HOUSTON, Jan 13 (Reuters) - Valero Energy Corp on Wednesday began shutting the large diesel-producing hydrocracker at the start of a multi-unit overhaul at its 335,000 barrel-per-day (bpd) Port Arthur, Texas, refinery, people familiar with plant operations said. A Valero spokesperson did not reply to a request for comment on Thursday. The 57,000-bpd vacuum-gas-oil 943 hydrocracker (HCU 943) is scheduled to be completely shut on Sunday and the 75,000-bpd 1241 gasoline-producing fluidic catalytic cracker (FCC) will begin shutting down on Monday and will be out of operation until Feb. 22, when it is scheduled to begin restarting to return to production by Feb. 26, the sources said. Simultaneous with the FCC, the 45,000-bpd gasoline Gulfiner hydrotreater and a sulfur recovery unit (SRU) will also be shut, according to the sources. Valero also plans to shut the 16,900-bpd alkylation unit when the FCC goes offline, the sources said. HCU 943 is scheduled to restart between Feb. 12 and Feb. 19. The Gulfiner and SRU are scheduled to restart a few days before the HCU, the sources said. Valero plans to restart the alkylation unit by mid-March. The alkylation unit converts refining byproducts into components that boost the octane content of gasoline. Hydrotreaters remove sulfur from motor fuels in compliance with U.S. environmental rules. SRUs process the extracted sulfur.