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.
Maintenance work scopes must be adhered to. If added work is approved, it must be tracked and reviewed to ensure that the equipment is inspected by operations personnel. Scheduled work and extra work should be listed and checked off for proper line-up.
When closing valves, ensure that they are closed completely. Bull plugs must be installed tight enough so that they will not loosen as a result of contraction/expansion occurring during hot and cold cycling and/or compressor vibration. When closing a valve, a valve wrench must be used to synch up by using force to turn the valve handle a 1/4 turn. When installing a bull plug, a wrench must be used to ensure that the bull plug is tight.
The recommissioning process used to reactivate the gas oil hydrotreater unit was modified to address bull plugs. Involve plant operations and health/safety personnel to develop an operational mechanical integrity checklist to ensure that all mechanical equipment components are addressed and then train personnel to use the checklist.