Union workers walked out of three more U.S. refineries this weekend, including the nation’s largest fuel-making facility, expanding a nearly monthlong strike.

After the latest negotiations ended late Friday, workers at the Motiva Port Arthur Refinery in Texas, the nation’s largest, walked off the job just after midnight. They were followed a day later by workers at the Motiva Convent, Motiva Norco, and Shell Chemicals Norco plants. Motiva Enterprises LLC is co-owned by Shell and a subsidiary of Saudi Aramco.

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