What is a possible reason for the Hydraulic Grade Line being below the pipe invert elevation?

The Hydraulic Grade Line (HGL) represents the potential energy of water in a system due to elevation and pressure. When the HGL is below the pipe invert elevation, it generally indicates that the pressure in the pipe is below atmospheric pressure, which is termed negative pressure. This situation can occur when the flow rate is high enough to create a significant velocity head or when there are sudden changes in elevation or pipe geometry that lead to a drop in pressure.

In essence, the HGL being below the pipe invert signifies that the water within the pipe is under suction, potentially leading to cavitation or other hydraulic issues if the pressure becomes too low. Keeping the HGL above the pipe invert is crucial to ensure that the system remains under positive pressure and functions correctly without causing disturbances such as air entering the system, which could lead to operational problems.