The Soudersâ€“Brown equation was named after Mott Souders and George Granger Brown, which is used to find the maximum allowable vapor velocity in vapour-liquid separation vessels. Use this online Souders-Brown equation calculator to calculate the apparent porosity for the given values of liquid density, vapor density and vapor velocity factor. Apparent Porosity (% AP) is defined as the open pore volume against the Bulk Volume expressed as a percentage.