Identifying failure modes in an oil refinery process unit

Alkylation increases the high-octane outputs of an oil refinery and hence enhances the value of gasoline products. Alkylation reactions are catalysed by strong acids, usually hydrofluoric acid (HF) or sulphuric acid (H2SO4).

Safety had a high priority for the company in this case. The HF Alkylation Unit of the refinery was subjected to hazard studies on a regular basis, due to the dual hazards of flammable hydrocarbons and highly corrosive HF.

The most recent studies had used a Hazard and Operability (HAZOP) process. This study used Failure Modes, Effects and Criticality Analysis (FMECA) as a complementary approach that might identify additional hazards. The issues raised during the FMECA were recorded in a hazard register, which was reviewed regularly in management meetings to monitor the implementation of the recommended actions.

The study identified 225 hazardous failure modes, but only eight were rated ‘Undesirable’, requiring immediate attention. A further 59 were rated ‘OK with Controls’, to be reviewed and the controls strengthened if necessary. The remainder of the hazardous failure modes were rated ‘Acceptable’ or were considered to present only minor risk. These were not rated.

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