The three burn victims were treated at the Christiana Care emergency room and transferred to Crozer Medical Center in Upland, Pennsylvania for precautionary treatment.
On Tuesday, three workers received burns at the Delaware City Refinery.
Chief Deputy State Fire Marshal Robert Fox said the plant suffered no damage and no down time of equipment as a result of the incident.
"Further investigation revealed that what was originally described as an explosion was actually a release of heated product under pressure," Fox said.
The New Castle County 911 Center received the call Tuesday afternoon about an industrial accident with multiple burn patients.
The Delaware City Fire Company and the Christiana Fire Company, along with New Castle County Paramedics, transported three burn victims to the Christiana Care emergency room.
The burn victims were treated and transferred to Crozer Medical Center in Upland, Pennsylvania for precautionary treatment.
The State Fire Marshal's Office was called to the scene and conducted an investigation. Investigators determined that during maintenance of equipment there was a release of heated product under pressure that caused the injuries to the workers, Fox said.
The Department of Natural Resources and Environmental Control Police also responded to conduct an investigation.