2.4.2 The condition line
The condition line is the locus of the conditions of the steam as it passes from the stop valve at the inlet to the HP cylinder, through all the turbine stages to the exit plane of the final moving blade of the LP cylinder (Fig 1.16).
Ideally the expansion of steam through the turbine will occur isentropically, that is, at constant entropy shown by the vertical dotted line. In reality the friction effects, such as profile and secondary loss, cause an increase in the heat and entropy of the steam.
Figure 1.17 shows a section of the condition line displaying one stage only.
H1 - H2 represents the isentropic heat drop and H, H3 the actual heat drop. Cylinder efficiency then is obtained for HP, IP and LP turbines by considering the heat drop across all the stages of the cylinder.