquid hydrogen temperature. If the valve has safety role, such as a pressure relief valve, the partial or total blocking of the flow can have safety-related consequences (see for example the extreme case of HIAD_609). 

However, the valve in this case had the main function to allow the flow of liquid hydrogen from the tank to the stationary storage system via a hose. It did not leak into the environment but into the hose when executing a purge. All safety-related systems functioned as designed.

The governmental investigation issued the following recommendations to the aerospace company:

1. To revise the procedure used for unloading the tank involved on the day of the incident to include instructions to follow in case of difficulty closing the tank's bottom valve.
2. To consider defining new meteorological parameters to be monitored and adhered to in order to carry out unloading.

An additional recommendation was issued to the hydrogen delivery company:

3. To  learn from the feedback provided by tankers drivers regarding the difficulty od closing the tank's bottom valve and integrate it into the generic unloading procedure.