Hydraulic Diameter Assessment

[ChihebG]

While calculation the corrosion rate of a crude oil pipeline with the deWaard prediction models, I have to use the hydraulic diameter in the calculations.
How can assess the hydraulic diameter knowning that I have all inputs (Temperature, Pressure, Velocity, Fluid contents...)?

Regards,

 

[bimr]

http://www.engineeringtoolbox.com/hydraulic-equivalent-diameter-d_458.html