(Updates number of injured and details about collapse)

By Erwin Seba

HOUSTON, July 18 (Reuters) - Four workers were killed when one of the world's largest cranes collapsed at a Houston refinery on Friday afternoon, a refinery spokesman said.

Seven workers were also hurt when the crane fell at about 1:30 p.m. local time (1830 GMT) but only two remained in area hospitals, and their injuries were not life-threatening, LysondellBasell Houston refinery spokesman David Roznowski told a news conference late on Friday.

Production at the 270,000 barrel per day (bpd) refinery was not affected by the collapse of the 600-foot (182.8-meter) tall crane, Roznowski said.

All of those killed were employees of the crane's owner Deep South Crane & Rigging of Baton Rouge, Louisiana, he said.

Deep South said it had few details about the accident. "Our thoughts and prayers are with our employees and their loved ones," the company said in a statement.

The crane was at the Lyondell refinery on the Houston Ship Channel to lift a giant steel derrick from a coking unit, which squeezes the last refinable material from a barrel of crude oil, as part of a seven-week overhaul, Roznowski said.

A crude distillation unit, which does the initial refining of crude oil coming into a refinery, was also shut prior to the start of overhaul.

Lyondell took the unusual step of planning an overhaul in July, when most U.S. refineries historically run at high capacity to maximize gasoline production for peak demand in the summer driving season.

"There are only a few cranes of this size in the world," Roznowski said. "You do the work when the crane's available. It was all about the availability of the crane."

The crane was not lifting anything when the collapse happened, Roznowski said. A test lift by the crane had been done earlier in the week.

The crane fell on a "break tent" at the refinery where workers doing the overhaul work could cool off and drink water out of the July heat, Roznowski said.

The crane collapse in Houston was the deadliest U.S. crude oil refinery accident since a 2005 explosion at BP Plc's <BP.L> giant refinery in Texas City, Texas, killed 15 workers and injured 180 other people. Texas City is located 50 miles south of Houston.

Following a two-year investigation of the BP blast, the federal Chemical Safety Board recommended that refiners not place workers in structures near process units incapable of withstanding catastrophic explosions.

The fatalities in Houston follow two recent deadly crane collapses in New York in March and May that killed a total of nine people.

Neither the city of Houston nor the state of Texas performs safety inspections of giant construction cranes, the Houston Chronicle newspaper reported on Friday.

The names of those killed have not been released pending notification of their next of kin.

LyondellBasell is a Netherlands-based chemical and refining company with annual revenues of $45 billion and 16,000 employees worldwide. (Additional reporting by Anna Driver, Eileen O'Grady and Bruce Nichols; Editing by Clarence Fernandez)

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