At 1:00 p.m., an employee was performing hot work (i.e. welding) on the dump pipe of a tanker truck when residual flammable vapours in the piping ignited. The flame propagated up the pipe and into the tank truck causing an explosion, killing 3 people (2 employees and a visitor to the site). The tanker truck delivered a load of petroleum containing water the previous day. However, flammable hydrocarbon vapours still resided in the tank car prior to the hot work, which subsequently contributed to the accident.