An explosion at an ExxonMobil oil refinery in California last year could have been prevented if managers had taken into account aging safety equipment and shut down a key, spark-generating part of the refinery before attempting repairs elsewhere, federal investigators said Wednesday. The unit being repaired had operated for five years without a maintenance overhaul and, as a result, a key valve failed and leaked volatile hydrocarbons into an electrostatic chamber where the material combusted, the U.S. Chemical Safety and Hazard Investigation Board said.