Two employees of a Signal Hill oil refinery were seriously burned when a tank of crude oil exploded, but the fire was quickly contained and did not spread to an adjacent Walnut Avenue elementary school, a county Fire Department official said. Michael Miller and Michael White, both 30, were preparing to weld a heater at the MacMillan Oil Co. refinery when the blast occurred. Both were treated at St. Mary Medical Center in Long Beach for second- and third-degree burns and then taken to the burn unit at Torrance Memorial Hospital. White was burned over 75% of his body; Miller over 35% of his. Cause of the fire was not immediately known and no damage estimate was available, Fire Department spokesman Rob Smith said.