A hydrogen leak occurred from a 1-inch gate valve on a makeup gas line in an oil refinery gas oil hydrotreater unit. When the leak was discovered, the gas oil hydrotreater unit shutdown procedures were immediately implemented and emergency response was requested. The refinery response team along with county response teams responded, and after approximately 1/2 hour, the gas oil hydrotreater unit was fully shut down. Shutdown consisted of sufficiently depressurizing the unit and adding nitrogen to allow safe closing of the leaking 1-inch gate valve and installation of the associated missing bull plug.

During this event, the 1-inch gate valve was found to be open roughly 10% with no bull plug in the valve, allowing the hydrogen to leak to the atmosphere. In addition, a 1-inch bull plug was found lying on the ground below the gate valve. This hydrogen leak vented and released approximately 133 pounds of makeup gas (~97% hydrogen) to the atmosphere, where it quickly dispersed. P

rior to the hydrogen leak occurrence, the gas oil hydrotreater unit and associated valve had operated at pressure (350 psi) for at least 36 hours. The gas oil hydrotreater unit involved in this incident had been recommissioned 18 months earlier after a company ownership change occurred and had undergone recent maintenance and testing prior to activation for operation. A post-event investigation determined the likely cause of the hydrogen leak was that the 1-inch gate valve was not completely closed prior to the start of operation. The investigation determined that a possible scenario for the loss of the valve bull plug was expansion and contraction during hot and cold cycling of the unit combined with vibration from a nearby reciprocal compressor over the 18 months since recommissioning.

The hydrogen leak from this event caused no hydrogen fire/explosion, no personnel injuries, no damage to the gas oil hydrotreater unit, no adverse environmental consequences, no offsite impacts, nor any significant threat to human health.