(Adds comment, updates share price in final graph)
By Selam Gebrekidan and Scott DiSavino
March 31 (Reuters) - A pipeline within a liquefied natural gas facility exploded in a rural area of Washington state on Monday and emergency workers continued to work into the evening to minimize the risk of further blasts from a leaking storage tank.
Shrapnel from the explosion within the Williams Companies Inc facility caused a leak in one of two LNG storage tanks at the site near Plymouth in southeastern Washington, said Joe Lusignan, a spokesman for the Benton County Sheriff's office.
"It's still volatile; it's still under investigation," Lusignan said late Monday afternoon.
"Because of the potential for explosion, we are focusing on making sure that our citizens are safe," Lusignan said.
After viewing video footage of the damage taken by a robot brought to the site by the Washington State Patrol bomb squad, Lusignan said law enforcement and hazardous material personnel were preparing to go to the site to evaluate damage first-hand.
"They will assess the level of damage, how to repair it and what safety issues we still have for our citizens," he said.
Road blocks and evacuation orders remained in effect and may continue through the night, Lusignan said.
Five workers were injured by the blast, he said. One worker was burned; four were hit by debris and taken to local hospitals, he said.
Williams Companies said leaking natural gas has evaporated and there was no indication dangerous vapors were drifting into town, which has about 400 residents.
Williams spokeswoman Michele Swaner said the company is investigating. The cause of the explosion has not been determined. Emergency responders evacuated workers and residents in a two-mile (3.2 km) radius, officials said.
Each storage tank stands 133.5 feet tall and holds up to 1.2 billion cubic feet of natural gas. Together, the Williams tanks can hold enough gas to meet nearly 3.4 percent of typical daily U.S. natural gas demand. Williams said each tank was around one third full before Monday's explosion.
An earlier fire at the site has been extinguished, but Lusignan said an ongoing gas leak could still ignite. Responders said they were containing liquid leaking from the tank.
Local media reported that the initial explosion at the gas storage site, which occurred around 8:22 a.m. Pacific time (1522 GMT), could be heard from 20 miles (32 km) away.
Experts played down the possibility of a natural gas vapor cloud formation that could explode.
"LNG is not as dangerous as some people think. When it is in a liquid state it can't ignite. In a gaseous state it mixes with air," said Kent Bayazitoglu, an analyst at Gelber & Associates in Houston. "The most likely remaining concern is if the gas is trapped" on site.
LNG accidents are rare, but Monday's incident could be held as an example of safety risks by groups opposed to building new U.S. LNG facilities, including export plants.
An explosion at a government-owned Sonatrach LNG facility in Algeria killed 27 workers and injured many more in 2004.
Companies hoping to build new U.S. LNG plants say safety technology, including the building of dams around storage tanks, has improved in recent years.
Williams said it has shut the connections from its main line, called the Northwest Pipeline, to the Plymouth facility.
The 3,900-mile-long (6,276-km-long) Northwest Pipeline, which delivers gas to several Western States, is still operating, Williams said.
The incident had no discernible impact on U.S. natural gas prices, which fell Monday on the West Coast.
Shares of Williams fell 34 cents, or 0.8 percent, to $40.58 on the New York Stock Exchange, while shares of most other energy utilities rose.
(Reporting by Selam Gebrekidan, Scott DiSavino and Joshua Schneyer in New York, Eileen O'Grady in Houston and Timothy Gardner in Washington D.C.; Writing by G Crosse, Grant McCool and Richard Chang)