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# What is the hydraulic grade line? How does it differ from the energy grade line? Under what conditions do both lines coincide with the free surface of a liquid?

Solution

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Hydraulic grade line(HGL) : the line which represents the sum of the static pressure and the elevation heads ($\dfrac{P \rho}{g} + z$).

Energy grade line(EGL): the line that represents the total head of the fluid ($\dfrac{V^{2}}{2g} + z$).

The difference between the heights of EGL and HGL is the dynamic head ($\dfrac{V^{2}}{2g}$) We note the following about the HGL and EGL.

When the velocity is zero and the static pressure is zero the energy grade line(EGL) coincides with the hydraulic grade line(HGL).

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