Sizing Vapor Lines

I use Crane and would recommend it. Use the formulas on page 3-4 (US unit book). I would not use the gas pipeline equations, they are more empirical in nature and are not intended for short lines.

The elevation changes are NOT included in the pressure drop formulas (can't be, there is nothing to enter for elevation differences is there?) but due to the much lower density of gases compared to liquids, they 'typically' aren't significant. They are easy enough to estimate if the pressure drop through the line is fairly small (eg. constant density of the gas). Divide the density of the gas (lb/ft3) by the elevation change (ft), that will give you lb/ft2 and change it to psi. But it won't be much. For example head losses due to acceleration also aren't included but you can include them if you want. If you really want to do the pressure drop, you need to go back to the overall momentum balance equation. The formulas in Crane are based on them for the hydraulic losses due to friction loss, assuming the acceleration head is insignificant and elevation differences are ignored.

as to the effects of changing density due to the pressure loss I suggest you to divide the pipeline into several pieces. Then calculate pressure drop for each piece after correcting density (you can be even more realistic by considering isoenthalpic tranformation, but isothermal behaviour can probably give you good results too)