* Relief well could intercept leaking well by end July
* Cap switch progressing
* New oil-capture vessel to start up late Sunday (Adds detail, quotes, byline)
HOUSTON, July 11 (Reuters) - The first of two relief wells is expected to intercept BP Plc's BP.LBP.N blown-out well in the Gulf of Mexico by the end of July and the leak could be plugged by early to mid-August, a BP executive said on Sunday.
“The anticipation is that toward the end of July is when we’ll be able to intercept the Macondo well,” Kent Wells, senior vice president of exploration and production for BP, told reporters.
Wells explained, however, that intercepting the well is only the first step to killing the leak and BP still expects to plug it with heavy drilling fluid and cement by early to mid-August.
Wells also said that BP made progress on a delicate undersea operation to install a new containment cap on the leaking wellhead, and planned to activate a new oil-siphoning system later on Sunday.
The new oil-capture system, a rig called the Helix Producer, had gone through all necessary startup procedures and was expected to begin siphoning oil, Wells said.
The new cap and startup of the Helix Producer are critical steps toward an upgraded oil-capture system with four vessels that can collectively handle up to 80,000 barrels of oil per day (3.4 million gallons/12.7 million liters), according to BP and the Coast Guard.
BP expects to have that four-vessel system in place within two to three weeks, Wells said.
BP on Saturday removed a cap on the well and began working to install the bigger cap and seal.
“If everything goes extremely well, we’ll be on the short end of four to seven days” to install the new cap, Wells said.
When fully operational, the new cap will capture virtually all of the oil spewing from the well, according to retired Coast Guard Admiral Thad Allen, the top U.S. oil spill official.
Meanwhile, oil is spewing mostly unchecked from the well. Another rig, the Q4000, is siphoning and burning off about 8,000 barrels a day from the leaking well, Wells said.
U.S. government experts have pegged the leak’s flow at up to 60,000 bpd (2.5 million gallons/9.5 million liters). (Additional reporting by Chris Baltimore and Lesley Wroughton, Editing by Sandra Maler and Alan Elsner)
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