Fire alarm and gas detector system (FGS) is the important safety barrier to prevent gas leakage. Combustible gas leaks and forms combustible gas cloud mixing with air

and its explosion belongs to volume explosion. The gas cloud has complexity and variability subject to a variety of factors

so it is converted into equivalent gas cloud and the threshold size calculation method is proposed. The threshold is key input index to realize the quantitative layout of detector network of FGS. The control room is selected as a receptor for analysis. Based on the critical value of shock wave overpressure which takes load of control room

the corresponding equivalent gas cloud size is calculated as the detecting threshold of combustible gas cloud by using multi-energy method in reverse order. Then the critical detecting time of the FGS is calculated by using equivalent air cloud computing method and Gaussian diffusion model. Through one LNG tank industry case analysis

the maximum cloud size which can be carried by the industry and the critical time of diffusion can be quantitatively determined. The numerical calculation shows that the equivalent gas cloud size threshold can not only be used as the quantitative input indicators for detector design of FGS

but also provide theoretical support for the detection time setting and the prevention and control measures of gas leakage and explosion.