Define specific volume of wet steam?

If the pressure of a wet vapour is ≤ 2000 kPa and the dryness fraction is ≥ 0.9, then (if you wish) you may use: Vx=XVg ; where Vx is the specific volume you are solving for, X is dryness fraction and Vg is the specific volume of the saturated vapour at the given state (pressure, temp, etc).

Otherwise use: Vx = (1 - X)Vf + XVg ; and in this case Vf being the specific volume of the saturated liquid at the given state.

The specific volume of wet steam is quite simply, the volume per given mass of the vapour at the given dryness fraction (or steam quality). In the case of wet steam it is solved for by a function of the relationship between the percentage saturated liquid, and the percentage saturated vapour in terms of specific volumes for the two obtained from a data chart.