We sell 3 types of UL tanks here. UL 142 Single Wall is a single wall tank that is built with a heavier gauge of steel and has special UL welds and testing methods. UL 142 double wall tanks are made with two walls, the second wall acts as a secondary containment. UL 2085 fireguards are built like a double wall but with a concrete encasing between the two walls. These tanks undergo a 2 hour fire test. 
All tanks come standard on skids but can also be ordered on saddles or skids with tow bars. 
*** Please note: if you are not sure which type of tank you need, you must talk to the fire department in the area which the tank will be set. Mills Equipment Company does not deal directly with fire departments and the rules for each city.