Abstract
A hydrogen vent is one of the fundamental equipment that ensures safe operation of hydrogen systems. Its venting direction could significantly affect the hydrogen concentration in the surrounding air. As the density of hydrogen is small compared to air, a hydrogen vent is typically installed away from personnel exposure area, which minimizes safety risk during venting process. However, a vent exit with an upward direction has a risk of taking in rainwater. Thus, this article studies the effect of the vent exit directions in hydrogen system from safety perspective; the considered directions are horizontal, vertical downwards, and a miter cut. The analysis was carried out by numerical simulation, in which the effect of discharge speed, wind speed, and nitrogen concentration were considered. The nitrogen concentration was considered because the system may be discharged with nitrogen gas before operation, some left-over nitrogen gas in pipelines may mix with hydrogen at the beginning of system operation. The safety margin in the study was taken as 25% of hydrogen lower explosive limit (LEL), which is 1% volume concentration of hydrogen. The results show that the vertical downwards vent results in the greatest risk distance. In contrast, risk distance is the smallest with horizontal vent. Compared to pure hydrogen discharging, hydrogen-nitrogen mixture gas would increase the risk distance. The distance reaches maximum for horizontal, vertical downwards, and miter cut when the nitrogen concentration is 95%, 75%, and 55%, respectively. The maximum distances are 0.8m, 0.5m, and 0.2m, respectively. It can be observed that the wind speed significantly affects the hydrogen diffusion at the vent exit, in which the increasing amount of vertical downwards vent is greater than horizontal vent at level 1 wind speed (0.3 m/s). The risk distance decreases at level 2 wind speed (1.6 m/s) because the hydrogen concentration is reduced due to diffusion. Among the studied vent exit direction, the horizontal vent has the best overall performance, while vertical downwards vent has the worst overall performance.
