Four years ago this April, seven people died in a massive explosion at the Tesoro oil refinery in Anacortes. How and why it happened—and how to prevent it from happening again—remained in question until a 158-page federal report dropped in late January. Here’s what it uncovered.

 

Remind me: What exactly happened?
Late on April 1, 2010, seven Tesoro employees were tasked with restarting three heat exchangers that were down for servicing. Early the next morning, with the labor-intensive process still under way, an adjacent, in-service heat exchanger ruptured, releasing gas and liquid in excess of 500 degrees. The explosion could be felt 350 feet away, and the fire it unleashed burned for three hours.

Okay, let’s back up: What’s a heat exchanger?
Put simply it’s a large, pressurized tube that heats up fluid used in petroleum refining.

Why did it rupture?
For starters it was old. Like, made-in-1971 old. And it was made out of carbon steel, a material considered unfit for the job because the heating process degrades it. So over time the steel is lined with tiny fissures, which become microcracks, which become cracks, which can lead to a rupture.

Surely someone—Tesoro, the state, the feds—knew that was possible and was inspecting the heat exchangers.
Sorta. In 2009, Washington state’s Division of Occupational Safety and Health finished an inspection of the Tesoro plant, which found 17 safety violations but none involved the heat exchanger that ultimately failed. And although Tesoro itself had a process in place, it lacked rigor and relied on a predictive model of the effects of heat on the exchanger—the so-called Nelson curve—that federal investigators have deemed inaccurate.

In other words, it wasn’t a question of if an explosion would occur at Tesoro, but when.
Exactly.