It is important to investigate the oil vapor leakage and emission from external floating-roof tanks for the loss evaluation and potential danger control. The oil vapor leakage from the annular space between the rim and the tank shell is one of the major sources for the evaporation loss. The vapor leakages from the rim of the single-tray floating roof are simulated numerically using FLUENT and measured experimentally. The effects of tank diameter

roof level and wind speed v on the pressure distribution above the roof surface are investigated. The results are as follows. (1) Numerical results agree with the experimental data. (2) When the ratio of the distance h between floating-roof and tank top to the tank diameter D

h/D

is greater than 1/4

the vapor is more likely to leak from the two opposite sides of the annular space perpendicular to wind direction and the side of the annular space in the upwind. (3) Tank diameter has little effect on the pressure distribution above the roof surface

while h/D and v affect the pressure distribution. Larger h/D lowers the wind pressure of roof center

and smaller h/D lowers the upwind pressure above the roof. When h/D varies from 1/20 to 1/2

the upwind and downwind pressures are negative

and with the decrease of h/D

the differential pressure above the roof increases. API loss formula for external floating-roof tank is recommended to consider the effect of ratio h/D to improve the accuracy in loss evaluation.