Tesoro Corp. refinery officials in Mandan said they expected to be back up and running Tuesday night after a loss of steam caused a shutdown earlier in the day.

Tesoro refinery spokesman Leif Peterson said the event happened about 7 a.m. Tuesday.

Peterson said the refinery, which has a production capacity of about 58,000 barrels a day, was in the process of replacing two of three burners on its boilers as a way to reduce emissions.

The refinery draws water from the Missouri River to feed boilers that produce steam to power the refinery.

Peterson said the steam also allows excess petroleum products to burn off more completely from the refining process.

He said workers were in the process of installing two temporary boilers when a malfunction occurred causing the refinery to trip, or shut down.

The shutdown resulted in a thick, black plume of sooty smoke that hung over the area, but did not present a health hazards, a state health official said.

Terry O'Clair, director of air quality for the State Health Department, said the refinery notified his department shortly after the refinery tripped Tuesday morning.

O'Clair said an emergency flare system designed to burn off gas and reduce pressure in the event of such an incident worked, but with the loss of steam, the gas burned off less completely than normal.

Other than looking bad, he said it caused no health risks because the flares are high enough in elevation.

"Aesthetically, it didn't look good," he said.

The refinery was able to resume producing steam a couple of hours after the shutdown, but Peterson said when bringing it back on line, start-up procedures need to be done sequentially and that would take between 12 and 18 hours.

He said engineers had an idea of what caused the shutdown but were still investigating Tuesday afternoon.

In layman's terms, he said, the loss of steam caused the shutdown.

"That's what caused the trip," Peterson said.

There were no injuries or equipment damaged, he said.

He said the pollution control project was scheduled to be completed by the end of October.

Peterson said the refinery was operating at near-capacity and had enough reserve supply on hand, so its customers would not be affected.

(Reach reporter Brian Gehring at 250-8254 or brian.gehring@bismarcktribune.com.)