Abstract: At approximately 11:30 p.m. on November 4, 1990, Employee #1, an assistant operator in the hydrocracker unit (HCU) of a refinery, was manually opening a bypass valve to increase the flow rate in the unit. The bypass was being used because the second-stage charge motor control valve was not working properly. As Employee #1 was opening the bypass valve, he heard a "whooshing" sound in the unit, to his west. A heat exchanger, identified as E2314, ruptured on the shell side, releasing vapors that were ignited by the first-stage prefeed heaters. Employee #1 sustained first-, second-, and third-degree burns over 70 percent of his body. The shell side of the exchanger had thinned to 0.1 inch, and failed at the normal operating pressure of 255 psi. The accident investigation revealed no connection between the failure of the heat exchanger shell and the operation of the manual bypass valve.