A phase diagram for water provides, among other things, the relation between pressure and temperature at which evaporation and condensation rates are the same and there’s no net flux. In fact, phase diagrams are only valid in equilibrium conditions, where there’s no net flux.
But there are almost no applications where phase diagrams are useful if you satisfy the zero net flux requirement.
That is, people usually refer to them because they want to evaporate or condense something, so they want to violate the equilibrium condition.
Why is this OK? Are the deviations from equilibrium neglible at practical flux rates?
There’s another issue that could be confused with this. If you have to add or subtract heat at the liquid vapor interface, you have to do heat transfer, usually by conduction through a gradient, so there is this problem of how you get the interface to the temperature you want. I understand that, and separate that issue from the one I’m asking about above. A very reasonable answer to it all might be that the equilibrium error is much smaller than the heat transfer problem in practice, so people can use phase diagrams as if they apply outside of equilibrium conditions, and also work the heat transfer, and get practically correct results.