ANS:D - may be above or below the centre line of conduit.

The Hydraulic Grade Line (HGL) represents the total energy level of the fluid flowing in a conduit or pipeline. It accounts for the pressure head, velocity head, and elevation head of the fluid at any point along the conduit. The position of the Hydraulic Grade Line relative to the centre line of the conduit can vary depending on several factors:

1. Elevation Changes: If there are elevation changes along the length of the conduit, the HGL will reflect these changes. For example, if the conduit slopes downward along its length, the HGL will typically fall below the centre line of the conduit at points downstream from the high point.
2. Frictional Losses: Frictional losses due to the roughness of the conduit's walls and the viscosity of the fluid can cause a pressure drop along the length of the conduit. This pressure drop can cause the HGL to fall below the centre line of the conduit.
3. Flow Regime: In different flow regimes, such as laminar or turbulent flow, the distribution of velocity and pressure within the conduit can vary, affecting the position of the HGL.
4. Obstructions and Fittings: The presence of obstructions, fittings, valves, or changes in the cross-sectional area of the conduit can also affect the flow pattern and the position of the HGL.