API plan 31 uses a cyclone separator that taps a small portion of discharge line process liquid for mechanical seal cooling process. Despite its simplicity, this piping plan requires comprehensive designing process to ensure its performances (flow rate to mechanical seal and pressure drop) are satisfactory. In this research, API 31 cyclone separator geometrical parameters (length cylinder, conical and vortex finder and radius overflow and underflow) effects onto pressure drop and seal flow rate are investigated using numerical method via ANSYS FLUENT software. It is found that increasing overflow and underflow radius and length of cylinder and conical decrease pressure drop by 15.94, 24.82, 20.30 and 11.42 respectively. Increasing vortex finder length meanwhile increases pressure drop by 7.77. Meanwhile, increasing radius overflow and length of vortex finder increase flow rate to seal by 43.79 and 9.63 respectively. Increasing radius underflow and length of cylinder and conical reduces flow rate to seal by 71.92, 6.41 and 8.93 respectively. It is also observed that a generalization of a particular cyclone separator geometry is not always applicable as it depends on the cyclone separator base design itself where in this case all geometrical factors are significant to pressure drop and flow to seal. © 2021 THE AUTHORS