Welding the C-channel on the tank top using the arc welding technique increased the metal
temperature above the steel melting point of 2800 °F (1538 °C). The surface of the metal
would quickly get “red hot” as welding progressed, a temperature in excess of 1200 °F (650
°C), which was significantly above the VF vapor autoignition temperature of 725 °F (385 °C).
The accumulated VF vapor inside the tank would ignite as soon as it contacted the extremely
hot steel or if welding sparks entered the tank through the agitator shaft opening.