Thanks to higher oil prices and favorable spreads in the crude market, the nation’s largest oil refiner has been on the move. In March, Valero agreed to buy Chevron’s Welsh refining operations for $730 million, a deal that will increase its oil output 220,000 barrels a day.The company also is stepping up production at a number of its U.S. refineries, a big reason why analysts anticipate double-digit earnings growth in the years ahead. After reporting a big loss in 2009, in 2010 the company swung to a profit. Investors approved: Valero stock is up almost 41% in the past year. —P.N.