Investigation of a pyrophoric iron fire in a Visbreaker fractionation column provides better cleaning work procedure

•Pyrophoric deposits may be present between tray and coke layer.•Coke renders pyrophoric iron water proof and provides combustible material.•Chemical decontamination of internals not a panacea in coking service.•Distillation column in coking service safe cleaning priorities may differ from those dictated by ease or speed.•Visbreaker/coking service column cleaning priorities should be re-evaluated.

An unexpected ignition of pyrophoric iron sulphide deposits took place in the fractionator of a Visbreaker unit. The distillation column had been chemically decontaminated prior to the event. The investigation of the event leads to conclusions concerning chemical decontamination limitations in vessels in coking service. Based on the lessons learned from this case, a prioritization of manual vessel cleaning works in coking service is proposed.