Essar operates a 400,000-barrel-per-day refinery at Vadinar in the western state of Gujarat.
"Our smaller CDU will go under maintenance shutdown in the fourth or the fifth week this month for about seven days. Our crude throughput during the maintenance period will be reduced by 350,000 to 400,000 barrels," the spokesman said.
However, industry and trade sources said the shutdown could last for about 10 days.
Essar has scheduled the shutdown to the later part of the month to take advantage of a planned cut in supplies of locally produced Mangala crude for about 10 days, industry sources said.
Cairn India Ltd, which operates an onland crude producing block at Barmer in Rajasthan, will shut its Mangala processing terminal for 10 days from Aug. 21 to add new units, they said.
No immediate comment was available from Cairn India.