St. Clair Fire Chief Roy Dewhirst said a Chemical Valley Code 8 precautionary alert was issued at approximately 3:30 p.m.
Shell Canada spokesperson Kristina Zimmer said the incident happened around 3:30 p.m. and the company immediately issued a CVECO Code 8 warning.
A Code 8 Advisory is issued by a CVECO member company for "an internal non-emergency situation that may be noticed by the public."
Zimmer said that because of the steam outage, a number of their process unit were adversely affected causing increased flaring at both Shell’s refinery and its chemical plant.
She added that with this particular incident, steam venting occurred which resulted in the unusual noise that people heard that lasted only a few minutes.
Shell personnel took every measure to ensure all of the site’s employees were safe and accounted, said Zimmer.
Officials said Shell is now working on protecting the integrity of the process units and repair the affected units.