Explosion during hydrogen supply in a chemical company

This event occurred when a hydrogen-carrying trailer was supplying hydrogen to a chemical plant.
Instead of connecting the trailer hydrogen outlet to the hydrogen stationary storage, the trailer operator connected it to the compressed air supply of the plant. Large quantities of hydrogen were then transferred from the trailer into the compressed air plant network. The explosive atmosphere ignited, exploded and opened a flange of the compressed air line, from which the hydrogen escaped and burned off.

The INITIATING CAUSE was the erroneous injection of hydrogen into a compressed air network, due to the confusion between hydrogen and compressed air connections.
During operation, trailers were filled for various consumers. The design of the trailer connections was standardised only for the filling and emptying lines. Other connections, such as auxiliary media (compressed air) or analysis caps, were designed differently for each trailer.
The incident occurred due to confusion between compressed air and sampling connections due to the mentioned different design of the analysis connection, similar to compressed air, on other trailers. On top of that, labelling was missing or unclear.

The ROOT CAUSE was related to an inadequate risk assessment, which failed (1) to recognise the possibility of mixing up connections and (2) to deploy measures aiming to avoid it.

compressed air network

Damage to the compressed air pipeline and the silo

Cooling of the pipes adjacent to the fire.
Flushing the affected section of the compressed air system with clearance measurement.
Flushing the silos containing the explosive mixture.

In this event, when transferring hydrogen from a hydrogen to a stationary storage, the personnel made the mistake to connect a hydrogen line to a compressed air line. Data on pressure are missing, nevertheless, it can be assumed for the pressure industrial standards, i.e. for hydrogen in the trailer a value between 200 and 300 bar, and for the compressed air a value below 10 bar. Therefore, hydrogen was released massively into the air pipelines, forming a flammable mixture.
The confusion between lines at the basis of this event should be excluded
(1) By design, i.e. a hydrogen line connector should not be compatible with any other gas lines connector, making mechanically impossible to create a working joint between different lines.
And/or
(2) By procedures, which requires clear guidelines, clear labelling of the lines and awareness among personnel of the hazards involved by the operations.

On the short term: to use a mobile compressor unit instead of the plant compressed air supply.
(i) The owner of the trailer to clearly label the nozzles for all media.
(ii) To include the compressed air and hydrogen connections in the plant operating instructions.
(ii) To equip the compressed air network with a backflow protection .
(iv) To communicate the event to the industrial gases association, to develop a connection design able to avoid mix-up.