This may have been answered already, but I haven’t been able to find a good answer…
I know that the corny keg posts are different, but I don’t know why it was ever setup this way. Most kegs are labeled inlet and outlet so you can tell which post/disconnect belongs where, but I don’t know why.
I recently inherited a new keg that has two liquid posts installed and caused some difficulty when I used it for the first time since I didn’t realize it. Once I figured it out, I got to thinking. Why not convert all of my kegs to liquid only posts and never run into issues again? Is there any good reason not to do this besides having to buy new parts?
Primarily I think it’s to avoid the possibility of inadvertently hooking the wrong QDs to the wrong posts even if they are labeled on the keg, just takes one easy error out of the equation. I know that I’ve tried to hook the wrong lines to the wrong posts on more than one occasion once the keg goes in the kegerator and the fact that the posts are different saved me from having a potential problem.
If you connect the gas line to a high pressure or recently jostled keg, you could foul your lines, regulator, and tank. For a homebrewer this may not be a huge deal, but for a restaurant using legs of pre-mix soda syrup, it could be a huge head ache.
I don’t have any real information on this just experience with with large systems and support.
Standards help avoid mistakes in fast moving environments so making the 2 posts different is an easy way to avoid errors out in the field. At he homebrewer level it probably isn’t important but on the scale these containers were designed for it likely solved some big issues along the way.
One idea comes to mind might be in a commercial system. Imagine a restaurant with 15 different sodas on the line that are stored in a room in the back. The CO2 tank is outside in a cage. You would need to manage up to 30 lines that may or may not have labels on them. With identical ends you have no obvious way to identify inputs and outputs. Maintenance would involve tracing lines (through walls, under fixtures, etc.) every time there was a problem. At least if you can easily tell the difference between gas and soda you can save some time trouble shooting.
Of the four kegs I have I am 99.999% sure there are no labels on the kegs identifying which post is which and even if there were labels there is always the chance that after cleaning I accidentally reverse the tubes.
Most of my kegs are scribed “in” on the gas side. Not all. Others have a bump or a divot on the rubber handle. I assume the bump means “out” and the divot means “in” but I’ve not tested that theory since it just came to me as I was typing this.