Shell spokesman Ray Fisher said operations at the Convent refinery were stable.
The 92,000-bpd gasoline-producing Fluidic Catalytic Cracking Unit (FCCU) was shut eight days ago when a compressor developed excessive vibration, the sources said.
The FCCU was just coming back into production after being shut for a planned overhaul from May 30 until Aug. 9.
The FCCU uses catalyst under high heat and pressure to convert gas oil to gasoline.
(Reporting by Erwin Seba; Editing by Tom Brown and Rosalba O'Brien)