Gas mixture release at a refinery

The incident occurred at the hydrocracker unit of a refinery. A mixture of hydrogen and hydrocarbon gas was accidentally released. The flammable gas formed a vapour cloud that ignited, resulting in an explosion and fire.
DETAILED SEQUENCE of EVENTS
(i) After a heavy rainstorm, the flow to the hydrocracker unit’s flare began increasing. The operators found that the emergency pressure-relief valve of the unit’s cold separator had malfunctioned and remained open, allowing flammable gas to flow into the flare system.
(ii) The plant manager gave the onsite managers the verbal approval to proceed with immediate actions to stop the flaring by reseating the safety device.
(iii) This was done by trying to close and seal the malfunctioning emergency pressure-relief valve by incrementally closing the inlet (upstream) isolation valve to lower the inlet pressure to the safety device. The procedure foresees that if the emergency pressure-relief device successfully closes (reseats), its inlet valve is reopened, which returns the safety device to its protective function.
(iv) One of the onsite managers worked with two field operators to perform the reseating activity while the board operator monitored the system pressure from the control room.
(v) The personnel raised safety concerns related to accessing either of the two 6-inch inlet valves. It was therefore decided to close the 20-inch outlet valve on the downstream side between the emergency pressure-relief valve and the flare system instead of trying to close one of the inlet valves.
The personnel and the manager did not recognise the hazard related to closing the outlet valve instead of closing one of the inlet valves. The two 6-inch inlet valves were designed for high-pressure conditions (2,470 psi, 170 bar). The 20-inch outlet valve to the flare system was rated for 275 psi (20 bar). Closing would subject to a pressure of about 2,100 psi (145 bar) from the flammable vapour flowing from the cold separator.
(vi) When the team had the outlet valve about 90% closed, the valve failed, releasing a high-pressure mixture of hydrogen and hydrocarbon vapour. The vapour cloud ignited, resulting in the explosion and fire.
Valero reported that about 49,000 pounds of the hydrogen and hydrocarbon mixture were released.
The consequences were a seriously injured worker and approximately $5.15 million in property damage.

The INTIATING CAUSE was the malfunctioning of a pressure-relief valve.
CONTRIBUTING FCTOR was the storm before the leak of the relief valve.
The ROOT CAUSE of the release, fire and explosion was probably the consequence of unclear procedures and a wrong human decision.
The CSB reports that the managers used the refinery’s “management of change process for isolating a safety device” to document Valero management’s approval for the operations team to perform the urgent reseating activity. However, after discussion in the team, they decided to use a valve which could not bear the pressure.

Pressure Relief Valve

The night before a heavy rainstorm had occurred. The CBS report does not explain the correlation between this event and the following series of events.

About 49,000 pounds of the hydrogen and hydrocarbon mixture were released.
The sources do not report the same values for the injuries. OHSA reports two, without hospitalisation, CBS only one sever injury.

In this case, a procedure was available, and a plant management of changes in place, to deal with the malfunctioning of a pressure relief valve. However, the procedure was still leaving a great amount of flexibility in its execution. This brought to the team decision to activate a valve which failed, instead of the more logic valves which could bear the pressure. The CSB reports that the decision was taken based on safety concerns related to accessing those valves. No details are given on these concerns, it could have to be related to the increased flow to the flare and the specific location of the valves.
Facts is that the managers on site did not recognise the risk of relying on the chosen valve. The consequence is that an incident with injuries occurred, by trying to address safety concerns. The occurrence highlight the need of a detailed hazard assessments, of robust training of onsite supervisors and managers and of procedure able to assist decision-making.