By Irene Klotz

CAPE CANAVERAL, Fla., Feb 6 (Reuters) - NASA on Friday delayed the launch of the shuttle Discovery's mission to the International Space Station by three more days, to Feb. 22, to allow more time to test potentially troublesome fuel valves.

The year's first shuttle mission had initially been planned for next Thursday, but was already delayed by a week to research issues with valves that keep the shuttle's fuel tank properly pressurized during the 8 1/2-minute climb to orbit.

A small piece of one valve broke off during the last shuttle launch in November. Although it posed no risk to the ship or its crew, NASA wants to check what would happen if a larger piece should chip off and become lodged in the lines that flow hydrogen back into the fuel tank for pressurization.

"We've put together a plan for testing that we didn't have the other day," said NASA spokesman Kyle Herring, explaining the decision to delay the launch further.

NASA plans eight more flights to the space station to complete construction, which has been under way for more than a decade. One final mission to service the Hubble Space Telescope also is planned before the shuttles are retired in 2010.

The station is a $100 billion project of 16 nations. Discovery's mission is to deliver its final set of power-producing solar wing panels. (Editing by Jane Sutton and Peter Cooney)

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