(Adds details about outage, background on unit, paragraphs 4-9)
HOUSTON, Aug 13 (Reuters) - The discovery of leaks in piping, which could pose a fire hazard, caused Saturday's shutdown of the largest crude distillation unit at BP Plc's 413,500-barrel-per-day (bpd) refinery at Whiting, Indiana, two people familiar with plant operations said on Thursday.
The leaks were found in overhead piping leading to heat exchangers on the 240,000 bpd CDU called Pipestill 12, said the sources, who requested anonymity because the information is not yet public.
The leaks may be in piping throughout Pipestill 12, which is configured to run crude oil from Canada's tar sands fields in Alberta. BP was continuing inspections on Thursday to determine the extent of the leaks.
Currently, repairs are expected to take at least a month, sources have told Reuters. But until the inspections are finished, it will not be clear how long it will take to repair damaged piping, the sources said on Thursday.
BP is also trying to determine if Pipestill 12's piping is more susceptible to corrosion from the crude or other substances running through the unit, the sources said.
BP continues to run the refinery at about 165,000 bpd or 40 percent of its rated capacity as two smaller CDUs remain in operation.
Pipestill 12 was the centerpiece of a $4 billion refit of the refinery to run more sour crude from Alberta's tar sands fields. The rebuilt Pipestill 12 came online in July 2013.
BP sold off refineries in Texas and California in 2013 to focus its U.S (Other OTC: UBGXF - news) . refining operations on plants with easy access to Canadian crude. Whiting is BP's largest U.S. refinery.
Crude distillation units do the initial refining of crude oil coming into a refinery and produce feedstocks for all other units. (Reporting by Erwin Seba; Editing by Meredith Mazzilli and Frances Kerry)