They are just draining the sludge from the bottom of the propane tank. No Problem
While we're at it a prototype propane tank has domed not dished ends due to the pressure they operate at.
I am fine with the minor points - looks well done. I have carbon dioxide tankers at my oil yard. Looks nicer than my effort to date. My bald guy is loading barrels, that's giving him a workout.
Nice job @Joe Fauty