Here are the references for the equation I gave above:
Druand, A.A & M. Marquez-Lucero, "Degermining Sealing Flowrates in Horizontal Run Pipes", Chemical Engineering, March 1998.
Another source is:
Sandler, H.J. & E.T. Luckie-wicz, "Practical Process Engineering", McGraw Hill, 1987
And as katmar stated, the diameter is in inches and the flow is in USgpm.