Abstract: At the gasoline blending unit of an oil refinery, a new product lineup was performed and the equipment was started. Overpressure developed in the product line and was relieved through a pressure relief system, but it was unable to contain the hydrocarbon volume with associated loss of containment. Seventy-three employees were tested for benzene exposure, with 72 negatives and one false positive. The predominant factor leading to the incident resulted from a temperature increase with pressure increase in the downstream body of a twin seal valve, causing the guide pin to shear upon opening. The valve appeared to be open but was closed.