A metal hydride storage system was refilled using compressed hydrogen in a closed lab environment. The tank system is an in-house development and is optimized for high hydrogen storage density and use with an air-cooled fuel cell. The system is equipped with a pressure relief valve that opens gradually at 35 bar to protect the tank from overpressure conditions. The tank itself is designed to adsorb 400 g of hydrogen at a pressure less than 15 bar.
For refueling, the secondary pressure on the compressed hydrogen supply container was set to 20 bar and the adsorption of the hydride was started without hydrogen flow limitation. Due to the exothermic nature of the hydride upon recharge, as expected a sharp increase in tank temperature was measured. The tank was uncooled because the temperature increase leads to an increase in plateau pressure, which automatically reduces hydrogen uptake.
After 3 min 30 sec, the overpressure device was triggered and hydrogen was released into the lab. The hydrogen supply from the compressed gas cylinder was cut immediately and the window was opened. The operator was on site at the time of the incident. The amount of hydrogen released is hard to quantify, but we estimate something in the range of 200-1,000 normal liters. The overpressure device closed after some minutes.
Analysis of the incident, which should theoretically not have happened, revealed the following. The used pressure reducer was made for 200 bar input pressure and 0-200 bar output pressure. The pressure gauge on the secondary stage was imprecise at the intended charging pressure of 20 bar. Checking with an electronic pressure gauge revealed a pressure of roughly 32 bar. Upon charging, the so-called "Supply Pressure Effect" (e.g., increase of secondary pressure with reduced primary pressure) of the pressure reducer led to an increased secondary pressure, which exceeded the limit of the pressure relief device. Also, some influence on the pressure due to the increased temperature cannot be ruled out, although that would be related to hydrogen in the gas phase rather than in the hydride.
As corrective steps, the charging pressure was measured with a digital pressure transducer instead of a mechanical gauge. The charging pressure was limited to 15 bar to slow down the charging.
Mechanical pressure gauges tend to be imprecise if only used in a narrow portion of the full scale. Digital transducers, although slightly more expensive, offer much more precision. The event happened because the set pressure was only 10% of full scale, and the error of the mechanical gauge was over 5%.
While this is not the reason for the event described, control of the charging pressure is one of the most crucial parameters. Although the storage is at low pressure, the pressure increase upon temperature increase can be much steeper than the ideal gas law would predict, depending on the charging conditions.
The chosen trigger point of 35 bar for the pressure relief valve is very low. Based on the tank design, at least 60 bar would be acceptable.
Additional discussion about working with reactive metal-hydride materials in the laboratory can be found in the Lessons Learned Corner on this website and in the Hydrogen Safety Best Practices Manual.