The accidental release and ignition of flammable vapours in petrochemical facilities generate overpressure and drag load which can impact the safety of installation and people. The intensity of the blast loads depends on many influencing factors including congestion, geometry, type & amount of fuel, leak size, and points of ignition among others. Given the stochastic nature of these parameters, it is obvious that the design for accidental load must be determined using a probabilistic method. This paper discusses a methodology known as “explosion exceedance diagram” and draws on recent developments in vapour cloud explosion research to determine the design accidental load (DAL). A case study demonstrates the application of the method.