Explosion when starting a hydrogen engine

Event

Event ID: 311
Quality: 3
Description: An explosion occurred during calibration of engine. The equipment was covered by a shed and when the valve was operated allowing hydrogen gas to flow through the engine, the gas ignited. The explosion damaged the shed and injured the personnel. The suggested overpressure was over 10 psi.
[Ordin, NASA (1974)]
Event Initiating system: Hydrogen System Initiating Event
Classification of the physical effects: Hydrogen Release and Ignition
Nature of the consequences: Explosion
Macro-region: North America
Country: United States
Date: 01/01/1950 - 31/12/1974
Main component involved?: Valve (Generic)
How was it involved?: Leak & Formation Of A Flammable H2-Air Mixture
Root causes: System Design Error
Root CAUSE analysis: The IMMEDIATE CAUSE was the leak of a valve allowing hydrogen to build in the propellant header.
When the valve was operated, the hydrogen was propelled through engine and collected under shed. The ignition source is unknown.

The ROOT CAUSE of the valve malfunctioning is unknown. Nevertheless, the escalation was caused by the shed allowing the formation of a pre-mixed flammable mixture, followed by an explosion. This suggests inadequate (safety) system design.

Number of injured persons: 2
Number of fatalities: 0
Post-event summary: The number of the workers injured is not given, but it is more than one.

Lesson Learnt: Unambiguous labelling of components and clear instruction for their maintenance and replacement should be in place. The workers involved should be well trained, also by making clear which would be the consequences of exchanging components or mounting them inverted.

Reference & weblink: Mishap no 6 in 
P. L. Ordin, Review of hydrogen accidents and incidents in NASA operations, 1974, NASA TM X-71565 
https://ntrs.nasa.gov/citations/19740020344
Lowesmith et al., Safety issues of the liquefaction, storage and transportation of liquid hydrogen: An analysis of incidents and HAZIDS, Int. J. Hydrogen energy (2014) https://doi.org/10.1016/j.ijhydene.2014.08.002
Hankinson and Lowesmith, Qualitative Risk Assessment of Hydrogen Liquefaction, Storage and Transportation, FCH JU project IDEALHY, Deliverable 3.10 (2013) 
confidential 
(accessed October 2025)
Hankinson and Lowesmith, HAZIDs for Hydrogen Liquefaction, Storage and Transportation , FCH JU project IDEALHY, Deliverable 3.11 (2013) 
https://www.idealhy.eu/uploads/documents/IDEALHY_D3-10%20HAZIDs_Liquefa… />
(Only summary publicly available, accessed October 2025)
References\HIAD_311%20H2TOOLS.pdf