How can one determine if a pipe is running full or is at its sealing flowrate?
The following equation gives a quick check to determine the sealing velocity for a pipe:
Q = 10.2 D^2.5
where:
Q is the liquid flowrate in gallons per minute
D is the pipe diameter in inches
If the current flowrate in the pipe is less than the value calculated for Q above, then the pipe is below its sealing flowrate or is said to be partially flooded. In order to calculate the velocity in this pipe you must use a set of flow area equation presented in Chemical Engineering magazine (March 1998, p. 129). The above equation is valid for liquid flow through a horizontal pipe.
Adopted from http://www.cheresources.com/chexpress/chexpress28.shtml