Explosion in the storage of a HRS

This event relates to a hydrogen release which took place from the high-pressure storage unit of a hydrogen refuelling station (HRS). A quantity of hydrogen estimated between 1.1 and 3.3 kg ignited and exploded. The cause of the leak was two untightened bolts on the plug flange of one of the high-pressure tanks.
The tank affected was the one closest to the ground. After breaching the high-pressure gasket, hydrogen quickly broke through the low-pressure gasket and rapidly leaked to atmosphere, forming a flammable cloud. Several fires followed.
The HRS was located on a busy roundabout. The blast was heard far away. Damage to surrounding buildings (windows broken 65 m away) indicate that a detonation occurred. There was no direct damage to people, but the explosion caused airbags on three cars to activate due to the shockwave.

The HRS manufacturer and owner shut down immediately all similar plants worldwide, pending inspection on bolts. Station operation stopped and was probably definitively shutdown (7 years later , the station is not back in function).

The INITIATING cause for the release were a flange assembly not properly tightened. The two bolts were screwed in place by hand but not tightened with the proper tool to the torque value required by the instructions. Post-incident inspection revealed the presence of corrosion on the surfaces participating to the connection. This could have contributed to the loss of confinement

The storage system passed the commissioning inspection because the pressurisation tests did not cause gasket to fail and did not reveal any leak. Despite the loose bolts, the flange’s gasket held for over 2 years against 950 bar pressure, before eventually giving way due to fatigue induced by pressure cycling and oscillations during refuelling. In the hours before the sudden release, the pressure records of the storage bank showed a slight decrease, which can be explained by a small leak of approximately 0.15 kg of hydrogen per hour.
The ignition source could not be identified with certainty: spontaneous ignition, interaction with the ground gravel and exposure to non-explosion-proof mechanical and electrical equipment are the possible candidates.

A contribution to the ROOT CAUSE was an error during assembly. This error had not been detected for years, despite pressure tests and a components certification performed by a 3rd party. Moreover, even though the pressure records could be used to detect the existence of a small leak, the station monitoring system was not programmed to detect leaks by monitoring abnormal pressure loss in high-pressure tank.

bolts, bolts

Event occurred shortly after a refuelling, as the compressor started refilling the high-pressure tank.

DESCRIPTION OF THE FACILITY
Scarce public information is available on the HRS characteristics.
The station was for light duty vehicles to be refilled at 700 bar.
The hydrogen was produced on site by an alkaline water electrolyser, capacity and power unknown.
While some sources mention a nominal value of 950 bar for the high-pressure storage, the other report only 900 bar. Total storage capacity, number and type of tanks unknown.
The strage system was srrounded by a gence, without roof.

Station likely unrecoverable. Material damages est. at 1 million euros.
Parts of the fence that absorbed the explosion were projected into the neighbouring roundabout, about 15-20 metres away, but o damage to vehicles. Damage to surrounding buildings (windows broken 65 m away) indicates that a detonation occurred.

There was no direct damage to people. The explosion caused airbags on three cars activating due to the shockwave. The drivers were checked in at hospitals, their injuries were light.
Neighbourhood was alarmed by the loudness of the blast, which was heard in a large area; one dog panicked and jumped from a balcony at the fourth floor of a neighbouring building (9 metres), breaking some bones but surviving.

The HRS manufactuer was fined NOK 25m.

17:30 Hydrogen leaked from tank and ignited
17:37 First emergency responders on the scene
17:40 Station manufacturer received first report of the incident
17:41 neighbouring highaways closed
17:47 Security zone of 500 meters established
19:28 Robot used to cool down site
20:14 Traffic partially reopened for traffic
20:14 Fire department confirmed fire under control

The tanks belonging to the high-pressure storage were manufactured and delivered by a sub-contractor to the company building the hydrogen refuelling station (HRS). This company assembled the tanks into the storage system. The tanks as part of the assembly were tested for leak, but these tests could not detect the lack of tightness, because the gasket hold the pressure, hiding the fault for over 2 years. It failed at a later stage during operation, under cyclic pressure conditions. It is unknown if additional tests and controls were part of the inspection after delivery of the tanks.
The event raises the question on how to improve certification and commissioning tests, to allows for a better (more accurate) assessment of the fit-for-use of the components. It raises also the question of the optimisation of quantity control and certification along the components supply chain.

According to the HRS manufacturer statement, the following tightening new procedure was adopted, respecting aerospace standards).
A dual-witness torque procedure, whereby
(1) first one fitter torques to nominal torque in a 3-step procedure and marks each specific bolt with a green line after torquing.
(2) Afterwards, a second fitter performs a control by re-torquing each bolt, marking it with a red line. This procedure is performed on the inner bolt holding the bush and on the outer bolts holding the plug.
Both fitters must sign-off in the Assembly Quality Control chart, noting the specific cylinder serial number, that they have been torquing the bolts, stating the torque value, and noting which torque wrench – all of which are calibrated – was used.

Moreover, the leak detection was improved by means of a software update able to perform more frequent detection measurements.
Better Ignition control measures were adopted, by guaranteeing more smooth surface in the storage unit and avoiding gravel, by improving ventilation and broadening the use of explosion-proof equipment.