Valve sizing

Valve size often is described by the nominal size of the end connections but a
more important measure is the flow that the valve can provide. And determining flow
through a valve can be simple. Using the principles of flow calculations, some basic
formulas, and the effects of specific gravity and temperature, flow can be estimated well enough to select a valve size—easily, and without complicated calculations

Gas Flow
Gas flow calculations are slightly more complex because gases are compressible
fluids whose density changes with pressure. In addition, there are two conditions that must be considered—low-pressure drop flow and high-pressure drop flow.

Effects of Temperature
Temperature usually is ignored in liquid flow calculations because its effect is too small. Temperature has a greater effect on gas flow calculations, because gas
volume expands with higher temperature and contracts with lower
temperature. But similar to specific gravity, temperature affects flow by
only a square-root factor