June 18 (Reuters) - U.S. refiner Phillips 66 said on Tuesday its margins have tightened after the expanded Trans Mountain pipeline project in Canada started up in May.

The $24.84 billion expansion has nearly tripled the flow of crude from landlocked Alberta to Canada's Pacific coast to 890,000 barrels per day (bpd).

"We are still exporting Canadian crude from the Gulf Coast, though that is the first thing to get trimmed back," Phillips 66 CEO Mark Lashier said during the J.P. Morgan Energy, Power & Renewables Conference on Tuesday. "It has tightened up those margins."

U.S. oil refiners and West Coast traders have flagged concerns about the quality of crude shipped on TMX, warning that high vapor pressure and acidity limits could deter purchases of Canadian heavy barrels. (Reporting by Sourasis Bose in Bengaluru; Editing by Krishna Chandra Eluri and Richard Chang)