the main object of this paper is to evaluate safety barriers intervening against overpressure implemented on a crude oil heater using layers of protection analysis approach suggested in IEC 61508 (International Electrotechnical Commission) Standard for the determination of safety requirements are illustrated. Accident scenarios are pre-identified using Hazard an Operability approach, Fault tree approach is required for an effective risk assessment process. In order to better appreciate accident scenarios, PHAST (Process Hazard Analysis Software Tool) is utilized to simulate them.