Leak at a filling hose of a hydrogen dispenser

A leak occurred at a dispenser hose of a hydrogen refuelling station.
On the day of the accident six FCVs had already been filled. Four minutes after the start of the filling of a seventh vehicle, (when the filling hose pressure was 79 MPa), an unusual hissing sound of gas leakage began to be heard. A popping sound was then heard at the end of filling.
At this point, neither the suction nor diffusion type stationary gas detectors had triggered an alarm.
Approximately five minutes later, the station used the residual pressure in the dispenser to check for leaks. While hydrogen was flowing through the filling hose, the gas detector at the end of the filling hose triggered an alarm, causing an emergency shutdown.

The investigation found that first a small crack in the inner layer at the dispenser side had developed, though which the hydrogen had diffused between inner and outer layer, building up pressure there, till the outer layer ruptured at a location at the other end of the hose, near the emergency breakaway coupler side.

The INITIATING CAUSE of the leak was the cracking of the internal layer of the hose, following by the diffusion of hydrogen into the space between inner-and the reinforcement layer and eventually the breaking oen the outmost layer under the internal pressure.

The crack of the inner layer occurred at approximately 15 mm from the end of the crimped portion of the ferrule on the dispenser side. Its length was 2 mm at the inner side and 1 mm at the outer side. Traces of foreign matter and burrs were also observed, suggesting a high likelihood of a mechanical damage. No significant deterioration in properties was found on the material of the inner layer, compared to an unused hose from the same production lot.

The outer layer of the filled hose was designed with microscopic holes allow hydrogen to permeate and escape to escape to the outside. Despite this design feature, it is believed that it broke open because the amount of hydrogen gas leaking through the crack in the inner layer was significantly greater than the amount of gas expected to permeate during normal use. As a result, the hydrogen pressure between the reinforcing layer and the outer layer, increased up to the rupture of the outer layer.

The hose has a guaranteed lifetime of 1,000 cycles or one year. It had been used for 547 cycles, and the one-year expiration was in September of that year. Therefore, it is unlikely that the expiration date was due to cycling.
The most plausible cause of crack generation was the presence of foreign objects introduced in the nitrogen flow used during the regular inspections. This conclusion was corroborated by the finding of metallic particles from the nitrogen container during a test using a 5 µm-mesh filter.

The ROOT CAUSE can therefore be identified in a suboptimal step of the inspection & maintenance procedures.

hose

DESCRIPTION OF THE FACILITY
The HRS had a hydrogen production capacity of 22,874 m3/day, a nominal operating pressure of 82 MPa and could refuel with hydrogen temperature ranging from -40 C to 10 C.
The hose which failed was of the short-length type.
It filled approximately 100 fuel cell vehicles (FCVs) per month with hydrogen.

The material of the inner layer of the hose was thermoplastic resin. With approximate dimensions: Inner Diameter 6.3 mm, Outer Diameter 14.8 mm, Length 800 mm.

No personal injury occurred. or property damage, beyond the costs of the investigation and need to replace the filing hose

According to the KHK report, could be considered a rare case of leak-before-burst failure. , a design requirement. Nevertheless, the design-related mechanism of a leak-before-burst event was considering a loss of confinement of the inner layer, followed by hydrogen diffusion through the reinforcement and the outer layer to be safely release outside. If a fatigue crack penetrated the airtight inner layer, hydrogen would easily pass through the non-airtight reinforcement layer and be released to the outside, resulting in a leak-before-burst. However, the outer layer maintained some capacity to retain hydrogen, and eventually burst open.
The lesson learnt from this case is that the external layers must be made more permeable to hydrogen. The creation of ‘micro-vents’ must be carefully chosen, to avoid build-up of pressure between layers.
In fact, the involved hose had been fabricated in February 2017, while the new generation of filling hoses manufactured after April 2018, had been improved and equipped with multiple ventilation holes.

More in general, considering also additional hoses failures occurred in the same period (see for example HIAD no 702, 1196, 1198), it was necessary to quantitatively examine the factors influencing the fatigue strength of the inner layer, including the involvement of external factors such as foreign matter.

Finally, the fact that hydrogen was released without that detector going off i highlights how challenging gas to is to place detectors in the most strategic positions in the surrounding of the dispenser, able also to catch releases from the hose during refilling.

(1) A portable gas detector was placed near the dispenser during filling to detect any hydrogen leaks and monitor the progress.
(2) To prevent the introduction of foreign matter during filling hose replacement, a 5μm filter was installed on the nitrogen piping during the nitrogen replacement process.