The maximum allowable gas velocity determines the required vapor space area above the highest liquid operating level. One method used to determine the required vapor space uses a variation of Souders Brown equation. The method takes into account particle settling velocity and horizontal gas velocity by ratioing them to the distance each must travel per unit time.
The equation is expressed as:
Vh = horizontal gas velocity, ft/sec.
Vs = particle settling velocity, ft/sec.
t = time in seconds
L = length between inlet and outlet, ft
D = vessel diameter, ft
The equation is further modified to provide five feet for entrance and exit flow stabilization.
The allowable horizontal gas velocity is then expressed as:
Re-writing the equation into the form of the Souders Brown equation, it is expressed as: