Turbine Expansion Lines are drawn on Mollier diagrams as steam expansion in power plant steam turbine. These expansion lines depict the thermal state of the steam as it expands through the turbine. These lines may be developed based on the throttle, and reheat conditions to determine the steam enthalpy at the various extraction points on the turbine. These lines are used in conjunction with a heat balance and the extraction stage shell pressure curves or constants to establish the extraction pressure at which to read the expansion line enthalpy for a given extraction point in the turbine. The expansion of the steam through the turbine results in the heat(enthalpy) and pressure energy conversion to kinetic energy in steam jets. The kinetic energy is change to mechanical energy in the rotationing buckets or blades. This expansion precess can be examined by plotting it on a Mollier chart.

Turbine expansion lines represents the Reheat turbine expansion lines. The overall IP and LP turbine internal efficiency curves are shown in this page. The curves are presented for various reheat turbine inlet temperatures depends on Design and Operation conditions. The reheat internal efficiency curves are plotted as a function of pressure ahead of the intercept valve. Intercept valve pressure is a reasonable parameter to plot efficiency against since it relates to steam flow. The ELEP(Expansion Line End Point) is defined as a function of intercept valve pressure and is based on a turbine exhaust pressure of 1.5inHg. The ELEP calculations can be corrected to other exhaust pressures base on the use of the correction to expansion line end points for exhaust pressure curve.