Exxon Mobil Corp’s (XOM.N) push to persuade workers at its Beaumont, Texas refinery to sign a five-year contract, nearly twice as long as the last one, is part of an effort to avert labor stoppages during a possible expansion that could make it the largest such plant in the United States, sources familiar with refinery operations said. As Reuters reported in July, before crude prices plunged, Exxon is evaluating a multibillion-dollar expansion to lift the refinery’s processing capacity to between 500,000 and 800,000 barrels per day from 344,600 bpd now.