What are Hydraulic Grade Line (HGL) and Energy Grade Line (EGL)?

Energy Grade Line (EGL) represents total energy head at any point: EGL = P/ρg + V²/2g + z. It decreases in flow direction due to friction losses and has sudden drops at minor losses. Hydraulic Grade Line (HGL) represents pressure head plus elevation: HGL = P/ρg + z = EGL - V²/2g. The vertical distance between EGL and HGL equals velocity head. HGL can rise or fall depending on velocity changes (pipes expanding/contracting). If HGL falls below pipe centerline, pressure becomes subatmospheric, risking cavitation. These lines are essential for pump placement and pipeline design.