Energy Grade Line

Terms Energy Grade Line and Hydraulic Grade Line are frequently used by hydraulic engineers. Let us express each of the terms of the Bernoulli equation as a head. We have seen that in absence of work and heat transfer,

$$z_1~+~p_1~+~{{\rho V_1^2 } \over 2}~=~ z_2~+~p_2~+~{{\rho V_2^2 } \over 2}~=~..=~...~=~ H~~$$ (3.74)

where term H is not to be mistaken for enthalpy and is to be taken as the total head. If the above equation is graphically represented we see that the total energy value being constant becomes a horizontal line as shown in Fig. 3.24 and is called the Energy Grade Line. One other line that is defined is the Hydraulic Grade Line, which is the Energy Grade Line take away the velocity head (i.e., V²/2g).

Figure 3.24: Energy Grade Line (EGL) and Hydraulic Grade Line (HGL) for an one-dimensional flow.

 

If the losses are taken into account the EGL will drop accordingly. Any work extraction along the path as with a turbine, will be seen as a sudden drop in the EGL. Any work addition will be reflected as a sharp rise. HGL follows similar trends.

(c) Aerospace, Mechanical & Mechatronic Engg. 2005
University of Sydney