US Lifts TransCanada Keystone Oil Pipeline Pressure Restrictions

May 3 (Reuters) - Pressure restrictions on TransCanada Corp's Keystone oil pipeline were lifted on Tuesday in a letter issued by U.S. pipeline safety regulators, a spokesman for the agency told Reuters on Thursday.

It was not immediately clear what current flow rates are, said Darius Kirkwood, a spokesman for the U.S. Pipeline and Hazardous Materials Safety Administration (PHMSA).

The company in November shut down its 590,000-barrel-per-day Keystone pipeline, which links Alberta's oil sands to U.S. refineries, after a spill in South Dakota and was ordered later that month to operate at reduced pressure.

Reduced flows on the pipeline had helped draw down inventories in the Cushing, Oklahoma, storage hub. A return to full capacity on the line is also expected to help relieve a bottleneck in the oil-producing province of Alberta, where increased output has run up against a shortage of pipeline and rail capacity.

The restrictions "really did have a minor impact on our throughput and so consequently, I don't anticipate seeing a tremendous increase in our throughput once it's lifted, based on some of the changes we've made already," TransCanada's head of liquids, Paul Miller, said on a conference call last week.

A spokesman for Calgary-based TransCanada could not immediately be reached for comment on Thursday.

"They've been moving historical volumes with DRA (drag-reducing agents). Just makes it easier for them to operate now," one U.S.-based crude trader said.

As of Dec. 1, pressure restrictions were in place only from the pipeline's Luverne pump station to Ludden pump station and on the segment that had the leak between Ludden to Ferney pump stations, near Amherst, South Dakota, the PHMSA letter showed.

On April 3, TransCanada requested that the remaining pressure restrictions on the pipeline be lifted and PHMSA approved the request effective on Tuesday.

(Reporting by Devika Krishna Kumar in New York; Editing by Grant McCool and Peter Cooney)