REDONDO BEACH, Calif. (AP) — 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. The resulting fireball injured four contractors, destroyed a large part of the refinery and sent a fine white ash raining down on nearby homes and cars.