(1)	$\begin{array}{l}\displaystyle \left( \frac{dp}{dl} \right)_f = - \frac{ j_m^2}{2 d} \cdot \frac{f}{\rho}\end{array}$

The friction losses depend on density and friction factor distribution along the pipe.

There are few popular practical approximations based on assumption of constant friction factor and linear density-pressure equation of state.

Approximations

(2)	$\begin{array}{l}\displaystyle \left( \frac{dp}{dl} \right)_f = - \frac{ j_m^2}{2 d} \cdot \frac{f_0}{\rho_0} = \rm const\end{array}$

(3)	$\begin{array}{l}\displaystyle \Delta p(L)=- \frac{j_m^2}{\rho_0} \cdot \frac{f_0 \, L}{2 \, d }\end{array}$

$\begin{array}{l}f(l)= f_0 = \rm const\end{array}$

$\begin{array}{l}\rho(l)=\rho_0= \rm const\end{array}$

(4)	$\begin{array}{l}\displaystyle \left( \frac{dp}{dl} \right)_f = - \frac{ j_m^2}{2 d} \cdot \frac{f_0}{\rho(p)}\end{array}$

(5)	$\begin{array}{l}\displaystyle \Delta p (L) =- \frac{j_m^2}{\rho_0} \cdot \frac{f_0}{2 \, d} \cdot \frac{ 1- \exp \left( - c^* \rho^* G \, L \right)}{c^* \rho^* G}\end{array}$

$\begin{array}{l}f(l)= f_0 = \rm const\end{array}$

$\begin{array}{l}\rho(l)=\rho^* \cdot (1 + c^* \, p)\end{array}$

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