A deputy says the evacuation could last overnight.
Benton County, Wash., Sheriff Steven Keane says plans call for sending up a helicopter for a better look at the damage to the plant.
Keane says a Washington State Patrol bomb squad robot equipped with video also will be sent in, because of safety concerns.
The 8:20 a.m. blast at the Williams Northwest Pipeline plant in the town of Plymouth, along the Columbia River, also punctured a liquefied natural gas storage tank.
Across the Columbia River, the blast shook the home of Cindi Stefani.
"It was just a very loud boom," she said. "I looked across the river and saw a giant mushroom cloud and flames at least a couple hundred feet high."
Animals on neighboring farms were running around, she added.
"At that point we were pretty scared. I was thinking, 'We need to get out of here."
Deputies went door to door throughout the town of Plymouth evacuating about 200 residents in a 2-mile radius.
Buses were provided for those without cars, and a shelter was set up across the river in Oregon at the Umatilla County Fairgrounds. As part of the evacuation, Highway 14 and railroad tracks were shut down.
The liquefied natural gas tank has a capacity of 1.2 billion cubic feet, but it was not full, and only a relatively small amount of gas leaked from the rupture and spilled to the ground in a containment area, the sheriff said.
The fire was later extinguished.
"I think they are working on the concern that the gas that spilled out into the containment area is not going to be a threat to ignite in case there is a spark or something," Keane said.
A Williams spokeswoman in Salt Lake City knew of only one employee injured. That worker is expected to recover from burns, said spokeswoman Michele Swaner. All 17 or 18 company employees were evacuated and accounted for, she said.
Swaner said it was too early to determine the extent of the damage or the cause of the explosion.
Williams works with local emergency responders, Swaner said, "so if something like this happens, we both know exactly what to do."
A secretary with the Patterson School District, about 7 miles away, said it provided three school buses to help with the evacuation. Students are on spring break, secretary Rachelle Munn said.
According to the company website, Williams operates about 15,000 miles of interstate natural gas pipelines.
The Plymouth facility cools natural gas into a liquefied state, company spokesman Tom Droege said.