The net head (sometimes called nett head) acting on a turbine is the difference in Total Head between the Inlet Reference Section of a turbine and the Outlet Reference Section. These Reference Sections have contractually-defined locations: the Inlet is located near the turbine inlet, and the Outlet near the end of the draft tube, upstream of the draft tube gate slot. The Reference Sections are perpendicular to the flow direction and are fitted with arrays of piezometers (in model turbines and prototypes) to measure the water pressure. The Reference Sections are in identical locations in the turbine model and prototype.
During a turbine efficiency test, an array of pressure taps around the Upstream and downstream Reference Sections are used to measure pressure (and hence hydraulic head), the velocity head is computed from the flow rate and the flow area at each section, and the elevations, z1 and z2, at the centre of each sections is determined by accurate survey. The head loss in the water conveyance is therefore excluded from the measurement in this instance, and the Net Head then reduces to the following:
Net Head = Total Head at the Inlet Reference Section – Total Head at the Outlet Reference Section.
The Total Head is the sum of the hydraulic pressure head, H, the velocity head, v²/2g, and the elevation head, z. If we use the suffix “1”: to denote the Inlet Reference Section and the suffix “2” to denote the downstream Reference Section, we arrive at the following equation:
Net Head = (H1 + v1²/2g + z1) – (H2 + v2²/2g + z2)