Okay: I have steam leaving a boiler at T = 873 K and P = 15 MPa and entering a turbine which it leaves at a pressure 2.1 MPa (these data are given), and I want to determine the other thermodynamic properties at this second point. Finding the enthalpy should be straightforward because I know that the isentropic efficiency of the turbine is 0.91. I want to start of course with s2 isentropic = s1 and work back from there.
Now my problem is this: I can't cross-tabulate the isentropic entropy with this pressure in the steam tables--there's no point in the steam tables where this entropy exists at that pressure--so I went instead to the tables for saturated water. Starting at P2 = 2.1 MPa, I then found the "quality" (or "vapour fraction") by applying the lever rule. When applying the lever rule, I can find results which are fairly close (like about 0.5% away from the printed result) from those the lecturer provided, but I am a bit skeptical because maybe it's some sort of coincidence?
My question is: when can I move to using the saturated water tables instead of the steam tables? Is there some kind of rule I can use to do this? It seems almost arbitrary but it has some important repercussions.