WOODS CROSS (AP) — Holly Corp. plans to shut down its Woods Cross refinery for 26 days, a move that could tighten the gasoline supply in Utah.

The plant is shutting down starting next week for maintenance and expansion.

The refinery processes about 26,000 barrels of crude oil each day and produces about 15 percent of the state's gas supply.

"We're still in the peak driving season, so a shutdown now could well tighten our fuel supplies," said Rolayne Fairclough, spokeswoman for AAA Utah. "For refineries, though, shutting down for maintenance is just something that has to be done occasionally. It is part of doing business."

Holly Corp. officials say that after the shutdown, the refinery should be able to have 20 percent more capacity.

"We are approximately three months from beginning to realize the benefits of our Woods Cross refinery upgrades," said Matthew Clinton, Holly's chief executive.

Shutting down a refinery is a common practice.

"Refineries are complicated, hard-working pieces of equipment that handle some real caustic chemical processes," said Lee Peacock of the Utah Petroleum Association. "They just have to be taken down occasionally so you can be sure they operate at peak capacity."

The timing of the work often depends on the availability of outside contractors to do the work.

"There is a lot of demand for their services," Peacock said.

Expansion at the Woods Cross refinery includes construction of a new unit designed to break apart petroleum molecules so they can be more easily refined into gasoline, kerosene and diesel fuel.

The refinery will also be able to process more "black wax," a gooey kind of crude that comes out of many Utah oil patches. Thick and viscous, it usually needs to be transported by truck instead of a pipeline.

Holly's refinery in Woods Cross processes about 5,000 barrels of black-wax crude a day. Company officials said the upgrade will allow twice that amount to be processed each day.