UPDATE 3-Fire-hit Chevron Calif crude unit seen shut 6 mths-IIR

* CDU may be shut 4-6 months after extensive damage-IIR Energy

* Refinery sources last week expected 3 month shutdown

* Chevron gives no timeline, says other units operating

NEW YORK, Aug 13 (Reuters) - The sole crude distillation unit at Chevron Corp's large Richmond, California, refinery may be shut for at least four to six months after last week's fire, industry intelligence group IIR Energy said in a report that suggests the outage may be twice as long as feared.

Investigators have found "extensive damage" in the cooling towers, pipe racks and heater tower, IIR Energy told clients on Friday.

Neither Chevron nor investigators have given any timeline for resuming full operations at the 245,000-barrel-per-day (bpd) plant, California's second largest, following an Aug. 6 fire at its crude unit that caused a spike in regional gasoline prices.

A Chevron spokesman on Monday reiterated only that the No. 4 crude oil unit -- the only one at the plant -- was still shut down while other parts of the plant were operating.

Sources at the refinery said last week the unit could be shuttered for up to three months. The fire erupted at a pump leaking a substance similar to diesel, according to a preliminary report the company filed with county pollution regulators last week.

IIR Energy, which employees hundreds of researchers to gather operational information, first hand, about industrial facilities across the world, said that Chevron had not yet decided whether it would continue to operate some secondary units that are running down on-hand feedstock inventories.

Chevron could buy intermediate feed from the market, although in most cases it is difficult for a refiner that has shut down its crude unit to secure enough of this type of oil to keep producing motor fuels at the same rate.

Last week, the plant was producing gasoline at over 50 percent capacity, sources said.

The fire damaged the only atmospheric tower on the CDU, the unit that begins converting crude oil to motor fuel. The CDU makes feedstock for all other units at the refinery.

The blaze, which broke out as workers and the refinery's fire department were evaluating a leak at the pump, sent 949 area residents to an area hospital complaining of respiratory problems and eye irritation among other symptoms.

The U.S. Chemical Safety Board announced on Saturday it will pursue a full investigation of the Aug. 6 fire.

The CSB said workers were attempting to stop a leak of gas-oil, an intermediate feedstock used to make gasoline, from a pipe connected to the crude distillation unit when the leak intensified, triggering a vapor cloud that exploded.

"Witness testimony collected by CSB investigators indicates that a large number of workers were engulfed in the vapor cloud," said CSB team leader Dan Tillema.

"These workers might have been killed or severely injured, had they not escaped the cloud as the release rate escalated and the cloud ignited, shortly thereafter."

One issue the Board will examine is why that portion of the pipeline which leaked was in kept in service after at 2011 maintenance overhaul of the refinery.