INCIDENT DUE TO "WE DO NOT KNOW WHAT WE DO NOT KNOW"

 On April 10, 2020, at 12:46 a.m., a mixture of hydrogen and hydrocarbon gas was accidentally released within the Hydrocracker unit of a refinery/ The flammable gas formed a vapor cloud that ignited, resulting in an explosion and fire (Figure 1) that seriously injured one operator and caused approximately $5.15 million in property damage.
After a heavy rainstorm, the flow to the Hydrocracker unit’s flare began increasing. Operators found that although the unit’s cold separator was operating at normal working pressure, its emergency pressure-relief valve had malfunctioned and remained open, allowing flammable gas to flow into the flare system. Two managers at the refinery phoned the complex manager, who gave the onsite managers verbal approval to proceed with immediate actions to stop the flaring by reseating the safety device.
The company uses the term “reseating” when referring to its practice of trying to close and seal a malfunctioning emergency pressure-relief valve by incrementally closing the inlet (upstream) isolation valve to lower the inlet pressure to the safety device. If the emergency pressure-relief device successfully closes (reseats), its inlet valve is reopened, which returns the safety device to its protective condition.

Onsite managers used the refinery’s management of change process for isolating a safety device for approval for the operations team to perform the urgent reseating activity. One of the onsite managers—the operations supervisor—worked with two field operators to perform the reseating activity while the board operator monitored the system pressure from the control room. Operations personnel raised safety concerns related to accessing either of the two 6-inch inlet valves, resulting in a decision 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 hazard of closing the outlet valve instead of closing one of the inlet valves was not recognized. Although the two 6-inch inlet valves were designed for high-pressure conditions—2,470 pounds per square inch (psi), the 20-inch outlet valve to the flare system was rated for just 275 psi. Closing the 20-inch outlet valve would subject this valve to about 2,100 psi of pressure from the flammable vapor flowing from the cold separator, much greater than the valve’s 275 psi pressure rating.
When the operations team had the outlet valve about 90 percent closed , the valve failed—releasing a high-pressure mixture of hydrogen and hydrocarbon vapor into the surrounding air. The flammable gas formed a vapor cloud, which ignited resulting in the explosion and fire.

Source: CSB.gov