Freezing of a valve on a LH2 tanker

A liquid hydrogen tanker was returning empty to the home terminal after delivery to a customer. At a stop, the drivers executed a visual inspection of the vehicle and noticed an accumulation of ice under the valve seat of the pressure control valve activating at 17 psig (pcv-1).
The liquid hydrogen tank was equipped with two pressure control (or relief) valves, one activating at 17 psig (pcv-1), and a second at 50 psig (pcv-2). The first one should be active only when liquid hydrogen is present in the tank. When the tanker contained only residual gaseous hydrogen, the 17 psig safety should be inactive.
In this case, it appeared that the ice build-up allowed the pcv-1 safety valve to remain active, in addition to the pcv-2. This would have caused the pressure relieving at 17 psig instead of 50 psig. The driver vented the tanker down to 2 psig which released 7,680 standard cubic feet of hydrogen to the atmosphere. Before resuming the journey, the drivers assured that the valve pcv-1 had completely thawed and returned to ambient temperature.
The valve and associated piping was inspected after returning to the home terminal. No defects were noted. Occasional ice accumulation is normal.

The INTIATING CAUSE was the formation of ice at pressure control valve, which made it remaining active against the procedure.
This event could be classified as near miss, because the safety measures were not compromised, which functioned as required. On the contrary, the fact that the low-pressure control valve was still active would have triggered an early venting, far away from the maximum allowed pressure in the vessel.
According to the transport company, ice accumulation is a ‘normal’ phenomenon.
There is nevertheless a lack of clarity on how the ice could impede the de-activation of the control valve, and consequently on the question if this can have additional negative impact on safeguards, such as the difficulty to operate other valves.

PRD