[topher]

This article claims that diesel engines were the source for other refinery fires:

Probe of Richmond refinery fire looking at spark sources

From that article:

The investigator said diesel-powered trucks and generators have been ignition sources at other large refinery fires, including at BP's Texas refinery explosion that killed 15 workers and injured dozens more.

Depends how credible this source is...

[philman_36]

Depending on what the vapor cloud consists of the heat from an exhaust manifold is enough to light off the cloud.

If the cloud is comprised of vapors requiring a high ignition point, such as a spark, then you rule that out.

Then you turn to the equipment and vehicles. Maintenance, static lines (if required), location at time of incident...

It's easy to lie in the reports to minimize lawsuits and it's hard to get a "stray" spark from an engine.

I would be more inclined to believe the cloud was released and expanded in volume and then was pushed by the wind and it advanced with no warning until it hit a welder striking an arc or somebody throwing sparks from grinding on something.

That particular plant has a "bad history" for me. And if it was the same management/people when I worked there from before BP took over...almost 200 hands don't walk off the job for nothing.