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# Sizing Vapor Lines

## Sizing Vapor Lines

(OP)
Hello,

I am trying to size a pipe for saturated ammonia vapor.  The flowrate is 1000 #/hr with an initial Pressure of 50-60 psig.  The pipeline will be about 400 ft,.   Any suggestions on what equations to use for pressure drop.  I have read crane 410 and there seem to be quite a few equations for compressible flow all with various assumptions.  Any suggestions will be appreciated.  Are elevation changes included in the pressure loss as they are for fluids, none of the compressible flow examples in 410 mention elevation changes.

Thanks
Replies continue below

### RE: 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.

### RE: Sizing Vapor Lines

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)

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