An explosion rocked a west Texas refinery Friday, injuring at least 19 people and sparking a blaze that sent a huge black cloud billowing into the sky.

Three people with burns were in critical condition Friday night, a nursing supervisor said. Sixteen others were hospitalized, mostly for smoke inhalation, and all but four were released by late Friday, a hospital official said.

No fatalities were reported and all employees at the Valero McKee Refinery were accounted for, said Valero Energy Corp.

More than 400 workers were evacuated from the Valero McKee Refinery after the explosion, authorities said.

Paul Jenkins, fire chief in the nearby community of Dumas, said Valero's fire brigade was fighting the blaze. Although the blaze had subsided substantially, it had not been extinguished late Friday, Valero said in a statement.

"The effort has been to isolate the units ... so there's nothing feeding the flames," said Mary Rose Brown, a Valero spokeswoman.

The refinery was shut down after the blast, as were pipelines in and out of the facility, the company said. Smoke could be seen from 60 miles away.

Valero said the fire was believed to have started at the refinery's "propane deasphalting unit," where fuel is processed at high temperatures.

Valero, the largest refiner in North America, operates 17 refineries, 16 in North America and one in Aruba. The McKee refinery, located in Sunray, is one of six in Texas and has a capacity of 170,000 barrels per day. Sunray has a population of about 2,000.