Heavy-duty vehicles (HDVs) and trains using hydrogen as fuel have significantly larger tanks than smaller fuel cell vehicles (FCVs). The requirement for depressurization time in case of fire may often be the same, thus there may be a need to dimension the thermal pressure relief devices (TPRDs) for significantly higher release rates than used in FCVs. This CFD-study assesses to what extent higher TPRD release rates than typically used for FCVs may be applied without compromising safety.
One conclusion from the study is that HDV TPRDs should be directed upwards rather than downwards to limit the size of reactive clouds and potential local risk to people around a release. It is further recommended to keep TPRD release rates below 200 g/s as the study indicated that explosion pressures would increase significantly for larger release rates. Risk for both releasing airbags of cars in motion throughout the tunnel and strong explosion wind pushing people in the tunnel off their feet may also increase if release rates above 200 g/s are chosen. For trains somewhat higher release rates may be tolerable if people remain inside the trains in an emergency, for people outside the trains a high explosion wind may be a concern for leak rates well above 200 g/s. For trains emergency venting times above 10 min could require additional fire protection to ensure that the tanks will survive an external fire.
Read more about these findings in Chemical Engineering Transactions