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Real fluid has viscosity. When fluid flows along a solid surface, the relative velocity between fluid and solid is zero on the solid surface. Getting away from the surface, velocity increases and shearing stress comes into being and forms a kind of friction. Fluid in a pipe losses pressure head when the friction happens as well as the pipe section area changes its size or the flow changes its direction. What is more, inlet box and outlet box will affect the results greatly together with boundary conditions. Namely, pressure drop is divided into 3 parts. They are inlet drop, outlet drop and inner drop of the pipe. Although there are many empiric formulas to estimate the pipe head loss and they are very easy to use, their accuracy is not high enough and the information is not detailed. Additionally these empiric formulas are limited to use for practical pipe with complex shapes.