The blast happened in the generator room of the Airgas-Norpac Inc. plant at 7700 14th Avenue S. about 1:30 p.m.
Firefighters rushed to the building, but the explosion had been contained by blast-proof safety equipment and there was no fire, officials said.
The building was evacuated and firefighters vented flammable gas from inside before allowing employees to return.
Airgas, Inc. spokesman Doug Sherman said that all gas has been shut off and there is no danger to the surrounding community.
He said the company has been operating at the South Seattle location for about 30 years.