The Hydraulic Grade Line is a graph of the pressure and gravitational heads plotted along the position of the pipeline or channel.
HGL Elevation = hp + hz
Where:
hp = pressure head (m or ft)
hz = gravitational head (m or ft)
Energy Grade Line
Th Energy Grade Line is related to the hydraulic grade line in that it contains the velocity head as well.
EGL Elevation = HGL + hv
Where:
hv = velocity head (m or ft)
The relationships between the hydraulic and energy grade lines are as such:
- If the pipe is not under pressure, the hydraulic grade line is simply the gravitational head.
- If the pipe is flowing full, the hydraulic grade line will always be below the energy grade line.
- In a reservoir (i.e. non-moving flow not under pressure) the hydraulic grade line will be equal to the energy grade line.
- A decrease in flow area will result in a decrease in the hydraulic grade line, and vice versa.
