A liquid hydrogen tank’s rupture disc failed prematurely, which caused the tank to vent its entire gas contents through the tank’s vent stack. Venting was very loud and formed a condensed moisture cloud visible from the top of the stack. Liquid air was also visible coming off the stack. Venting ceased after approximately 5 minutes. On-site staff called the fire department, which arrived promptly and evacuated the area. Normal operations resumed after the Fire Department was able to determine there were no unsafe conditions.
A redundant safety circuit was put into service and the tank returned to normal operation after the failure. Both rupture discs were replaced and the tank was inspected.
Important to note that the safety system functioned as intended. Most prevalent learning is that liquid air will form and fall off the vent stack due to very high flow of very cold gas when the relief systems are flowing at capacity. Liquid air may also splash off flanges or other pieces and will cause a small vapor cloud as it falls from the stack. Falling liquid air was mistaken for liquid hydrogen during the event.