UPDATE 3-BP to start pressure testing Gulf well cap Tuesday

* If new system works, BP says no oil will leak

* Relief well on track to meet blown-out well by end-July (Adds quotes from briefing, professor)

By Kristen Hays

HOUSTON, July 13 (Reuters) - BP Plc (BP.L) (BP.N) said it will begin pressure tests at its undersea oil gusher in the Gulf of Mexico on Tuesday afternoon after it placed a new containment cap and seal on the wellhead late on Monday, the top official overseeing the spill said.

Kent Wells, senior vice president of exploration and production for BP, told reporters at 2:30 p.m. CDT (1930 GMT) the test would begin in "a couple of hours."

"It looks really promising right now," said Satish Nagarajaiah, professor in civil and mechanical engineering at Rice University in Houston.

Nagarajaiah said that even if the well integrity test fails, BP has options for containing the flow, including siphoning and the relief wells.

Retired U.S. Coast Guard Admiral Thad Allen said earlier that before starting the test, BP would shut down two oil-capture vessels and shut three escape routes for crude on the new stack as well as a hydraulic valve inside of it.

If the well can sustain up to 9,000 pounds per square inch of pressure for the first six hours of the 48-hour test, the cap could shut off all flow until a relief well kills the leak by mid-August, Allen said.

But if pressures linger for the first six hours at 6,000 psi or less, "we'll know we can't sustain that in the long run," he said.

Either way, BP will have a four-vessel oil-capture system in place by mid-July that can handle up to 80,000 barrels a day, he said. If the cap can shut off all flow, the vessels will be in place as a backup, Allen said.

Wells told reporters Tuesday morning that BP and government scientists would monitor well integrity test at "very minute intervals."

High pressures would indicate the wellbore is intact after the April 20 blowout, but low pressures would be a sign oil and gas are leaking out the sides, Wells said.

"Everybody hope and pray we see high pressures here," Wells said.

He said relief wells remain the sole means to permanently kill the leak. The first of those wells has another 30 feet (9 metres) to drill before inserting pipe and preparing to intercept the blown-out well by the end of July, Wells said. (Additional reporting by Alyson Zepeda and Chris Baltimore; editing by John O'Callaghan and Jerry Norton)