If i didnt have a clue i would use the higher temperature.

Location is important as is distance between compressors. Most gas transmission lines will have compressors far enough apart that incoming gas becomes close to ground temperature plus the JT effect can bring it down even further. However 15C looks a bit low to use for sizing - 20 to 25C is better, but if you get it wrong you under or oversize your machines quite quickly. I would start with a higher temp and then look for analysis as to why it should be lower.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

Thank You So much. Very Helpful ;)

When the units fail because they were specified for too low a summer temperature (which will be the worse case for efficiency and sizing) what is your plan to continue gas delivery to your customers?

Location along a pipeline is important because higher inlet pressures (denser gas) allow the use of smaller machines, and since inlet pressures drop off more and more with distance along a pipeline due to the disproportional increases in friction with lower pressure, transmission efficiency is quickly lost when distances between compressor stations become excessive. Pipeline transmission is more efficient at high pressures and cold temperatures. It is also desirable to have all compressor stations use similar equipment (equal compression ratios), hence a particular characteristic distance between compressor stations of equal compression rations can be maintained for any given volumetric flow rate, inlet temperature and inlet pressure condition.

The required compression ratio is the key to your study. Machine size is affected by inlet temperatures, with the higher inlet temperatures and lower inlet pressures both requiring larger machines for any given compression ratio. Let the inlet pressure fall too much, or inlet temperatures climb too high, and machine sizes increase and distances between compressor stations become too close.

On the other hand, since denser gas enters the compressor at lower inlet temperatures and higher inlet pressures, smaller machines may be used, but more power will be required with lower inlet temperatures for any given discharge pressure.

you must get smarter than the software you're using.

If you are off by 10 degrees F (higher than design), you will lose 2% of the machines capacity and discharge temperature will change 12 degrees.