A third, and largest, explosion occurred when a vessel containing butylene, isobutane, and butane violently ruptured. This explosion appeared to be a secondary event caused by the fire. The explosion caused one 38,000 lb. fragment of the vessel to fly over the Schuylkill River, and two other fragments weighing 15,500 lbs. and 23,000 lbs. to land onto other portions of the refinery site.