Darcy's equation is the equation of choice for incompressible fluids. Nevertheless, it can be used to accurately predict pressure drops in compressible fluids, such as air, steam, etc. The following restrictions should be observed in applying the Darcy equation:
1. Because of the basis of isothermal flow (constant density), if the calculated pressure drop is less than about 10% of the inlet pressure, reasonable accuracy will be obtained if the specific volume used in the formula is based upon either the upstream or downstream condtions, whichever are known.
2. If the calculated pressure drop is greater than about 10%, but less than about 40% of inlet pressure, the Darcy equation may still be used with reasonable accuracy by using a specific volume based upon the average of upstream and downstream conditions. Otherwise, the "Modified" Darcy formula (which includes the "Y" compressibility factor) is to be used.
3. For greater pressure drops (>40% of inlet pressure), such as are often encountered in long pipe lines, the methods detailed out in Crane's Technical Paper #410 should be employed. This includes the "Modified" Darcy equation.
The Panhandle equations, Fritsche's, Weymouth's, etc. are all empirical variations and attempts to define compressible flow. All these equations have their trade-offs and their flaws, as well as inaccuracies. There is no known "super" or compressible equation for all seasons.
Unless you have a bias against using Crane's Tech Paper #410, I would go with what quark suggests: use Crane. Crane even gives you specific examples of how to do the calculations with steam as the fluid. I do not believe that the modified Darcy equation is limited to very low pressures. Crane does not mention this nor do the examples calculations they offer.